Journal of Non-Crystalline Solids 353 (2007) 1070–1077 www.elsevier.com/locate/jnoncrysol
Structural properties of Cr2O3–Fe2O3–P2O5 glasses, Part I A. Sˇantic´
a,*
, A. Mogusˇ-Milankovic´ a, K. Furic´ b, V. Bermanec c, C.W. Kim d, D.E. Day e
a Rud-er Bosˇkovic´ Institute, NMR Center, 10000 Zagreb, Croatia Rud-er Bosˇkovic´ Institute, Department of Materials Physics, 10000 Zagreb, Croatia c Faculty of Science, Geology Department, 10000 Zagreb, Croatia d Korea Institute of Geoscience and Mineral Resources, 305-350 Daejeon, South Korea University of Missouri-Rolla, Graduate Center for Materials Research, Rolla, MO 65409-1170, USA b
e
Received 12 July 2006; received in revised form 17 November 2006 Available online 16 February 2007
Abstract The structural properties of xCr2O3–(40 x)Fe2O3–60P2O5, 0 6 x 6 10 (mol%) glasses have been investigated by Raman and Mo¨ssbauer spectroscopies, X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The Raman spectra show that the addition of up to 5.3 mol% Cr2O3 does not produce any changes in the glass structure, which consists predominantly of pyrophosphate, Q1, units. This is in accordance with O/P 3.5 for these glasses. The increase in glass density and Tg that occurs with increasing Cr2O3 suggests the strengthening of glass network. The Mo¨ssbauer spectra indicate that the Fe2+/Fetot ratio increases from 0.13 to 0.28 with increasing Cr2O3 content up to 5.3 mol%, which can be related to an increase in the melting temperature from 1423 to 1473 K. After annealing, the 10Cr2O3–30Fe2O3–60P2O5 (mol%) sample was partially crystallized and contained crystalline b-CrPO4 and Fe3(P2O7)2. The SEM and AFM micrographs of the partially crystallized sample revealed randomly distributed crystals embedded in a homogeneous glass matrix. EDS analysis indicated that the glass matrix was rich in Fe2O3 (39.6 mol%) and P2O5 (54.9 mol%), but contained only 5.5 mol% of Cr2O3. These results suggest that the maximum solubility of chromium in these iron phosphate melts is 5.5 mol% Cr2O3. 2007 Elsevier B.V. All rights reserved. PACS: 61.43.F3; 78.30.j; 76.80.+y Keywords: Raman scattering; Mo¨ssbauer effect and spectroscopy; Phosphates
1. Introduction In the last 30 years, phosphate glasses have been investigated because of their interesting properties. Compared to conventional silicate and borate glasses, phosphate glasses generally have a higher thermal expansion coefficient, lower melting and softening temperatures, and high ultra-violet transmission [1–4]. On the other hand, a property limiting their usefulness in some application is their relatively poor chemical durability. However, several studies have shown that the chemical durability of phosphate glasses can be improved by the addition of oxides such as *
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[email protected] (A. Sˇantic´).
0022-3093/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.12.104
Al2O3 and especially Fe2O3 [5,6]. The high chemical durability of iron phosphate glasses is attributed to the replacement of the bridging P–O–P bonds by more moisture resistant P–O–Fe bonds [7–9]. As a result, iron phosphate glasses are of interest for several technological and biological applications. One of the most attractive applications of iron phosphate glasses is as an alternative wasteform for high-level nuclear waste vitrification [1,3,7,8]. In some instances, iron phosphate glasses may be more technically suitable and less expensive than the borosilicate glasses now being used to vitrify certain types of high-level nuclear wastes [10]. This is especially true for those wastes containing significant amounts of sodium, sulfate, phosphate, iron oxide, chrome oxide and heavy metal oxides, such as Bi2O3, La2O3 and
A. Sˇantic´ et al. / Journal of Non-Crystalline Solids 353 (2007) 1070–1077
U3O8. These oxides generally have a lower chemical solubility in most borosilicate glasses and this lowers the waste loading, such that the radioactive waste volume becomes undesirably large [3,10]. Chromium oxide, Cr2O3, has a low solubility in many alkali alumino borosilicate glasses and typically 61 mass% [11]. However, Huang et al. [12] recently reported that a simulated high-level waste containing 4 mass% of Cr2O3 was easily vitrified in an iron phosphate glass at 1150 C and at 1250 C for waste loadings of 55 and 75 mass%, respectively. Even at these high waste loadings, these iron phosphate wasteforms had an excellent chemical durability that met all current standards. In this respect, iron phosphate glasses appear promising for vitrifying nuclear wastes that contain significant Cr2O3. The chromium ion, when dissolved in a silicate or borate glass matrix in small quantities, colors the glass and changes the optical properties of the glass significantly. Extensive investigations on the optical absorption, luminescence and ESR spectroscopy of the Cr3+ ion in a variety of oxide glasses have been made in recent years in view of their technological importance in the development of tunable solidstate lasers and new luminescence materials [13–15]. In glasses, the chromium ions can occupy a variety of sites of different crystal field strength due to site variability and compositional disorder. Although, chromium ions may exist in different valence states, as Cr3+, Cr4+, Cr6+, in silicate and borate glasses, only Cr3+ ions are present in phosphate glasses. Murata et al. [13] suggested that the Cr3+ ions are selectively incorporated into certain sites which break the P@O double bonds, so the relative content of Cr3+ is independent of the composition in phosphate glasses. The aim of the present study was to investigate the effect of Cr2O3 on selected structural properties and the glass forming characteristics of iron phosphate glasses. Glasses of the general composition xCr2O3–(40 x)Fe2O3– 60P2O5, 0 6 x 6 10 (mol%) were prepared by conventional melting and investigated by Raman and Mo¨ssbauer spectroscopies, differential scanning calorimetry (DSC), X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM).
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2. Experimental The xCr2O3–(40 x)Fe2O3–60P2O5, 0 6 x 6 10 (mol%) glasses were prepared from appropriate mixtures of reagent grade Na2CrO4, Fe2O3, NH4H2PO4 which were melted between 1423 and 1473 K for 3 h in air in high purity alumina crucibles. Each melt was stirred 3 or 4 times with a fused silica rod to insure chemical homogeneity and then poured into a steel mould to form rectangular bars (1 · 1 · 5 cm3). The bars were placed in a furnace and annealed for 2 h at 723 K and then cooled to room temperature. The absence of crystallinity in the samples was confirmed by XRD, SEM and AFM. The room temperature XRD patterns were collected with an X-ray diffractometer (Philips PW 3710) using nickel filtered Cu Ka radiation. The Joint Committee on Powder Diffraction Standards (JCPDS) data were used to identify the crystalline phases present in the partially crystallized Cr10 composition, see Table 1 and Fig. 1. The limit of detection for the XRD analysis of crystalline phases was 1 wt.%. The chemical composition and microstructure were evaluated by SEM (JEOL T300) equipped with energy dispersive X-ray (EDS) detector (Vega TS 5136 Tescan). The nominal composition of the glasses was determined by EDS analysis. A disc of each sample was analyzed at six different positions and the mean composition is given in Table 1. The AFM measurements were performed using Multimode AFM with a NanoScope IIIa controller (Veeco Instruments, Santa Barbara, USA) with a vertical engagement (EV) 10 · 10 lm scanner. Measurements were carried out in the contact mode. Commercially available sharpened Si3N4 tips (NP-20, Veeco) were used. The density of each glass was measured at room temperature by the Archimedes method using water as the buoyancy liquid. The estimated error is ±20 kg m3. The glass transition (Tg) and crystallization (Tc) temperatures for the glasses were measured by DSC, TA Instruments, DSC Model 2010. The DSC curves were obtained by placing 50 mg of glass particles, size
