Preceramic Paper-Derived Ceramics

June 16, 2017 | Autor: Tobias Fey | Categoria: Materials Engineering, Mechanical Engineering, The American

Descrição do Produto

J. Am. Ceram. Soc., 91 [11] 3477–3492 (2008) DOI: 10.1111/j.1551-2916.2008.02752.x r 2008 The American Ceramic Society

Journal

Preceramic Paper-Derived Ceramics Nahum Travitzky,w,z,y Hans Windsheimer,z,y Tobias Fey,z and Peter Greilz,y z

University of Erlangen-Nuremberg, Department of Materials Science (Glass and Ceramics), Erlangen, Germany y

University of Erlangen-Nuremberg, Centre for Advanced Materials and Processes, Fuerth, Germany

A novel class of preceramic paper may serve as a preform to manufacture lightweight as well as multilayer ceramic products. In this article, we discuss the formation, microstructure, and properties of preceramic papers and their conversion into ceramic materials. Oxide as well nonoxide ceramics were processed into single-sheet, corrugated structures, and multilayer ceramics. A high ﬁller loading and uniform distribution of ceramic ﬁllers involves control of colloidal surface interaction including electrostatic, electrosteric, and mechanical retention. Sintering of an oxide-loaded preceramic paper in air results in highly porous products, with the porosity shape and distribution templated by the pulp ﬁber used in papermaking. In the case of carbide-loaded paper, dense composite materials are obtained by reactive inﬁltration processing. Anisotropic properties of machine-fabricated preceramic paper and bonding interfaces in multilayer stacks give rise to anisotropic mechanical properties of the resulting ceramic composites. Noncatastrophic failure was observed when tensile loading stress was applied parallel to the interfaces (in-plane loading). Applying well-established paper processing technologies, including laminated object manufacturing, ceramic structures of complex shape and size can easily be processed, offering a high potential for economical manufacturing.

I. Introduction

C

papers are well-known and widely used materials in various types of refractory and sealing applications because of their high-temperature stability and resistance against moisture, acids, and bases. Usually, ceramic paper is made up of inorganic ﬁbers of silica, alumina, zirconia, mullite, and cordierite and loaded with inorganic powders including natural silicates or synthetic ﬁllers.1–4 The recent work, however, has focused on conversion of bioorganic-based paper into inorganic materials such as ceramics. ERAMIC

D. Green—contributing editor

Manuscript No. 24882. Received June 22, 2008; approved September 5, 2008. The work was ﬁnancially supported by the Federal Ministry of Education and Research (BMBF), Germany and the Federation of Industrial Research Associations (AiF) ‘‘Otto von Guericke e.V.,’’ Germany. w Author to whom correspondence should be addressed. e-mail: Nahum.Travitzky@ ww.uni-erlangen.de

Bio-organic-based paper is a sheet or a continuous web of material commonly formed by the deposition of wood pulp or plant ﬁbers (usually preprocessed lignocellulosic ﬁbers) from an aqueous suspension. The paper may contain a signiﬁcant fraction of powdered minerals, known as ﬁllers, such as pigments, clay (kaolin), talc, Al(OH)3, CaCO3, or TiO2, and even some fraction of synthetic ﬁbers. The processing approach used for the deposition of ﬁbers or their mixtures with or without the addition of ﬁllers deﬁnes the paper properties. Paper-derived ceramics were fabricated by coating paper and pulp with ceramic slurries. The organic fraction of the paper substrate was burned out during ﬁring in air, leaving a porous ceramic residue.5 An additional approach that involved conversion of natural plant ﬁbers like sisal, jute, and hemp6,7 and wood tissue8–12 into ceramic materials of various phase compositions was used for fabrication of biomorphous ceramics from ﬁber boards and paper made of preprocessed lignocellulosic ﬁber preforms. Lignocellulosic ﬁbers are biopolymeric materials with great diversity. Their chemical compositions make use of only minor variations of principally two different monomeric repeat units: monosaccharides (pentoses and hexoses), forming cellulose and hemicelluloses, and phenylpropanoids present in lignin. Cellulose-rich ﬁbers can be separated, isolated, and puriﬁed by aqueous deligniﬁcation and mild hydrolysis in an acidic or an alkaline medium.13 Conversion into nonoxide or oxide ceramic products may be achieved by: (i) pyrolytic decomposition, resulting in a porous carbon replica (template) that may subsequently be reacted to form carbide phases or may be inﬁltrated with nonreacting sols or salts, which can further be processed to yield oxide reaction products; and (ii) inﬁltration of chemically preprocessed native lignocellulosic preforms with gaseous or liquid organo-metallic and metal–organic precursors and subsequent oxidation to remove the free carbon phase.12 Highly porous SiC high-temperature ﬁlters, for example, were prepared by chemical vapor inﬁltration of silanes into carbonized paper preforms.14 Processing of dense composites involves reactive or nonreactive metal melt inﬁltration into carbonized porous ﬁber network.15 Inﬁltration of the laminated paper structures and corrugated cardboard with different preceramic polymer suspensions and postheat treatment resulted in the formation of low-density Si–Al–C–O ceramic composites.15,16 The paper-derived composites in the system Si–Al–C–O were characterized by low shrinkage upon thermal processing of typically 2%–8% (initial polymer volume fraction of 30%–60%) and a noncatastrophic failure behavior with a stress–strain curve similar in

Feature

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shape to that of ﬁber-reinforced ceramic matrix composites. In order to prevent crack formation during consolidation, a high packing density of inﬁltrated powder is preferred, which results in low matrix shrinkage. Dense particle packing around the ﬁbers within a ﬁber preform requires control of slurry rheology. Fiber packing and ﬁber surface properties (e.g., texture, ﬁber-toﬁber bonding, hydrophily, and surface charge) play a major role in controlling the permeability and accessibility of the preform structure. In order to ensure good particle packing, the particles should be much smaller than the ﬁber diameter17 and an external pressure might be necessary to promote effective ﬁlling of the interﬁbrillar space.18,19 In the following, an overview of fundamental aspects relevant for manufacturing ceramics and ceramic composites from highly loaded paper-based preforms (hereinafter called preceramic paper) is presented. In the preceramic papers, inorganic ﬁller materials such as ceramic or metal particles were incorporated into the paper sheet during the papermaking process. As a consequence of the conversion process from the bio-organic ﬁber preforms into ceramic products, this novel class of preceramic paper might offer a versatile and economic approach to process lightweight materials with tailored macro- and microscopic porosities for a broad range of applications. Our focus is on the scientiﬁc issues associated with ﬁller loading of a porous ﬁber sheet, conversion into the ceramic product by heat treatment, inﬁltration reactions, and the structure–property relations dominated by the speciﬁc ﬁbrillar preform architecture. The inﬂuence of the structural anisotropy of paper sheets, caused by the machining process, on the mechanical properties of the resulting lightweight ceramics will also be discussed. Potential applications of the preceramic paper for fabrication of ceramic parts will be demonstrated.

II. Preceramic Paper Preceramic paper is a multiscale composite material. The fabrication of the preceramic paper commonly involves the use of a paper machine and can be subdivided brieﬂy into: (i) preparation of an aqueous feed suspension containing ﬁller and wood pulp or cellulosic ﬁber, (ii) coagulation of the ﬁber and ﬁller in the suspension using polymeric additives, and (iii) formation of the paper sheet by dewatering the feedstock. The properties of paper such as smoothness, porosity, dimensional stability, pore size distribution, density, stiffness, strength, and compressibility are strongly dependent on the bonds between the ﬁbers. These bonds are primarily hydrogen bonds. A high loading of the inorganic ﬁllers will modify the ﬁber bonding and hence the properties of the preceramic paper.

(1) Fibers Pulp ﬁbers are most commonly used in papermaking.20 Pulp ﬁbers can be made from a variety of wood plants like spruce, pine, eucalyptus, and many others. Pulp ﬁbers are extracted from wood in chemical, mechanical, or chemi-mechanical processes. Typical ﬁbers used in papermaking are about 1–4 mm long and roughly 10–30 mm wide. Figure 1 shows the structure of pulp ﬁbers used in papermaking. The ﬁbers themselves have a cellular structure with a hollow lumen surrounded by the cell wall having a typical thickness of 2–5 mm, Fig. 1(a).21 The cell wall is also a composite material with three signiﬁcant layers and, in each layer, there exist up to 50 vol% of largely crystalline cellulose microﬁbrils embedded in a mixture of amorphous biopolymers (mainly hemicellulose (25%–40%) and lignin (15%– 30%)). Figure 1(b) schematically shows the macroscopic habitus of pulp ﬁbers, which may attain curled or kinked shapes depending on the processing conditions. Cellulose (C6H10O5)n is the main load-bearing component in ﬁbers and is able to attain a high stiffness and tensile strength due to intra- and intermolecular hydrogen bonding. Figure 2 shows intra- and intermolecular hydrogen bonds in two adjacent cellulose molecules.22

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Fig. 1. Structure of the pulp ﬁbers used in paper making: (a) model of the cell wall structure (ML, middle lamellae; P, primary wall; S1 and S2, secondary walls; T, tertiary wall; and W, wart layer)21 and (b) macroscopic habitus of pulp ﬁbers.

The degree of polymerization n in a cellulose ﬁbril of a native softwood ﬁber is above 10 000, declining in chemical pulping to 500–2000. The cellulose crystal is one of the strongest and stiffest of organic molecules with a modulus of 145 Gpa23 and a theoretical tensile strength estimated at 7500 MPa.24 The orientation angle of the cellulose ﬁbrils with respect to the ﬁber axis affects the mechanical properties of single ﬁbers and therefore has a signiﬁcant inﬂuence on the mechanical behavior of the paper sheet. Owing to variation of wood source, growth conditions, and chemical and mechanical treatment, the strength of ﬁbers varies over a wide range and is commonly described by means of Weibull distribution characterized by low Weibull moduli of 3–5.20 For example, the elastic modulus and tensile strength of pine ﬁber were reported to be 14.8 GPa and 600 MPa for early or spring-wood (the part of the wood in a growth ring of a tree that is produced earlier in the growing season) and 20 GPa and 1020 MPa for late or summer-wood (the part of the wood in a growth ring of a tree that is produced later in the growing season).25 An additional complication is that the deformation of the ﬁber inﬂuences the tensile strength of the ﬁbers and the bonding ability of the ﬁbers in the paper structure, which can cause nonlinear stress–strain behavior in the paper upon loading during machining and shaping.26

Fig. 2. Intra- and intermolecular hydrogen bonds in two adjacent cellulose molecules (according to Klemm et al.22).

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(2) Paper Formation Sheet formation from the low concentrated (0.5–5 wt% of solid content) suspension is the key step in preceramic papermaking. It is desirable to retain as much as possible of the ﬁller and ﬁber in the sheet, which, in the dry state, may attain a ﬁller-to-ﬁber weight ratio of 5:1–10:1. Thus, control of ﬁller-to-ﬁller and ﬁllerto-ﬁber interaction is essential for attaining a high ﬁller loading of preceramic paper that still exhibits the good ﬂexibility and strength necessary for paper-based shaping and machining procedures. For the fabrication of preceramic paper, a papermaking suspension comprising of ﬁbers, ﬁllers, retention aids, and binder is transferred to the paper machine, where a continuous sheet of paper is then formed by dewatering as shown schematically in Fig. 3. Water is removed by drainage through the voids in the wire by applying a vacuum under the wire. As a result, a wet web consisting of pulp ﬁbers and ﬁller particles is formed. After this initial dewatering, the web is then transferred to the press part of the paper machine, where the water content is further reduced by mechanical pressing of the web between metal rolls. After the mechanical pressing, the paper web is transferred to the drying part of the paper machine, where it is transferred between metal cylinders and dried by warm air. After the drying step, typically a water content of about 5 wt% remains in the paper sheet. In addition, at the dry end of a papermaking machine, the preceramic paper can be smoothed and polished between sets of rollers. This step is called calendering. The paper sheet is then wound on rolls for further processing/transportation. On an industrial scale, paper machines operate at high velocities up to 35 m/s. The typical thickness of common paper varies from 100 to 300 mm and paper density, which is characterized by weight per area (grammage), varies from 20 to 300 g/m2 (e.g., typical writing paper has a density of 70–80 g/m2). Compared with writing paper, preceramic paper may contain a substantially higher ﬁller fraction 480 wt% of inorganic powder, resulting in a density of 4300 g/m2. Because the ﬁller particle size is substantially smaller than the average papermaking fabric pores (70–100 mm), the retention of ﬁller in a paper sheet is a ﬁltration process. Retention produced through ﬁltration, or mechanical entrapment, involves catching the ﬁllers in the matrix of ﬁbers as the web forms on the wire. As the ﬁber mat builds, more ﬁller is caught in the denser network, which provides more opportunities of entrapment. In modern high-speed paper machines, due to the short residence time, the contribution of mechanical entrapment for ﬁller retention is signiﬁcantly reduced. The importance of colloidal aggregation therefore increases. (A) Retention: Retention is the key to efﬁcient papermaking. Chemically assisted retention uses the processes of coagulation and ﬂocculation to increase the effective size of the ﬁller particles. The lignocellulosic-based ﬁbers used for making paper develop a negative surface charge in water as a result of the dissociation of carboxyl and sulfonic acid groups

þ

½COOH þ H2 O ! ½COO þ H3 O ; ½SO3 H þ H2 O ! ½SO3 þ H3 Oþ ; þ

pKa ¼ 3 4

Cations may be attached by electrostatic and adsorptive forces (van der Waals forces) to these surfaces. The negative charge on the surface is partially neutralized by these cations, causing the net potential energy of the system to reduce rapidly in this region as explained by the electric double-layer model.27,28 (a) Coagulation: Coagulation is the initial step in the retention process. The principal mechanisms include neutralization of surface charges and screening of electrostatic repulsions by increasing the electrical conductivity of the solution. During coagulation, the electrostatic sphere of charge that surrounds the small particles and keeps them well separated is neutralized by a cationic source (coagulant). Reducing the extent of the repelling forces allows the particles to come closer together. Effective coagulation is reached when the distance that separates the particles is sufﬁciently small that a high-molecular-weight (HMW) polymer (ﬂocculant) can span between the particles to form a ‘‘bridge,’’ producing agglomerates that can be retained by ﬁltration through the forming web. Coagulants include inorganic or organic polymers. Alum is the most common inorganic coagulant for acid (pH 4.0–5.5) papermaking systems. Papermaker’s alum is aluminum sulfate. Concentrations and addition amounts are usually based on either (a) the equivalent amount of Al2O3 or (b) the equivalent amount of the hydrate, Al2O3 14H2O. At a higher pH ( 5.5), alum is only weakly cationic, and becomes much less effective. Polymeric organic coagulants are designed to perform over a broad pH range. They have been used effectively in systems ranging from pH 4.0 to 8.5. Organic coagulants are highly cationic, low-molecularweight polymers with molecular weight o200 000. Representative coagulants are shown in Table I. (b) Flocculation: Flocculation is the second step in the retention process. The word ‘‘ﬂocculation’’ is more often used when one is referring to treatment with retention aids that are capable of forming shear-resistant bonds between ﬁne particles, ﬁber–ﬁnes, and ﬁbers. HMW polymeric ﬂocculants are added to bridge the neutralized particles (after coagulation) and hold them in the sheet. Flocculants are usually polyacrylamide, polyethylene, or polyethylenimine-based polymers with a molecular weight from about 500, 000 to tens of millions. Unlike coagulants, which are always cationic, ﬂocculants can be cationic, anionic, or nonionic. The mechanisms for attachment of an HMW polymer to the particle surface are not completely understood. Two mechanisms are probably most important: hydrogen bonding and ion pairing. Acrylamide polymers have a multitude of hydrogen bonding sites that enable the polymers to attach to the surfaces

Table I. Coagulation and Flocculation Agents Used in Preceramic Paper Making29–31 Retention aid polymer

H

pKa ¼ 1 2

½A 1 (where pKa 5 log10 Ka and Ka ¼ ½H½HA for HA - H 1A ).

Monomer

Polyvinylamine

Mw (106 g/ mo1)

pKb

o0.05

10.53

H N CH CH 2 n

NH 2

Polyacrylamide (cationic)

C

O

CH

1–10

0.5

0.5–2

10.98

CH 2 n

H

Polyethylenimine

CH 2

CH 2

N n

Fig. 3. Schematic representation of preceramic paper sheet manufacturing.

With pKb 5 log10 Kb and Kb ¼ ½BH ½½BOH pKb 5 14–pKa.

for B1H2O-BH1OH; in water

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of ﬁllers. Cationic polyacrylamides can become attached to oppositely charged particles through ion pairing. The retention process may be characterized by deﬁning a degree of retention, DR. Measurements of the zeta potential have been used to quantify the surface charge of particles, and the colloid titration ratio of the anionic to the cationic demand of pulp slurries was evaluated. Both techniques have proved convenient to monitor the retention level. The DR should exceed 90% in order to achieve a high ﬁller loading and a homogeneous preceramic paper microstructure.

(3) Preceramic Paper Composition For the fabrication of preceramic paper, the type of pulp used may depend on the desired ceramic product composition. For the fabrication of carbide ceramics, pulp ﬁbers attaining higher carbon yields after pyrolysis are favored in order to increase the overall ceramic yield. For the fabrication of oxide ceramics, pulp ﬁbers with low ash contents are chosen to minimize the introduction of impurities into the fabricated engineering ceramics. Depending on the papermaking process, for example, rolling and pressing, porosity typically ranges from 35% to 55%. If preceramic paper is sintered in an air atmosphere, the pulp ﬁbers will decompose and be oxidized, leaving an additional porosity with a morphology, distribution, and orientation according to the ﬁber template. Figure 4 shows a compositional diagram for preceramic paper with major constituents ﬁbers, ﬁllers, and porosity. While inorganic ﬁller loadings of 20–30 wt% (which corresponds to 5–10 vol%) of kaolin, talc, alumina, lime, or titania are used for producing common printing and writing paper,32 signiﬁcantly higher loading fractions of ﬁller up to 90 wt% (30– 40 vol%) are necessary in preceramic paper to attain consolidation as well as high ceramic yields upon the organic–inorganic transformation. Inorganic ﬁllers include oxides, carbides, and nitrides as well as metals or intermetallics. Filler particle sizes should not exceed 30 mm in order to avoid sedimentation during sheet forming which can cause gradient structure in the ﬁllerloaded sheet. Solid ﬁller powders should exhibit positive surface charge in aqueous media for proper incorporation into paper microstructures by the retention mechanisms discussed above. Organic and other additives such as defoamers and dry strength agents are added to modify the paper structure and to improve the mechanical properties of the paper sheet. Table II presents the typical compositions of a preceramic paper compared with conventional writing paper.

0.0

1.0

0.2

fp ef

0.6

SiC preceramic paperw

20–30 0.5–2 30–50 110 80

83 0.8 40 200 350

80 4.5 47 240 310

w

Fabricated by Papiertechnische Stiftung.

(4) Anisotropy of Paper Owing to the continuous nature of the papermaking process, the orientation distribution of the ﬁbers is generally weighted in the direction of manufacturing. The symmetry of the resulting anisotropic materials is usually described with a three-principal orthogonal axis coordinate system as shown in Fig. 5(a). The ﬁbers that are curled and kinked when consolidated in the sheet are primarily oriented in the plane; furthermore, within the plane, ﬁbers are more highly oriented in the machine (rolling) direction (MD) than the transverse direction (CD). Figure 5(b) shows machine-fabricated Al2O3 and SiC-ﬁlled preceramic papers. Common methods to characterize ﬁber orientation include mechanical testing, light diffraction, small-angle light scattering, and ultrasonic propagation. Phase-contrast X-ray microtomography, followed by complex image processing can be used for acquiring binary three-dimensional images of paper with a resolution down to 1 mm. Stereological principles were applied to extract information from sets of cross-sectional images of different orientations were used to develop a conformal representation of orientation and density of ﬁber and porosity structures.33 Light-scattering measurements in the three principal directions of the paper sheet34 can reveal the average shape of the porosity, which is represented by an ellipsoidal pore.35 The principal axes of the equivalent pore are considered to be useful structural parameters, from which models were derived to describe the effect of paper structure on the paper properties.36 Figure 6 shows SEM micrographs of an Al2O3-ﬁlled preceramic paper (fabricated by the Papiertechnische Stiftung - Paper Technology Specialists (PTS), Munich, Germany) for two principal orientations. While the ﬁbers in the CD–MD plane only reveal a low degree of preferential orientation, the ﬁbers are highly oriented perpendicular to the ZD direction. Characterization of the spatial variation of density, that corresponds directly to the ﬁller distribution in the sheet, is of particular importance in order to control shrinkage upon sintering and to avoid residual stress formation, which can cause delamination during processing of multilayer laminate ceramics. Figure 7(a) shows a transmission microwave (90 GHz) amplitude scan37 of an alumina-ﬁlled preceramic paper. The paper

preceramic paper

0.8

(a)

(b) ZD

res

Vo

0.4

fib

writing paper

of

lum

Al2O3 preceramic paperw

n tio

rac

tio

0.6

Filler content (wt%) Mean particle size (mm) Porosity (%) Sheet thickness (mm) Areal density (g/m2)

Filler (CaCO3)loaded writing paper

rac

no

0.4

ef

lum

or

Vo

es

0.8

Table II. Composition of Al2O3- and SiC-ﬁlled Preceramic Paper in Comparison with Common Writing Paper

0.2

MD CD

1.0 0.0

0.0 0.2

0.4

0.6

0.8

1.0

Volume fraction of filler particles Fig. 4. Compositional diagram showing the range of preceramic paper compared with common writing paper.

Al2O3

SiC

Fig. 5. Principal directions with respect to paper machining and ﬁber orientation directions (a) Al2O3- and SiC-ﬁlled preceramic papers; (b) schematic representation of the preceramic paper).

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(a)

(Fig. 8). Usually, the total strain e is split into elastic eelast and plastic eplast parts

(b) MD

MD

filler

cellulose fiber CD

e ¼ eelast þ eplast

ZD

(1)

filler

100 µm

cellulose fiber

Fig. 6. Typical microstructure of a preceramic paper loaded with 80 wt% Al2O3 ﬁller (PTS, Munich, Germany): (a) top surface view and (b) cross-sectional view (right).

e¼

(5) Mechanical Properties of Paper In terms of mechanical properties, paper is described as an orthotropic material with only minor property differences for inplane orientations but pronounced property changes observed in out-of-plane orientations. For example, the anisotropic elastic modulus ratio EMD/ECD typically ranges from 1 to 6 and EMD/ EZD is greater than 100.38 Load–displacement curves of preceramic paper (80 wt% Al2O3) in the three principal directions show similar behavior with large differences in fracture stress and strain between the out-of-plane and the in-plane orientations as shown in Fig. 8. Compared with ceramic foils and sheets prepared by tape casting of ceramic powder slurries, which, depending on the solvent and the organic ﬂocculent, binder, and plasticizer systems, may attain a tensile strength of 1–5 MPa and a strain-to-failure of 5%–20%,39,40 the ﬁbers in preceramic paper structure lead to higher strength at least in the MD direction. Paper exhibits complex mechanical behavior, commonly characterized by a highly anisotropic linear elastic response under a moderate mechanical load and a nonlinear plastic response at higher loads. Similar to common papers the preceramic paper also follows the parabolic stress–strain relation for plastic strain

Amplitude Attenuation (a.u.)

Y-Position (mm)

60 50 40 30 20 10 0

(2)

where E is Young’s modulus and E0 and n 1 are the hardening modulus and the hardening exponent, respectively. For the Al2O3-loaded preceramic paper: n 5 2.3–2.5 for CD and 2.0– 2.8 for MD; E0 5 1.1–1.9 GPa for CD; and 0.79–1.1 GPa for MD. Continuum modeling of the mechanical behavior of paper sheet properties is based on anisotropic elastic–plastic constitutive models. Most models use orthotropic elasticity extended by an isotropic strain-hardening plasticity term.41,42 Because the inplane elastic–plastic material behavior under tensile loading conditions is of particular relevance for most handling, shaping, and machining processes with paper sheet preforms, orthotropic symmetry signiﬁcantly reduces the complexity of the determination of the material parameters,43 without much loss in the generality of the model. Compared with common writing paper, which can attain a tensile fracture strength of 470 MPa in the MD direction and 420 MPa in the CD direction,44 the lower volume fraction of ﬁbers present in the highly ﬁller-loaded preceramic paper results in reduced values of 7 MPa in the MD and o4 MPa in the CD directions, respectively. The mechanism of paper fracture involves, in the ﬁrst stage, rupture of the interﬁber links. Fracture initiation is therefore a ﬁber network failure process.45 Further on the weakest ﬁbers begin splitting, cracks appear, and when stress redistributes, they grow to be the main crack.44 Micromechanical modeling of the paper fracture commonly implies the development of a paper network constitutive model based on force equilibrium or strain energy methods.46 The plasticity of the ﬁber–ﬁller network is caused by stretch and pull-out of the ﬁbers. When the ﬁber network is loaded, the stresses are transferred from one ﬁber to its adjacent ﬁbers and ﬁller particles. (A) Analytical Models: The analytical or closed-form models for the mechanical properties of paper can be roughly divided into two groups, with one dominated by ﬁber properties

sheet investigated was then subdivided into pieces of 10 mm 10 mm, and the geometrical density of each individual piece was determined by the Archimedes method for reference. Plotting a density map from these data, Fig. 7(b), reveals that the density of the Al2O3-ﬁlled preceramic paper varies in the range of 1.5 and 2.3 g/cm3. The mean density of the preceramic paper calculated for a porosity of 45% equals to 1.9 g/cm3. Therefore, in the paper, the relative density variation is 720%.

(a)

n s s þ E E0

17

(b)

2.3 2.2 16

2.1 2.0 1.9

15

1.8 1.7 1.6

14 0

10

20

30

40

50

X-Position (mm)

60

Density (g/cm3)

100 µm

Uniaxial strain e in the principal material directions of most paper grades under increased loading stress s is well described by the Ramberg–Osgood relation, which, in a slightly modiﬁed form is given as41,42

1.5 0

10

20

30

40

50

60

X-Position (mm)

Fig. 7. Microwave transmission amplitude scan (a) and density distribution derived from a microwave scan and referenced to density measurements (b).

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ﬁller-loaded common paper (Fig. 9(a)) will be reduced in number and substituted by ﬁber–ﬁller–ﬁber bonds at the ﬁber crossings (Fig. 9 (b)). Signiﬁcant research has focused on the relative importance of the mechanical properties of the ﬁbers versus those of the interﬁber bond in inﬂuencing the fracture behavior of a paper sheet. The failure of a weakly bonded sheet is predicted to occur when the sheet strain reaches a critical strain at the maximum point of the stress–strain curve. In the weakly bonded sheets, most ﬁbers are pulled out intact. Sheet strength is obtained by calculating the stress corresponding to the critical sheet strain. Strongly bonded sheets fail when the sheet strain reaches the failure strain of ﬁbers. Thus, for example, the tensile strength of a sheet, TS, was described to correlate with the ﬁber tensile strength, sf: Ts ¼ ksf

Fig. 8. Stress–displacement curves of uniaxial tensile loading in the principal material directions of an Al2O3 (90 wt%)-loaded preceramic paper.

and the other group assuming that the structure of paper can be treated as a ﬁber-reinforced composite. This class of models are called pull-out models. The main feature of common ﬁber strength models such as the models of Cox,47 Page,48 or Allan and Neogi49 is that the paper sheet properties depend on the properties of the ﬁbers and a structural function G. Thus, the elastic modulus for a sheet, Es, is given by 1 r Es ¼ Ef G 3 rf

(3)

where r and rf are the apparent densities of the sheet and the ﬁbers, respectively, and Ef is the elastic modulus of the ﬁbers. The structural function G mainly accounts for the ﬁber length, ﬁber orientation (e.g., fraction of ﬁbers in the loading direction), and interﬁber bonding. Stochastic network models were developed to consider the complex multiscale composite microstructure of paper and to account for the effect of ﬁbrous-particulate joints on the deformation and failure behavior of particle-loaded ﬁbrous structures.50 The ﬁber bonding strength is strongly related to the ability of cellulose-based pulp ﬁbers to form H-bonds at the ﬁber crossings. The theory of H-bond-dominated solids adopts a molecular approach to describe the macroscopic behavior of paper.51 Elasticity is assumed to arise from interatomic potentials and the straining of molecular bonds. The H-bond theory provides a reasonable qualitative and quantitative description of the effects of moisture, temperature, and time on the elastic modulus of paper based on thermodynamic principles.52 A principal weakness of this theory, however, is that, when compared with structural theories of paper elasticity, it ignores the contribution of ﬁber morphology and ﬁber distribution toward the elastic modulus. Furthermore, due to ﬁller loading in the preceramic paper ﬁbers are likely to be surrounded by a layer of ﬁller powder and the direct ﬁber-to-ﬁber bonds that dominate in low

(4)

where k was given as a constant (k 5 1/3) as in the early Cox model47 or formulated as a complex function that accounts for interﬁber bonding shear strength and microstructure parameters such as ﬁber length, ﬁber orientation, etc. in the more recent models of Page,48 Allan and Neogi,49 Kallmes et al.,53 and Kane.54 In a preceramic paper containing a substantial amount of ﬁller particles (up to 40 vol%), tensile strength based on ﬁber pull-out as the dominating mechanism generally scales with a frictional strength, sfric, the number of ﬁbers per unit area, nf, and the square of the average ﬁber length, lf. For weakly bonded sheets, the sheet strength is given by54 Ts / sfric nf lf2

(5)

while for strongly bonded sheets: Ts / sfric nf lf2 þ sf nf lf2

an additional contribution is given by the ﬁber strength, sf, and the number of ﬁbers, nf , of embedded length greater than half the critical length, lf . Substitution of sfric by the effective shear strength representing the friction stress developed by particleparticle or particle–ﬁber interaction in the pull-out models may be used to derive critical conditions for the fracture of a ﬁllerloaded preceramic paper structure. Figure 10 shows a representative fracture surface of an Al2O3-ﬁlled preceramic paper. All of the models discussed above assume that all bonds or segments reach the failure threshold simultaneously. However, because the sheet is not a homogeneous structure, as for example shown by the variation of density, and the stresses in ﬁbers are not uniform because of stress transfer, the local structure of the ﬁber network in paper will strongly affect the mechanical behavior. Hence, recent models have focused on incorporating local bond structure at ﬁber crossings,55 ﬁber length and orientation distributions,56 the ratio of ﬁber length to ﬁber strength,57 and the speciﬁc packing structure of the ﬁbers in the paper sheet based on probability density function analyses of ﬁber orientation.58 While the tensile strength of ﬁller-free paper has been studied extensively, there is only limited information about the inﬂuence of inorganic ﬁllers on the properties of papers. The ﬁllers gener(a)

(a)

(6)

(b)

(b) CD

ff

MD

ff

fp

fp 100 µm

100 µm

Fig. 9. Fiber–ﬁber (ff) and ﬁber–ﬁller particle (fp) bonding in: (a) common paper and (b) SiC ﬁller-loaded preceramic paper (right).

1 mm

10 µm

Fig. 10. Fracture surface of an Al2O3-ﬁlled preceramic paper showing extended ﬁber pull-out of ﬁbers oriented along the principal tensile load applied (a); the ﬁbers are covered by ﬁller powder (b).

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Fig. 11. Multilayer and corrugated preform structures built up from sheet and corrugated paper.

ally act as a debonder between ﬁbers and may therefore, lead to strength loss of the paper. Experimentally, the tensile strength decreases linearly as the ﬁller-speciﬁc surface area increases.59 Different methods such as preﬂocculation, lumen loading, polymer-treated ﬁllers, etc., have been attempted in order to increase the ﬁller content without decreasing the tensile strength of the sheet.

(6) Paper Conversion The mechanical properties of the preceramic paper have a signiﬁcant inﬂuence on handling, machining (e.g., embossing, calendering, and cutting), shaping, and joining of paper sheets. Because the paper conversion may be described as a process combining two partial processes: shaping (compression, bending, or a combination of both) and shape ﬁxing (plastic deformation or joining), an elastic–plastic behavior and a high mechanical stability are desired for achieving and maintaining the shape of the preform during conversion into the ceramic component by sintering and/or inﬁltration at elevated temperatures. A single paper sheet, a multilayer laminate, and a corrugated board offer a wide range of fabrication routes for thinwalled lightweight ceramic components of complex shape or large volume as shown schematically in Fig. 11. Rapid prototyping techniques based on paper sheet cutting and lamination offer an elegant and ﬂexible approach for CAD-controlled manufacturing of three-dimensional components. Stacking of preceramic paper of different compositions facilitates fabrication of components with a gradient variation of composition and properties, for example, functionally graded materials. (Sidebar A).

III. Ceramic Formation Conversion of the preceramic paper preform into a ceramic product involves removal of the bio-organic pulp ﬁbers and consolidation of the inorganic ﬁller powder compact. Depend-

Sidebar A. Laminated Object Manufacturing of Multilayer Ceramics Laminated object manufacturing (LOM) generates three-dimensional objects by sequential stacking, laminating, and shaping of paper sheets. It can be considered as a hybrid between additive and subtractive processes: a part is built up in a layer-bylayer lamination approach (additive process), but each layer is individually cut by a special tool (subtractive process) in the shape of the cross section of the part. Each layer is bonded to the previous layer with a thermoplastic adhesive coating on the bottom side of the paper sheet, which is activated by heat and pressure during the LOM processing (Fig. A1). Computer-numerical controlled (CNC) cw-CO2-laser proﬁle cutting is used in LOM of preceramic paper. Coating of the paper preform with adhesives may easily be achieved by spraying or screen printing. Adhesives are based on thermoplastics, which may additionally contain ﬁne-grained ﬁller of the same composition as in the preceramic paper to accommodate for differential interface shrinkage upon bonding. Adhesive layers with an initial thickness of approximately 60 mm, which after LOM processing reduces to 40 mm, were developed based on an aqueous mixture of polyacrylate, polymethacrylacid, and polysiloxane containing a submicrometer-sized Al2O3 ﬁller. A typical composition is given in Table A1. Lamination process was simulated using FE calculations (FEMLAB, COMSOL, Sweden) of stress and strain distribution during roller compression. The simulation model consisted of two continuously adhesive-backed paper layers on a rigid support. Properties used for calculation are given in Table A2. The lamination force transmission was performed by a salient in the topmost paper layer showing the actual radius of curvature of the lamination roll, Fig. A2. Including the roller weight, the lamination pressure was adjusted to 50 N and transmitted to the stack through the salient. For FE calculations of stress/strain distributions, the domain was subdivided in 3763 Lagrange—quadratic default elements of triangular shape. The mesh density was increased in the area below the roller salient. A static analysis of the stress/strain distribution was conducted using a stationary linear solver. Considerable compressive stresses were found in the paper layer below the roller. The peak stress attained is 95% of maximum stress expected while adjacent areas are subjected to only minor stresses. Cutting of each layer by local CO2-laser irradiation is based on thermal-induced local oxidation (combustion) of the organic constituents. Geometrical precision of laser optical cutting of a single sheet depends on the laser beam interaction with the preceramic paper. Insufﬁcient laser cutting of a paper layer may lead to problems during the removal of the part out of the paper block (demolding). If the depth of a cutting notch exceeds the sheet thickness, deterioration of the layers underneath may result

Table A1. Composition of the Adhesive for LOM Processing of Al2O3-Filled Preceramic Paper

Polyacrylate dispersion Al2O3 ﬁller (d50 5 0.8 mm) Dispersant Defoaming agent Water Fig. A1. Schematic setup of laminated object manufacturing.

Wet (wt%)

Dry (wt %)

Dry (vol%)

60.8 19.5 0.9 0.6 8.2

64.1 32.5 1.5 1.0 —

83.4 14.0 1.5 1.1 —

LOM, laminated object manufacturing.

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Sidebar A. Continued in decreased surface quality or shape accuracy. The cutting behavior of Al2O3- and SiC-ﬁlled preceramic paper is dictated by the optical properties of the ﬁller particles with respect to the wavelength of the incident laser radiation. Table A3 lists relevant properties of the preceramic paper constituents. Figure A3 shows a plot of the line energy versus degree of single layer cutting for two different preceramic papers ﬁlled with SiC and with Al2O3, respectively. The line energy of the laser beam during cutting operation, EL, was calculated from the laser power, PL, and the cutting speed, vL: EL ¼ PL =vL

ðA:1Þ

The degree of single layer cutting, for example, the cutting ratio, L: L ¼ x=d

ðA:2Þ

Table A2. Properties of the Materials Used for Simulation of the Lamination Process Young’s modulus E (GPa)

Poisson ratio n

15 1.5 200

0.25 0.4 0.33

Paper layer Adhesive polymer layer Rigid support (Fig. A1)

Fig. A3. Degree of single-layer cutting as a function of line energy for a CO2-laser beam cutting for Al2O3- (top) and SiC-ﬁlled (bottom) preceramic papers. Fig. A2. FE modeling of stress and strain during the roller lamination process: net generation (top) and distribution of equivalent compressive stresses (bottom).

Table A3. Optical Properties of Paper Constituents at the CO2 Laser Wavelength (10.6 mm)60–62 Optical property

Relative absorptivity (%) Relative reﬂectivity (%)

Al2O3

SiC

Cellulose

90 10

40 50

85–90 10–15

Fig. A4. Top view of laser-cut laminated object manufacturing-processed components with complex geometry: turbine rotor (diameter: 60 mm) prepared from an Al2O3-ﬁller preceramic paper (left) and gear wheel (diameter 50 mm) manufactured from SiC-ﬁlled preceramic paper (right).

was determined by optical measurement of the depth of the cutting notch, x, and the thickness of the paper sheet, d. Both paper grades show an increasing cutting ratio L up to a limiting laser power, EL , beyond which complete sheet separation occurs during laser cutting. The minimum laser energy necessary for successful Al2O3 paper sheet cutting is lower ðEL ¼ 180 J=mÞ than the minimum power necessary for SiC sheet cutting ðEL ¼ 450 J=mÞ. This result can be explained by higher reﬂectivity of SiC under CO2-laser irradiation (Table A3). Examples of laser cut components prepared from preceramic papers applying LOM processing are shown in Fig. A4.

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ing on the kind of ﬁllers and the desired product phase composition three major processing routes have been investigated: i. Oxide ceramics are formed by annealing an oxide-ﬁlled preceramic paper preform in air to decompose and oxidize the ﬁbers in the temperature range of 3001–8001C, followed by sintering at elevated temperatures (12001–16001C); ii. Carbide ceramics are obtained after pyrolysis in an inert atmosphere at 3001–8001C to form a biocarbon template preform and subsequent reaction with carbide forming ﬁllers like Si, Ti, or intermetallics at temperatures below the melting temperature of the ﬁller phase; iii. Nonoxide composite ceramics involve the formation of a highly porous biocarbon template preform into which a liquid or a gas phase is inﬁltrated and there is ﬁnal consolidation during cooling (melt) or decomposition (gas phase); and because the ﬂuid inﬁltrant ﬁlls up the remaining pore space without changing the overall preform dimensions, a near net-shaped composite can be obtained which is distinguished by low residual porosity. Controlled thermal decomposition of the cellulose-based wood ﬁbers to form a biocarbon template is strongly affected by the kinetics of the transport of gaseous decomposition products via the open pore network system.12 The mechanisms involved in the conversion of cellulose into carbon are: (a) desorption of adsorbed water up to 1501C, (b) dehydration of cellulose between 1501 and 2401C, forming fragments with C 5 O and C 5 C bonds, (c) chain scissions, or depolymerization, and breaking of C–O and C–C bonds within ring units evolving H2O, CO, and CO2 between 2401 and 4001C, and (d) aromatization leading to highly disordered graphitic layers of carbon above 4001C. Heating of pulp ﬁbers, which, in addition, contain lignin and hemicellulose generally, gives rise to the same products. Pyrolysis occurs in a stepwise manner, with hemicellulose breaking down ﬁrst at 2001–2601C, cellulose next at 2401– 3501C, and lignin at 2801–5001C.63 Between 2001 and 4001C, almost 80% of the total weight loss occurs which may vary between 40% (lignin) and about 80% (cellulose). Water, carbon dioxide, acids, carbonyl groups, and alcohols are the major volatile species, whereas clusters and free radicals of carbon species are present in the carbon residue. When the preceramic paper is annealed in oxidizing environment, thermal decomposition of the ﬁber into a carbon ﬁber is immediately followed by oxidation. Compared with graphitic carbon, where oxidation tends to start at 5001–6001C, the highly disordered structure of the biocarbon phase leads to oxidation temperatures as low as 3001C as shown in Fig. 12. Additional organic additives including binders and retention aids decompose in the same temperature range and contribute to the total weight loss according to their initial weight fraction. Owing to the short gas transportation distances for typical sheet thicknesses of o1 mm, the decomposition did not lead to formation of cracks even when heating rates as high as 501C/min were used.

(1) Sintering The presence of an anisotropic ﬁber network during incorporation of ﬁller particles and its decomposition and oxidation during pyrolysis, results in both an anisotropic pore structure and a spatial variation in powder packing. These two together lead to variation in densiﬁcation shrinkage. Because the ﬁbers are oriented preferentially perpendicular to the ﬁltration and pressing direction applied during paper sheet formation, for example, MD–CD in-plane orientation, elongated pores are formed during oxidation in the sheet plane. These pores can be clearly seen in Fig. 13(a), which is a representative cross section of the sintered microstructure of a multilayer stack processed from an Al2O3-ﬁlled (80 wt%) preceramic paper (16001C, 2 h). Figure 13(b) will be discussed in the next subsection. The shrinkage anisotropy can be quantiﬁed by deﬁning and measuring the anisotropy factor, K, deﬁned by three separate parameters, one for each plane, that is, x–y, y–z, and x–z64 eyy exx ; Kyz ¼ 100 1 ; Kxy ¼ 100 1 eyy ezz exx and Kxz ¼ 100 1 ezz

Anisotropic shrinkage was measured for Al2O3-loaded paper sheets after sintering in the temperature range between 15001 and 16501C. For these paper-derived ceramics, the most signiﬁcant shrinkage is in the thickness direction (ezz). The in-plane shrinkages are signiﬁcantly lower. The effect of sintering temperature on the three anisotropy factors is plotted in Fig. 14(a) (all samples sintered for 1 h) and the effect of sintering time in Fig. 14(b) (all samples sintered at 16001C). Several interesting points can be noted from these results. First, as the sintering temperature increases, all anisotropy factors decrease. Also, as the sintering time increases, Kx–z and Ky–z increase but Kx–y decreases. The most important observation is the values of these factors. The relatively small value of Kx–y implies that the shrinkage in the two in-plane directions is approximately equal. However, the shrinkage in the out-of-plane direction is almost a factor of 3 higher than that in the plane. Qualitatively, these results on shrinkage are consistent with microstructural observations of pore shape. Owing to the process used to make paper, the microstructure is almost isotropic in the x–y plane (MD–CD plane). There is a slight preference for ﬁbers to orient in the machine direction (MD), leading to a small but measurable anisotropy in the shrinkage. The microstructure is very anisotropic in the x–z (or the y–z) planes (MD–ZD or CD– ZD planes), resulting in large differences in shrinkage. In the literature, many causes of anisotropic shrinkage have been investigated and reported. In general, it has been associated with anisotropy in powder packing density, particle shape (and alignment of particles), and pore orientation.64–70 Recently, an anisotropic continuum formulation for sintering bodies has been developed.71 This formulation has been developed for a transversely isotropic body. This is the correct symmetry for these (a)

(b) MD

MD CD

ZD

50 µm Fig. 12. Thermogravimetric analysis of two different preceramic papers (heating rate 10 K/min, Al2O3: measured in air, SiC: measured in N2). Compositions of the preceramic papers are given in Table II.

(7)

50 µm

Fig. 13. (a) Polished surface of paper-derived sintered Al2O3 (side view of the multilayer stack), and (b) polished surface of paper derived Si–SiC (top view of the multilayer stack).

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dominated matter transport, the time for disintegration of the cylindrical pore into discrete spherical pores is expressed by: t

kT z4 r0 4 r0 ln 2 2 d A0 Ds gs z 4p

(8)

where k is the Boltzmann constant, T is the absolute temperature, Ds is the surface diffusion coefﬁcient, gs is the speciﬁc energy of solid–vapor interface, z 5 l/r0, where l is the wavelength of the perturbation and r0 is the initial radius of the cylindrical pore; d is the diameter of an atom and A0 is the initial amplitude of the perturbation. From Rayleigh analysis, it follows that l4lmin 5 2pr0 with a maximum value of lmax 5 9.02 r0. Thus, for an Al2O3-ﬁlled preceramic paper sintered at 16001C for 1 h, the critical radius for collapse of ﬁber-derived pores r0 was estimated from r0

Fig. 14. Shrinkage anisotropy factors Ki–j as a function of (a) sintering temperature and (b) sintering time (at 16001C) of an Al2O3-ﬁlled preceramic paper. Presented results with a mean standard deviation of 73% were calculated from Eq. (7).

ceramic papers. The primary cause of anisotropic shrinkage in these systems is the preferential orientation of anisotropic pores. The sintering shrinkage of a compact with elliptical pores has been modeled.65,72,73 These models clearly show that oriented elongated pores lead to anisotropic shrinkage. In Ch’ng and Pan73 model calculations for Al2O3 showed that an elongated pore shrinks faster in width than in length direction which coincides with our experimental ﬁndings of larger shrinkage in ZD direction compared with in-plane shrinkage. The numerical results also show that the shrinkage anisotropy rate is controlled by kinetic factors like surface diffusion, grain-boundary diffusion, and grain-boundary migration in addition to pore shape and orientation. For example, increasing the grain-boundary migration rate increases the shrinkage anisotropy rate, whereas, increasing the surface diffusion, reduces the shrinkage anisotropy rate. It is difﬁcult to use these calculations to explain quantitatively the effect of time and temperature on anisotropic shrinkage in this study because the shrinkage rate was not measured and also because the effect of temperature on all the kinetic parameters is not known. As shown in Fig. 13, large elongated pores remain from the pulp ﬁbers upon oxidation and sintering in air. An important factor to investigate is the stability of these long elongated pores. At elevated temperatures, long elongated pores may undergo morphological instability driven by a reduction in surface energy (Rayleigh instability). A long cylindrical pore will eventually collapse into a row of isolated spherical pores that have a lower total surface energy than the elongated pore of the same volume.74 The critical conditions for pore disintegration were analyzed by Stu¨we and Kolednik.75 For surface diffusion-

1=4 d tDs gs 2 kT lnðr0 =A0 Þ

(9)

taking the following materials’ data: gs 5 0.95 J/m2, d (Al31) 5 0.05 nm, Ds 5 8.22 107 exp[577730 (kJ/mol)/RT] 5 6.66 109 (cm2/s).76 Assuming a variation of the ratio r0/A0 from 100 to 1.1 a critical initial pore radius of r0 9.5–25.2 nm results. Hence, Eq. (9) may be considered as an upper bound for pore instability criteria, indicating that the signiﬁcantly larger pores derived from the wood pulp ﬁbers (r45–15 mm) are unlikely to disintegrate upon sintering and maintain their elongated shape as it has been conﬁrmed by the microstructural analyses (Fig. 13(a)). In preparing multilayer stacks of laminated preceramic paper sheets, an adhesive bonding layer is used. This layer has ceramic particles in an organic matrix. The successive paper layers can be stacked in any in-plane orientation. However, because the inplane shrinkage is almost isotropic, the only shrinkage mismatch of interest is that between the paper and the adhesive layer. The effect of sintering rate mismatch between layers has been theoretically analyzed by several investigators. Using continuum mechanics formulations, it has been shown that in-plane stresses are generated in the mismatched sintering layers.77–79 These stresses lead to modiﬁcation of densiﬁcation behavior, which has been investigated experimentally.68,80 The most important consequence of the shrinkage mismatch stresses is the possibility of generating defects in the sintering ﬁlms and at the interfaces. However, it has been shown theoretically that the tendency to form cracks during sintering decreases as the layer thicknesses decreases.81 Because it is the adhesive layer that is in tension during sintering (slower densiﬁcation rate), one of the design goals of this approach is to use thin adhesive layers with shrinkage rates closely matched to that of the paper.

(2) Inﬁltration and Reaction Bonding Conversion of the pulp ﬁbers into carbon ﬁbers may be achieved by annealing the preceramic paper in an inert atmosphere at temperatures above 8001C. The remaining carbon ﬁber network offers an interconnected porosity that can be used for inﬁltrating the pyrolyzed preform with a low-viscosity liquid, for example, a metal or a polymer melt or a sol, or with a gas phase. Reaction bonding process between the pyrolyzed carbon ﬁber preform and a solid carbide-forming ﬁller such as Si, Ti, B, or intermetallics at temperatures below the melting temperature of the ﬁller phase results in porous carbide products. Another approach is to use a SiC-ﬁlled paper, pyrolyze in an inert environment and, then form reaction-bonded SiC by inﬁltrating the pyrolyzed preceramic paper preform either with liquid Si melt at T414501C or gaseous Si at 10001–15001C. Figure 13(b) shows a typical microstructure of liquid Si-inﬁltrated and reactionbonded SiC derived from a SiC-ﬁlled preceramic paper. The reaction product may contain unreacted carbon due to slow reaction rates and diffusional material transport. Compared with

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by constrained slow diffusion (Knudsen) and viscous frictional drag in pores smaller than approximately 1 mm,12 the large macroscopic channels in paper-derived corrugated preforms with a channel porosity volume 450% offer accelerated access for liquid as well as gaseous precursors. Thus, large size components can effectively be converted into ceramic components. Alternative to an external precursor source, the paper can be dip coated by a low viscosity polymer or slurry to provide local and uniform distribution of reactants in the internal channel surfaces. Compared with capillary inﬁltration from an external precursor source, the local inﬁltration offers short transportation distances and hence reduced high-temperature treatment time. Using a polyalkylsiloxane (CH3SiO1.5)n with n 5 300–400 as a binder phase for Si, Si/Al, and SiC powders it was demonstrated that the dip-coated preform may achieve superior mechanical properties after curing at 1501–2501C.15 The polysiloxane may also be used as an adhesive bonding for lamination and joining. During annealing, the polysiloxane decomposed to an amorphous Si–O–C phase above 6001C, leaving an inorganic residue of more than 70 wt% which at temperatures above 12001C crystallized to SiO2, SiC, and C.15

Fig. 15. Top: Stress–strain curves obtained from bending loading of porous (volume fraction of porosity B31%) multilayer Al2O3 ceramics derived from a preceramic paper (loading orientations (a) and (b) see Table II). Bottom: Effect of loading direction versus machining direction, for example, preferential pore orientation for in-plane loading (a) and (b).

sintering, however, reaction bonding offers near-net-shape capability with only minor changes in preform dimensions. In contrast to the conversion process of native plant tissue templates where vapor and liquid inﬁltration kinetics are limited

(3) Mechanical Behavior of Preceramic Paper-Derived Ceramics Unlike ﬁber and layer ceramic matrix composites with interfaces designed for triggering energy dissipation mechanisms based on delamination of the weak interphase and then frictional sliding,82,83 preceramic paper-derived multilayer ceramics show brittle fracture with almost no crack deﬂection at interlaminar interfaces. Figure 15 shows representative stress–strain plots, in bending, for paper-derived Al2O3 multilayer ceramics for different loading directions. Lamination of the multilayer stacks was achieved by means of an Al2O3-ﬁlled thermoplastic adhesive, resulting in bonding layers approximately 40 mm in thickness. The specimen with tensile stress applied parallel to the layer orientation remained linear elastic up to the peak load, that is, fracture strength. Examination of the fracture surface indicated that cracks were only slightly deﬂected at the layer interfaces, which may be correlated with only a minor difference in the fracture resistance of the interface compared with the sheet material.84 Examination of specimens with different angles between the principal tensile stress orientation and ﬁber alignment, for example, machine direction, reveals only a minor inﬂuence of pore alignment on the fracture anisotropy of in-plane-loaded multilayer Al2O3 ceramics (Fig. 15). In contrast, out-of-plane loading perpendicular to the lamination planes gives rise to a pronounced reduction in failure stress. Table III shows the an-

Table III. Anisotropy of Bending Strength (sc) and Fracture Toughness (KIC) as a Function of Loading Direction Versus Bending Plane Orientation of Paper-Derived Multilayer Ceramics

Loading direction

Si–SiCw a b c Al2O3z a b

Bending strength, sc (MPa)

Young’s modulus, E (GPa)

Fracture toughness, KIC (MPa m1/2)

315738 315727 150739

252715 — —

3.871.1 3.570.8 1.370.2

96714 10472

164712 —

1.970.1 2.170.1

w Phase content: 45.1 vol% SiC, 54.9 vol% Si, mean SiC particle size 4.5 mm, porosity o1%; Si-interface bonding layers; zMean Al2O3 particle size 1.5 mm after sintering, porosity 31%.

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Table VI. Potential Applications of Paper-Derived Ceramics Field

Examples

Transportation

Energy

Environment Fig. 16. Inﬂuence of cylindrical pore orientation on the porosity dependence of elastic modulus as derived from Eqs. (10–12).

isotropy of bending strength (sc) and fracture toughness (KIc) as a function of loading direction versus bending plane orientation of paper-derived multilayer Si–SiC and Al2O3 ceramics fabricated by laminated object manufacturing—LOM (see Sidebar A). Experimental details on the fabrication of Si–SiC material from SiC-ﬁlled preceramic paper by LOM are given in Windsheimer et al.85 The mechanical properties of the obtained materials strongly depend on the layer orientation with respect to the loading direction. For example, for the Si–SiC composite, bending strengths and fracture toughnesses varied between B150 and B315 MPa and from B1.3 to B3.8 MPa m1/2, respectively. For comparison, commercially available Si–SiCs fabricated by other processing routes, such as REFEL or SILCOMP, exhibit typical bending strength and fracture toughness of B350 MPa and B4 MPa m1/2, respectively.86–88 Several approaches have been developed to model the effect of porosity on the strength. Speciﬁc and general models have been developed to account for the effect of volume fraction of porosity, pore shape, and size and its connectivity (for a review see Rice89). The basic approach has been used to calculate the effective stress on the solid phase. Several modiﬁcations have been made to this approach including fracture mechanics-based analysis, which showed a strong effect of the grain size and a weak effect of the pore size.90,91 The widely used exponential relationship between the porosity (fp) and the elastic modulus (E) as well as the fracture strength (sc), which is based on the minimum bond area between particles (10) E ¼ E0 exp bfp and sc ¼ s0 exp bfp was extended to account for nonspherical pores.92 b was shown to be primarily a function of pore shape and alignment with

Support structures

Diesel particle ﬁltration Exhaust gas catalytic conversion Light metal reinforcement Friction materials in clutches Porous burner Integrated coal gasiﬁcation combined cycle Pressurized ﬂuidized bed combustion Gas regeneration for HT fuel cell Thermal insulation Heat exchanges Solar thermal storage Photovoltaic substrates Water cleaning Exhaust gas puriﬁcation Catalytic reactor inserts Kiln furniture

respect to the stress axis. On the other hand, b was found to be independent of pore size. The approach used was an analysis of the strain and strain energy change associated with incrementally introducing pores into a loaded ceramic volume92 baligned

2 1 u2 2 k 2 2 cos f þ sin f ¼ 1þ p E ð kÞ k

(11)

and 4 1 u2 E ðkÞ þ pk brandom ¼ 1 þ 3p kE ðkÞ

(12)

where k is the aspect ratio of an ellipsoidal pore (k 5 c/a where c is the major and a the minor axis and are equivalent to the ﬁber length and thickness, respectively) and f is the tilt angle of the major pore axis (c) to the normal stress axis. For ﬁber-like pores, c a and k-N. Hence from Eqs. (11) and (12), b-1 for aligned pores parallel to the applied stress axis (f 5 01), b 2.9 for aligned pores perpendicular to the stress axis (f 5 901), and b 2.3 for a random pore orientation. Figure 16 shows the inﬂuence of porosity on the decay of fractional elastic modulus (elastic modulus of porous ceramic to that of dense material, E/E0) as calculated from Eqs. (10–12) for different loading orientations and pore morphologies. Experimentally, a Young’s modulus of E 5 155–175 GPa (E/E0 5 0.41–0.46) was measured by acoustic vibration analysis for a sintered Al2O3 paper (porosity fp 5 0.31) in the a) and b) directions (01–901), which is in the range characteristic for a random pore orientation. From

Sidebar B. Corrugated lightweight structures Corrugated board is made from a combination of two sheets of preceramic paper (liners) glued to a corrugated inner medium (ﬂuting). Adhesive systems for sheet bonding are based on common thermoplastics, which may contain ﬁne ﬁllers of same composition as in the preceramic paper. Stacks and rolled structures of corrugated board can easily be fabricated into three-dimensional bodies containing directed macroscopic pore channels with a minimum diameter down to approximately three to ﬁve times the single layer thickness (200–300 mm). Examples are shown in Fig. B1. The hierarchical porosity structure with large pore channels in the mm-range and the struts with interconnected pores of effective sizes in the mm-range provide suitable conditions for designing catalyst carrier structures. Owing to the ﬁbrillar strut structure mimicking the initial cellulose ﬁber arrangement turbulent ﬂow is induced perpendicular to the main ﬂow direction along the macroscopic channels and hence a high effective surface is available for catalytic reaction. Generally, the material properties of honeycomb like porous bodies with directed porosity structure scale with the fractional density r ð¼ rcell =rstrut Þ by a power law relation94 E ¼

Ecell Estrut

¼ Ci ðr Þn

ðB:1Þ

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Sidebar B. Continued where Ci is a constant, Ecell is the Young’s modulus of the macroporous cellular structure, and Estrut is the elastic modulus of the strut material given by Eq. (10), respectively. Representing a honeycomb like structure of corrugated paper by equilateral triangular cells the fractional density may be expressed by pﬃﬃﬃ ! pﬃﬃﬃ t 2 3t ðB:2Þ r ¼2 3 1 l 2 l

20 mm

35 mm

Fig. B1. Paper-derived corrugated ceramic structures: laminated Al2O3 heat exchanger (left) and rolled SiC catalyst carrier (right).

Table B1. Characteristic Values of n Direction of loading

Young’s modulus Strength Toughness

Out-of-plane (axial) (n)

In-plane (radial, tangential) (n)

1 1 1.5

3 2 1.5 Fig. B3. Maximum principal tensile stress variation calculated by FE for the rolled corrugated structure shown in Fig. B2: t, strut thickness; h, wave amplitude; and l, wavelength.

with t and l denote the strut thickness and the edge length, respectively. Depending on the direction of loading n may attain characteristic values as given in Table B1 94: While compressive loading in axial direction primarily results in compressive stress states, radial compressive loading gives rise to local bending of struts and hence tensile stresses. Fracture is likely to start at the segments where the maximum tensile stress exceeds local tensile strength in the strut region that is controlled by the strut porosity. The high porosity in the struts, which may exceed 30%, limits the loading capacity in radial direction whereas substantially higher failure stresses are achieved under axial Fig. B2. FE stress distribution analysis for a corrugated paper-derived loading conditions. Optimization of rolled corrugated SiC/Al6Si2O13 composite structure subjected to radial compression loadstructure design was achieved by simulation of stress ing. distribution as a function of strut thickness t, wave length l, and amplitude h. The rolled corrugated structure was simulated by generation of a two-dimensional Archimedian spiral, Fig. B2(a), which subsequently was expanded in three dimensions. The algorithms were implemented in PERL scripts. Stress distribution was calculated by FEM (MARC/MENTAT, MSC Software Corporation, CA) in axial as well as in radial directions. While axial loading is mainly dominated by the area fraction of solid and the properties of the solid strut material, radial loading is strongly dependent on the structure design parameters. Figure B2(b) shows the distribution of principal stress (sxx) calculated for loading a SiC/Al9Si2O13 corrugated structure in radial direction applying a nominal stress of 10 MPa (y-direction). Loading in radial direction is considered to be more critical compared to axial loading in many applications including particle ﬁlter and catalyst systems. The calculated maximum tensile stresses are plotted in Fig. B3 as a function of geometric parameters. They remain below a critical value set to 100 MPa for the wave amplitude being o400 mm, the wave phase distance o270 mm and the strut thickness being 250 mm. The FE calculations show a high strut thickness and a low wave height causes reduction of the stresses under compressive radial stress. These results can be used to optimize the design for these highly porous-designed structures. Eq. (10), it is clear that the dependence of fractional strength is the same as that for fractional modulus. The simpliﬁed analytical approach does not account for intersecting pores and pore shape changes due to sintering or for the distribution of pore

shapes and orientations that would be expected for most materials. However, both analysis and experimental results suggest that a preferential ﬁber texture generated by controlled machining of paper sheet might be favorable for preceramic-derived

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multilayer ceramics subjected to in-plane loading parallel to the initial ﬁber orientation.

Papiertechnische Stiftung - Paper Technology Specialists (PTS), Munich, Germany. We would like to thank them for this support.

References

IV. Potential Application 1

The orientation dependence of mechanical behavior will have a pronounced inﬂuence for the design and manufacture of corrugated lightweight structures. Preceramic paper may be easily processed into planar and corrugated lightweight structures of variable size, geometry, and composition. Depending on the residual porosity, single sheet, multilayer laminates, and corrugated products offer a wide variety of potential applications for paper-derived ceramics, Table IV. Thermal, chemical, and mechanical stability makes preceramic paper-derived ceramics promising candidates for thermal shielding applications in high-temperature furnaces or as burning aids. Recently, it was shown that Al2O3 sheets with micrometer-sized cylindrical pores can be also fabricated by coextrusion of Al2O3 powder and nylon ﬁbers as pore forming agents.93 Thermal treatment of the extruded parts resulted in porous alumina ceramics with porous microstructures similar to the microstructures observed in preceramic paper-derived Al2O3. The fabrication approach by extrusion may give rise to a ﬂexible shaping method. However, the paper technological fabrication of preceramic paper-derived ceramics allows a fast and low-cost production. Preceramic paper-derived ceramics may be used for gas or liquid ﬁltration applications, for example, diesel particulate ﬁlters or water cleaning systems, respectively. The pore size and volume fraction of porosity may be easily controlled by the raw material used for papermaking. Structures made of preceramic paper-derived ceramics can be optimized by FE modeling. For instance, the optimization of a rolled corrugated structure based on FE modeling of local stress and strain distribution with respect to minimizing local tensile stress loading by variation of strut thickness, wave amplitude, and wavelength was demonstrated for a SiC-based catalyst carrier design. (Sidebar B).

V. Conclusions Preceramic paper may serve as a preform to manufacture lightweight corrugated as well as multilayer ceramic products. Based on well-established paper processing techniques, fabrication of cellular components of variable geometry, size, and complexity can be achieved. While the cellulose ﬁber network generated during papermaking provides good ﬂexibility and machinability, ﬁller loading controls the paper-to-ceramic conversion via sintering or reaction inﬁltration. In order to improve the mechanical properties, reduction of density variations at high ﬁller fractions and tailoring of ﬁber template porosity in the paper sheets are key factors to be optimized. While the preferential alignment of ﬁber-templated pores with an elongated morphology in the paper sheet exerts a minor inﬂuence on the anisotropic mechanical behavior, multilayer stacking of sheets gives rise to a pronounced difference in mechanical properties when loading is applied parallel (in-plane) or perpendicular (out-of-plane) to the sheet directions. Owing to the ﬂexibility in shaping, stacking and the ability to apply computer aided manufacturing (e.g., laminated object manufacturing) and the versability of chemical compositions, the preceramic paper offers an economical approach to process lightweight ceramics with tailored macro- and microscopic porosities for a broad ﬁeld of applications.

Acknowledgments The authors gratefully acknowledge Rajendra Bordia, who is currently visiting Humboldt professor at the University Erlangen-Nuremberg, for helpful discussion. We gratefully acknowledge H. Dannheim for support and discussions concerning microwave characterization of the preceramic papers. Samples of various preceramic paper compositions were manufactured and provided by the

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Nahum Travitzky earned his Master degree in Semiconductors and Dielectrics from the Institute of Steel and Alloys in Moscow. He obtained M.Sci. and D.Sci. degrees in Material Science and Engineering at the Israeli Institute of Technology, Technion, Haifa. He was senior research associate at Technical University of Hamburg, Harburg, Germany, before he joined as a Senior Lecturer the University of Tel Aviv, Israel, in 1990. In 1999, he joined as a visiting Professor the Technical University of Hamburg, Harburg. Since 2002, Travitzky is Leader of Advanced Ceramic Processing and Rapid Prototyping Group at the University of Erlangen-Nuremberg, Department of Materials Science, Glass and Ceramics, Germany. His current research activities pertain to design, advanced processing and characterization of ceramic-based materials with the focus on preceramic paper and polymerderived ceramics and ceramic–metal composites. He has over 70 scientiﬁc papers and six patents. Hans Windsheimer studied glass and ceramic at the Friedrich-Alexander-University of Erlangen-Nuremberg, Germany and at Alfred University, Alfred, NY, USA. He received his Dipl. Ing. in materials science in 2005 from the University of Erlangen-Nuremberg. Afterwards he worked as a research fellow at the Institute of Glass and Ceramics at the University of Erlangen-Nuremberg. His research focused on processing (including laminated object manufacturing) and characterisation of preceramic paper-derived ceramics. He is author and co-author of various papers and patent. In 2008, H. Windsheimer joined Linn High Therm, Eschenfelden, Germany as a project engineer for the development of special industrial furnaces.

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Tobias Fey received his Dipl. Ing. and Ph.D. in material science from the University of Erlangen-Nuremberg, Germany in 2001 and 2008, respectively. For nearly 5 years he worked as research associate at the University of Erlangen-Nuremberg. His research focused on biomorphous ceramics derived from cellulose and their characterization including mechanical and thermal FEM-simulation on real mCT-models. In 2006, he joined SEMIKRON Elektronik GmbH & Co. KG. In 2008, T. Fey joined the chair of glass and ceramics at the department of Material Science of the University of Erlangen-Nuremberg, Germany as a senior research assistant. Peter Greil is a Professor of Materials Science and chair of Glass and Ceramics at the Department of Materials Science of the University of Erlangen-Nuremberg, Germany. He received his Dipl. Ing. in mineralogy/ crystal chemistry in 1980 and a Ph.D. in metallurgy in 1982 from University of Stuttgart. Before joining Erlangen-Nuremberg University in 1993 he worked for the MaxPlanck-Institute for Metals Research, Stuttgart, and held a position as Associate Professor for advanced ceramics at the Technical University of HamburgHarburg. Greil’s research activities on advanced processing of ceramics centered on nonoxide and polymer-derived high-performance ceramic-based composites. Biomorphous cellular ceramics derived from natural tissue and polysaccharide templates are investigated in order to tailor the mechanical properties of lightweight cellular ceramics and composites. He has published more than 270 papers and 35 patents.

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Preceramic Paper-Derived Ceramics

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