14945
2007, 111, 14945-14947 Published on Web 09/25/2007
Direct Synthesis of Metastable Nanocrystalline ZrW2O8 by a Melt-Quenching Method Maria N. Mancheva,†,* Reni S. Iordanova,† Yanko B. Dimitriev,‡ Kostadin P. Petrov,† and Georgi V. Avdeev† Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria, and UniVersity of Chemical Technology and Metallurgy, 1756 Sofia, Bulgaria ReceiVed: June 22, 2007; In Final Form: August 11, 2007
Metastable orthorhombic (γ) ZrW2O8 was obtained by fast cooling of the melt of a stoichiometric mixture of ZrO2 and WO3. Heating of the as-quenched sample at 600 °C for 15 h led to the full transition of the orthorhombic (γ) to the cubic (R) ZrW2O8 polymorph. The average crystallite sizes of the obtained ZrW2O8 phases were in the range of 50-80 nm. The morphology of the powdery products was characterized by scanning electron microscopy.
ZrW2O8 received considerable attention in the past several years because of its isotropic negative thermal expansion over a wide temperature range (from -273 to 777 °C).1-3 NTE is one of the important properties of highly functional materials such as electronic components, optical substrates, low-temperature thermocouples, and catalyst supports.4-6 According to the phase diagram, cubic (R) ZrW2O8 is thermodynamically stable in the 1105-1257 °C temperature range.7 Below these temperatures, it is metastable and decomposes to WO3 and ZrO2. High-pressure-high-temperature synthesis has resulted in metastable orthorhombic (γ) ZrW2O8 only.8,9 There are several known routes to the synthesis of ZrW2O8 polymorphs: solid-state synthesis,10,11 high-pressurehigh-temperature synthesis,8,9 coprecipitation,12-14 hydrothermal route,15 nonhydrolytic sol-gel synthesis,16,17 mechanochemically assisted solid-state synthesis,18,19 and quick cooling.20,21 The synthetic method employed influences the morphology and type of polymorphs. In this connection, accumulation of new knowledge of the experimental conditions and application of new techniques to the synthesis of ZrW2O8 continue to be topical. Recently, S. Nishiyama et al. used quick cooling for the synthesis of cubic (R) ZrW2O8.21 The purpose of this work is to verify the applicability of the fast-quenching roller technique to the preparation of ZrW2O8 polymorphs. The meltquenching method is well known as being appropriate for the preparation of amorphous or metastable phases and also for the design of the so-called glass ceramics.20,22 It is impossible to predict the results without experiments. These facts motivated our studies. For our experiments, we used literature data on both DTA analysis15,11,20 and phase diagrams7 in order to select the appropriate melting temperature. A stoichiometric mixture of monoclinic (m) ZrO2 (Reachim) and monoclinic (γ) WO3 (Merck) in a 1:2 molar ratio was homogenized in an agate mortar and melted for 20 min in air at 1300 °C in a corundum * Corresponding author. E-mail:
[email protected]. † Bulgarian Academy of Sciences. ‡ University of Chemical Technology and Metallurgy.
10.1021/jp074870f CCC: $37.00
TABLE 1: Quantitative Phase Composition and Mean Crystallite Size of ZrW2O8 Samples Obtained by Melt-Quenched Method samples quenched sample
phases and vol %
crystallite sizes of ZrW2O8 (nm)
(γ) ZrW2O8 (73%) (R) ZrW2O8 (12%) WO3 (11%), ZrO2 (4%) (γ) ZrW2O8 (60%)
53 37
(R) ZrW2O8 (40%) calcination of quenched (R) ZrW2O8 (100%) sample at 600 °C for 15 h calcination of quenched (R) ZrW2O8 (78%) sample at 600 °C for 20 h WO3 (17%), ZrO2 (5%)
23 40
calcination of quenched sample at 600 °C for 5 h
50
78
crucible. The melt was quenched at high cooling rates (104105 K/s) using a roller quenching technique, which is a modification of the original equipment development by H. S. Chen et al.23 Visually, the quenched sample obtained by the roller quenching technique presents fragmented 1-3 cm flat pieces, about 50-100 µm thick, and a very small amount of droplet-like pieces. The quenched sample was ground and the powders were calcined at different temperatures from 150 up to 600 °C. Powder X-ray diffraction patterns were registered at room temperature with a DRON diffractometer using Cu KR radiation in the 10° < 2θ < 90° range. The X-ray diffraction data were analyzed using the “PowderCell” program to determine the volume percentage of the phases present in the samples and to estimate the average crystallite size of ZrW2O8.24 The morphology change of the particles of ZrW2O8 powders was investigated by scanning electron microscopy (SEM-JEOL-JSM-6390). The as-quenched sample (Figure 1A and B) was polyphase, containing mainly orthorhombic (γ) ZrW2O8 (73%) (JCPDS card 56566). The calculated average crystallite size of orthorhombic (γ) ZrW2O8 was 53 nm. The quenched sample was subjected to heat treatment at selected temperatures. Below 600 °C, phase transitions were not observed. Heat treatment at 600 °C for 5 h led to full interaction between WO3 and ZrO2 © 2007 American Chemical Society
14946 J. Phys. Chem. C, Vol. 111, No. 41, 2007
Letters
Figure 2. XRD pattern of ZrW2O8 obtained by fast-quenching technique and heating at 600 °C for 20 h.
Figure 1. (A) XRD pattern of ZrW2O8 obtained by fast-quenching technique (experimental data). (B) XRD pattern of ZrW2O8 obtained by fast-quenching technique.
and a change in phase ratio of the orthorhombic (γ) and cubic (R) ZrW2O8 phases (Table 1). The amount of orthorhombic (γ) ZrW2O8 decreased to 60% because of an orthorhombic (γ) f cubic (R) ZrW2O8 transition, while the amount of cubic (R) ZrW2O8 increased up to 40% probably as a result of both phase transition and chemical reaction between the initial oxides (Table 1). The orthorhombic (γ) f cubic (R) ZrW2O8 transition was accomplished during heat treatment up to 15 h. With increasing time of heat treatment, the cubic (R) ZrW2O8 (JCPDS card 280117) started to decompose to monoclinic (γ) WO3 and monoclinic (m) ZrO2 (Figure 2). According to the phase diagram, at this temperature the cubic (R) ZrW2O8 did not exist as a thermodynamically stable compound.7 The calculated average crystallite size of cubic (R) ZrW2O8 was 78 nm (Table 1). Obviously, the transformations proceed too long and our efforts to detected transformation temperature of metastable to stable phase by DTA were unsuccessful. For comparison, T. Hashimoto et al.,11 O. Xing et al.,15 and Y. Morito et al.20 show DTA data above 1000 °C concerning the solid-state reaction of cubic (R) ZrW2O8 and peritectic melting. SEM images of the powder sample show formation of polyhedral-like grains, which indicates the occurrence of the crystallization process after fast quenching. However, zones of probably amorphous spherical grains can be observed (Figure 3a and b). After heat treatment at 600 °C, several twinned cubic
Figure 3. (A and B) SEM images of ZrW2O8 obtained by fastquenching technique.
crystals surrounded by many smaller aggregates were established. That is why it is impossible analysis to determine the real dimension of the separate crystals by SEM. In our case, the fast cooling rate led to direct crystallization of the metastable orthorhombic (γ) ZrW2O8. As was mentioned above, this polymorph was obtained by high-pressure-hightemperature synthesis.8,9 Using cooling in water, air, liquid nitrogen, and a furnace, S. Nishiyama et al.21 synthesized mainly cubic (R) ZrW2O8. These results confirm that the preparation of different polymorphs of ZrW2O8 may be controlled by selecting the cooling rate of the melt.
Letters
J. Phys. Chem. C, Vol. 111, No. 41, 2007 14947 References and Notes
Figure 4. (A and B) SEM images of ZrW2O8 obtained by fastquenching technique and heating at 600 °C for 20 h.
The fast quenching rate of the melt is appropriate for the preparation of the metastable orthorhombic (γ) ZrW2O8 polymorph. The phase transformation of orthorhombic (γ) to cubic (R) ZrW2O8 was observed after heat treatment at 600 °C. SEM results revealed the transformation of the polyhedral grains to cubic ones during heat treatment.
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