Materials Letters 57 (2003) 3775 – 3778 www.elsevier.com/locate/matlet
Ceramic system based on ZnOCuO obtained by freeze-drying Jusmar V. Bellini a,*, Marcio R. Morelli b, Ruth H.G.A. Kiminami b b
a UEM, Departamento de Fı´sica, Av. Colombo 5790, CEP 87020-900, Maringa´-PR, Brazil UFSCar, Departamento de Engenharia de Materiais, C.P. 676, CEP 13565-905, Sa˜o Carlos-SP, Brazil
Received 7 February 2003; accepted 26 February 2003
Abstract Compositions of powders containing ZnO and copper(II) acetate monohydrate [(CH3COO)2CuH2O, denoted CuAcH2O] equivalent to ZnO + x mol% Cu (x = 0, 0.05, 0.5 and 5.0) were obtained by freeze-drying. The powders were characterised by thermogravimetry (TG) up to 700 jC and differential scanning calorimetry (DSC) up to 1150 jC, in static air with heating rate of 5 jC/min. Pellets of the freeze-dried powders were compacted without pressing additives and sintered in air at 950 jC/1 h. After crushing, the crystalline phases of the sintered samples were characterised by powder X-ray diffractometry (XRD). Microstructure and Cu-element mapping of sintered ZnO + 5.0 mol% Cu samples were characterised by scanning electron microscopy (SEM) with microprobe apparatus. TG and DSC indicated the presence of three important thermal events. Event I: (100 – 190 jC) corresponds to dehydration of CuAcH2O, giving rise to copper(II) acetate [(CH3COO)2Cu, denoted CuAc], with an endothermic peak at 125 jC. Event II: (190 – 350 jC) corresponds to thermal decomposition of CuAc giving rise to CuO, with two exothermic peaks at 237 and 261 jC. In air, above 350 jC, the ceramic system is ZnOCuO. Event III: (800 – 1000 jC) corresponds to a solid-state reaction involving ZnOCuO, with an exothermic peak at 895 jC. DSC, XRD and SEM indicated that, after sintering at 950 jC/1 h, the resulting ceramic matrix is composed by Cu-doped ZnO grains (ZnO/Cu) for ZnO + x mol% Cu (x V 1) and by ZnO/Cu grains with CuO-inclusions (ZnO/CuCuO) for ZnO + x mol% Cu (x>1). D 2003 Elsevier Science B.V. All rights reserved. Keywords: Ceramics; Freeze-drying; Zinc oxide; Copper(II) acetate monohydrate; Powder technology; Sintering
1. Introduction Several studies on the effect of Cu additive on the electrical and microstructural properties of polycrystalline ZnO have appeared recently in the literature [1– 5]. Non-linear electrical characteristics were observed for sintered ZnO/Cu with various Cu concentrations [1,2,5]. In general, it is observed that the gradual
* Corresponding author. Tel./fax: +55-44-263-4623. E-mail address:
[email protected] (J.V. Bellini).
addition of Cu decreases the leakage current, but increases the breakdown electric field of the ZnO/Cu ceramics. Moreover, segregation of a second phase containing excess Cu precipitated from ZnO matrix was observed in the microstructure for samples doped with 2.6 wt.% Cu [2] and 3.9 wt.% Cu (5.0 mol% Cu) [5], respectively; although its crystalline structure was not clearly understood. In this research, the results of the thermal, structural and microstructural characterisation of ZnO-based ceramics obtained from freeze-dried mixtures of powders containing ZnO and CuAcH2O are presented. The
0167-577X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(03)00177-0
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2.2. Characterisation
Fig. 1. TG analyses for freeze-dried powders of ZnO + x mol% Cu (x = 0, 0.05, 0.5 and 5.0).
The resulting powders were studied by thermogravimetry (TG) (Netzsch, model STA 409C) and differential scanning calorimetry (DSC) (Netzsch, model 404) in the following conditions: atmosphere of static air; heating/cooling rate of 5 jC/min; 250 mg of powder; crucibles of Al2O3. The microstructure was analysed by scanning electron microscopy (SEM) (Leica, model StereoScan 440) with a microprobe apparatus (Oxford, model eXL) for Cu-element mapping. After crushing, the crystalline phases of sintered samples of ZnO + x mol% Cu (x = 0, 0.05, 0.5 and 5.0) were identified by powder X-ray diffractometry (XRD) (Siemens, model D5000) using CuKa radiation.
purpose of these studies was to clarify the thermal decomposition route of these mixtures in order to explain the structural and microstructural changes in the resulting Cu-doped ZnO ceramics.
2. Experimental procedure 2.1. Processing Initially, specific compositions of powders containing ZnO (99.9%, Uniroyal, with particle mean diameter D 5 0 c 0.5 Am) and C uAcH 2 O (98.7 %, Mallinckrodt) were dissolved in 250 ml of distilled – deionised water. The aqueous mixtures of ZnO + y wt.% CuAcH2O ( y = 0, 0.12, 1.2 and 12.0) equivalent to ZnO + x mol% Cu (x = 0, 0.05, 0.5 and 5.0) were transferred to appropriate glass flasks, frozen in a continuous system at 50 jC (Edwards, Shell Freezer) in 45 min and freeze-dried (Edwards, Micromodulyo) for 16 h. The freeze-dried powders were uniaxially pressed into a disk shape using a steel die without any pressing additives and sintered in conventional electric furnace (EDG, model 3000), in air, at 950 jC/1 h, with heating rate of 5 jC/min. Sintered samples of ZnO + 5.0 mol% Cu were previously polished with 0.3 Am Al2O3 powder to a mirror-like surface. Thus, after thermal attack at 900 jC for 15 min, the surfaces were coated by Au-evaporation for microstructural analyses.
Fig. 2. DSC analyses for a freeze-dried powder of ZnO + 0.5 mol% Cu: (a) 100 – 400 jC; (b) 400 – 1150 jC.
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3. Results and discussion Fig. 1 shows the TG analyses up to 700 jC with constant heating and cooling rate of 5 jC/min. For pure ZnO and ZnO + 0.05 mol% Cu, the mass loss was difficult to measure as it was below the instrument sensitivity. The total mass losses for the powders ZnO + x mol% Cu (x = 0.5 and 5.0) were about 1.6% and 8.4%, respectively. Fig. 2a and b shows the DSC analyses in the range 100 –400 and 400– 1150 jC for the powder ZnO + 0.5 mol% Cu. These results obtained by TG and DSC indicate that the material decomposes via two processes of mass loss (events I and II) over the temperature range 100 – 350 jC, and a third one (event III) 800– 1000 jC. Events I and II have been reported previously [6,7] for the thermal analysis of freeze-dried CuAcH2O powders. Event I: (100 –190 jC) corresponds to dehydration of CuAcH2O, with an endothermic maximum peak at 125 jC, giving rise to CuAc. Event II: (190 – 350 jC) corresponds to thermal decomposition of CuAc. In this range, DSC showed two exothermic peaks at 237 and 261 jC. These peaks are related to a complex interface solid –gas– solid reactions involving the production of Cu ! Cu2O ! CuO, during heating in air, and volatile or gaseous products [7]. Event III: (800 – 1000 jC) is
Fig. 4. SEM microstructure (above) and Cu-element mapping (below) for ZnO + 5.0 mol% Cu sintered in air at 950 jC/1 h.
Fig. 3. Powder XRD obtained from samples of ZnO + x mol% Cu (x = 0, 0.05, 0.5 and 5.0) sintered in air at 950 jC/1 h.
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probably due to a solid-state reaction involving ZnO and CuO with Cu-diffusive into the ZnO lattice, and sintering may occur [4]. In this range, an exothermic peak is observed at 895 jC. Fig. 3 shows the powder XRD spectra of crushed pellets obtained from samples of ZnO + x mol% Cu (x = 0, 0.05, 0.5 and 5.0) sintered in air at 950 jC/1 h. From powder diffraction files (PDF) of the joint committee on powder diffraction standards (JCPDS), the range 25j < 2h < 45j was chosen to include the diffraction planes of relative maximum intensity of the crystalline phases. The observed phases were ZnO (Zincite, PDF 36-1451) and CuO (Cuprite, PDF 50661). For ZnO + x mol% Cu (x = 0, 0.05 and 0.5) only the ZnO phase is observed. Above 1.0 mol% Cu, which is the estimated solubility of Cu into the ZnO lattice, the ZnO grain is regarded as Cu-doped [3]. Thus, for ZnO + 5.0 mol% Cu, the XRD spectrum presents traces of CuO as secondary phase (insoluble CuO). Fig. 4 shows the SEM microstructure (above) and respecting Cu-element mapping (below) for ZnO + 5.0 mol% Cu, sintered in air at 950 jC/1 h. The microstructure shows pores (black regions) that originated from inclusions (in white) pulled out during polishing. These inclusions are the CuO phase, which was previously identified by XRD. Analysis of the Cuelement (in grey) reveals a homogeneous Cu-distribution into the ZnO matrix with Cu-rich regions corresponding to the inclusions in the microstructure. The existence of that particle was confirmed using a larger counting time, in a small region of the sample (not showed here).
4. Conclusion During heating of freeze-dried homogeneous mixtures of ZnO+(CH3COO)2CuH2O powders, in air, DSC and TG indicated that after dehydration followed by thermal decomposition of (CH3COO)2Cu, above 350 jC, the resulting system is ZnOCuO with Cudiffusive and sintering occurring above 800 jC. XRD and SEM indicated that, after sintering at 950 jC/1 h, the resulting ceramic matrix is composed by Cu-doped ZnO grains (ZnO/Cu) for ZnO + x mol% Cu (x V 1) and by ZnO/Cu grains with CuO-inclusions (ZnO/ CuCuO) for ZnO + x mol% Cu (x>1), respectively.
Acknowledgements The authors wish to thank the financial support of CAPES/PICDT and CNPq.
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