J Mater Sci (2012) 47:8061–8066 DOI 10.1007/s10853-012-6698-2
Zr12xYbxWMoO82x/2 (x 5 0, 0.04) ceramics fabricated by in situ synthesis from trigonal polymorph: preparation, sintering process, and negative thermal expansion properties Ruiqi Zhao • Xi Chen • Hui Ma • Xishu Wang Xinhua Zhao
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Received: 4 April 2012 / Accepted: 22 June 2012 / Published online: 11 July 2012 Ó Springer Science+Business Media, LLC 2012
Abstract In this study, negative thermal expansion (NTE) Zr1-xYbxWMoO8-x/2 (x = 0, 0.04) ceramics were fabricated by in situ synthesis from trigonal polymorphous precursors for the first time. Phase transition was studied by means of powder X-ray diffraction. Study on the sintering process of Zr1-xYbxWMoO8-x/2 (x = 0, 0.04) ceramics was performed by calcining a series of precursor pellets at 950, 980, and 1000 °C for different times, varying from 1 min to 1 h. The results indicate that the sintering process can be mainly divided into three stages: phase transition from trigonal precursors to cubic Zr1-xYbxWMoO8-x/2 (c-Zr1-xYbxWMoO8-x/2, 0–5 min), densification of c-Zr1-x YbxWMoO8-x/2 (6–30 min), and the final sintering stage with little densification ([30 min). Densification reaches almost the maximum in the duration of *30 min at assigned temperature. Temperature has little influence on densification of
R. Zhao (&) College of Physics and Chemistry, Henan Polytechnic University, Jiaozuo 454003, Henan, People’s Republic of China e-mail:
[email protected] R. Zhao X. Chen X. Zhao College of Chemistry, Beijing Normal University, Beijing 100875, People’s Republic of China X. Chen School of Civil Engineering and Communication, North China University of Water Source and Electric Power, Zhengzhou 450011, Henan, People’s Republic of China H. Ma Analyzing and Testing Center, Beijing Normal University, Beijing 100875, People’s Republic of China X. Wang Department of Engineering Mechanics, Tsinghua University, Beijing 100084, People’s Republic of China
ZrWMoO8, but improves that of Zr0.96Yb0.04WMoO7.98 evidently. In addition, densification of ZrWMoO8 can be promoted markedly by introduction of Yb3?.
Introduction Cubic ZrW2O8 and relative compounds have attracted considerable interest because of their considerable isotropic negative thermal expansion (NTE) over a wide temperature range [1–9]. However, there is an order–disorder phase transition in c-ZrW2O8 at about 155 °C [2], around which there is a sudden change in coefficients of thermal expansion (CTEs) because of the a-b phase transition (CTE is about -9 9 10-6 °C-1 for the ordered a-ZrW2O8 and -5.5 9 10-6 °C-1 for the disordered b-ZrW2O8 [10]). It is necessary to avoid the abrupt change of CTEs in engineering applications and many efforts have been made in this aspect. Fortunately, c-ZrWMoO8, whose a-b phase transition occurs at -3 °C [11], also exhibits NTE property with a constant CTE of about -5 9 10-6 °C-1 from room temperature to 600 °C [12]. There are many methods reported to prepare c-ZrW2O8. The phase diagram of WO3–ZrO2 system [13] shows the thermodynamically stable range (1105–1257 °C) of c-ZrW2O8 and c-ZrW2O8 can be obtained in the above temperature range by sintering its corresponding oxides, i.e., ZrO2 and WO3. It is reported that c-ZrW2O8 can also be prepared by sintering its trigonal phase in its thermodynamically stable temperature range [14]. However, there are less methods reported to prepare c-ZrWMoO8 due to the apt sublimation of MoO3, i.e., combustion route [15], dehydration of the precursor route [11, 16, 17], hydrothermal method [18, 19], and melt quenching technique [20]. The existing methods listed above are either uncontrollable or time consuming to
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prepare phase-pure c-ZrWMoO8. Besides, there are few reports on fabrication of dense NTE ceramics [5, 21] and there is no paper concerning the sintering process of NTE ceramics. High density is correlated closely with many mechanical properties, such as fracture toughness, strength, and so on, which are needed to consider in practical applications. Consequently, it is necessary to study the sintering process and optimize the conditions of preparing dense NTE ceramics. In this work, NTE Zr1-xYbxWMoO8-x/2 (x = 0, 0.04) ceramics were fabricated by in situ synthesis from trigonal polymorphous precursors at different conditions, and the sintering process was studied thoroughly. The study of the sintering process reveals the influences of sintering temperature, time, and substitution of Zr4? with Yb3? on densification of Zr1-xYbxWMoO8-x/2 ceramics.
Experimental procedures Fabrication of Zr1-xYbxWMoO8-x/2 ceramics (x = 0, 0.04) Zr1-xYbxWMoO8-x/2 (x = 0, 0.04) precursors were prepared by the route described previously, except for some modifications [22]. Briefly, 0.02 (1 - x) mol Zr(IV) and 0.02 mol Mo(VI) were dissolved in 50 mL deionized water and then added dropwise and simultaneously into a slurry of 0.02 mol W(VI) to obtain white co-precipitate. 0.0004 mol Yb2O3 was dissolved in nitric acid, then dropped into the co-precipitate. After stirring for 3 h, the co-precipitate was dried, ground, and sintered at 600 °C for 3 h. The resulting powders were reground and sieved to be used as precursors. The green body (*1 g) with a diameter of *10 mm was formed by dry-press using a steel mold under a pressure of *4 MPa. Then, the pellet was put in a platinum crucible covered with a Pt foil and sintered in a preheated furnace under designed conditions. Zr1-xYbx WMoO8-x/2 (x = 0, 0.04) ceramics were obtained by quenching the pellets in ambient atmosphere. The ceramics used for thermal expansion and FE-SEM measurements were fabricated by sintering precursor pellets for 1 h at 1000 °C.
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method and indexed by the Treor90 program suited in PowderX software [23]. Diameters (d) and heights (h) of each pellet were measured 6 times at different positions. The average data were used to calculate bulk density (q) of samples from the equation q ¼ pd4m2 h. Since c-ZrWMoO8 is isotropic NTE material, the negative value of Mq q is used to represent 0
shrinkage of pellets during calcinations, where Mq equals q q0 , q is the bulk density of each ceramic, and q0 is the average density of precursor pellets (2.84 g cm-3). Microscopic images of precursor powders and the fractured ceramic surfaces were captured using a field emission scanning electron microscope (FE-SEM) with HITACHI s-4800. Dimension change of Zr1-xYbxWMoO8-x/2 (x = 0, 0.04) ceramics was measured from 25 to 600 °C by thermal mechanical analysis (TMA Q400, TA-Instruments) with heating and cooling rates of 3 °C min-1 and a constant force of 0.5 N. Each ceramic was held for 5 min at 600 °C before cooling.
Results and discussion Figure 1 shows XRD patterns of ZrWMoO8 precursor and the precursor calcined 1 h at 950 °C. As shown in Fig. 1a, the starting material is indexed with trigonal phase ˚ , c = 17.538(3) A ˚ ), which is very similar (a = 9.8748(3) A ˚, c = to that of trigonal ZrW2O8 (a = 9.8101(1) A ˚ 17.602(2) A [24]). The formula number of trigonal ZrWMoO8 (hereafter denoted as t-ZrWMoO8) in the unit cell is 9 and the theoretical density is 5.03 g cm-3 [25]. The precursor calcined for 1 h at 950 °C is indexed with cubic ˚ , reference to that of b-ZrW2O8 structure (a = 9.1406(2) A ˚ [15]). The theoretical denc-ZrWMoO8, a = 9.1400(5) A sity of c-ZrWMoO8 is 4.34 g cm-3. According to the
Characterizations The ground ceramic powder was characterized by X-ray diffraction (XRD) with Cu Ka radiation (X-Pert MPD Philips, 40 kV and 40 mA) by means of a step size of 0.0167° (2h) and counting time of 20 s per step. The data used for indexation were collected from 10° to 90° (2h) and from 15° to 30° (2h) for phase identification. The reflections (2h/°) were calibrated by means of the line-pair
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Fig. 1 XRD patterns of a ZrWMoO8 precursor and the b precursor calcined 1 h at 950 °C
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results shown in Fig. 1, the phase identification of Zr1-xYbxWMoO8-x/2 (x = 0, 0.04) ceramics fabricated under different conditions is shown in Fig. 2. The content of cubic phase in ceramics increases as sintering time prolongs and phase-pure ZrWMoO8 is obtained in 5 min at 950 °C and in 3 min at temperatures from 980 to 1000 °C (Fig. 2). The relationship between shrinkage of Zr1-xYbxWMoO8-x/2 (x = 0, 0.04) pellets calcined at different temperatures and durations is shown in Fig. 3. From Fig. 3, it can be observed that Zr1-xYbxWMoO8-x/2 (x = 0, 0.04) precursor pellets have similar shrinkage trends at temperatures from 950 to 1000 °C, except that temperature has more influences on the shrinkage of Zr0.96Yb0.04WMoO7.98 (Fig. 3b) than on that of ZrWMoO8 (Fig. 3a). Combined
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with the XRD results and shrinkage of Zr1-xYbxWMo O8-x/2 (x = 0, 0.04) ceramics, the sintering process is mainly divided into three stages: phase transition stage (0–5 min), densification of c-Zr1-xYbxWMoO8-x/2 (x = 0, 0.04) (6–30 min), and the final sintering stage ([30 min). In the first stage (0–5 min), there is a little expansion appearing in the shrinkage curves of ZrWMoO8 and the expansion decreases as sintering time increases (Fig. 3a). This shrinkage feature can be illuminated by the factors influencing densification of ceramics. There are two contrary factors influencing densification in this stage: sintering time and phase transition. In this stage, sintering time is of benefit to densification, while the phase transition has an opposite influence. The negative influence of phase transition can be proved by the XRD result (Fig. 2) in which
Fig. 2 XRD patterns of ZrWMoO8 (a–c) and Zr0.96Yb0.04WMoO7.98 (d–f) precursor pellets calcined for different time at different temperatures
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Fig. 3 Shrinkage of Zr1-xYbxWMoO8-x/2 ceramics with a x = 0 and b x = 0.04 versus sintering time. Each point represents a ceramic fabricated under corresponding conditions
Fig. 4 FE-SEM images of a ZrWMoO8 and b Zr0.96Yb0.04WMoO7.98 precursor powders. c and d are microscopic images of corresponding fractured ceramic surfaces
the phase transition from t-ZrWMoO8 (theoretical density: 5.03 g cm-3) to c-ZrWMoO8 (theoretical density: 4.34 g cm-3) is clearly presented. However, there is no
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expansion observed in shrinkage curves of Zr0.96 Yb0.04WMoO7.98 ceramics (Fig. 3b), although the same phase transition occurs in this stage (Fig. 2d–f).
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Furthermore, the shrinkage of Zr0.96Yb0.04WMoO7.98 pellets increases obviously as duration prolongs. This shrinkage feature may be attributed to the substitution of Zr4? with Yb3?. The introduction of Yb3? induces lattice distortions (the radii of Yb3? and Zr4? are 0.858 and ˚ , respectively [26]) and oxygen vacancies, both of 0.720 A which can facilitate the densification markedly as reported in the Al2O3–TiO2 system [27]. In the second stage (6–30 min), the sintering process becomes that of c-Zr1-xYbxWMoO8-x/2 (x = 0, 0.04) (Fig. 2), and there is only sintering time influencing the densification at the same assigned temperature. The absolute values of shrinkage increase almost linearly in duration of 6–20 min and they increase continuously as sintering time extends until it reaches *30 min (Fig. 3). In this stage, temperature has little influence on the shrinkage of ZrWMoO8, but enhances that of Zr0.96Yb0.04WMoO7.98 obviously. The difference in densification can also be illustrated from the microscopic images of the precursors and corresponding fractured ceramic surfaces (Fig. 4). ZrWMoO8 grains grow from *0.1 to *3 lm after calcinations of 1 h (Fig. 4a, c). And, Zr0.96Yb0.04WMoO7.98 grains become much larger than those of ZrWMoO8 after calcinations in the same conditions (Fig. 4b, d). In the final sintering stage ([30 min), the shrinkage increases little as duration prolongs continuously (Fig. 3). This feature can also be demonstrated from the microscopic images of fractured ceramic surfaces. As shown in Fig. 4c, d, most pores in the ceramic become isolated ones after calcinations of 1 h. The isolated pores are hard to remove by simply extending sintering time. Consequently, the densification of c-Zr1-xYbxWMoO8-x/2 (x = 0, 0.04) is improved little by prolonging duration in this stage. In our previous work, it has been reported that the phase transition from t-ZrWMoO8 to c-ZrWMoO8 occurs at temperatures from 913 to 1000 °C [22]. The key procedure of preparing phase-pure c-ZrWMoO8 is to reduce the sublimation of MoO3, which can be accelerated by both duration and sintering temperature. In the present study, c-ZrWMoO8 is obtained in 5 min at 950 °C and in 3 min at 1000 °C. Combined with the ceramic shrinkage features (Fig. 3), it can be concluded that dense NTE ZrWMoO8 ceramics can be fabricated by calcining t-ZrWMoO8 precursor pellets for 30 min at temperatures insuring the phase transition. The NTE properties were measured by TMA. The dimension changes of Zr1-xYbxWMoO8-x/2 (x = 0, 0.04) ceramics during the treatment of heating and cooling are shown in Fig. 5. As expected, dimensions of both ceramics decrease linearly as temperature increases and vice versa. The CTEs of Zr1-xYbxWMoO8-x/2 (x = 0, 0.04) ceramics are about -4.3 9 10-6 °C-1 (x = 0) and -4.7 9 10-6 °C-1 (x = 0.04) from room temperature to 600 °C,
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Fig. 5 Dimension change curves of ZrWMoO8 (solid line) and Zr0.96Yb0.04WMoO7.98 (dash line) ceramics in the treatment of heating and cooling
showing a little difference with -5.5 9 10-6 °C-1 reported by Allen and Evans [10]. The difference may be ascribed to samples used to measure dimension changes. In Allen and Evans [10], thermal expansion curves were derived by numerical differentiation of cell parameters obtained from neutron diffraction data. However, in our present work, there are a lot of pores and cracks in ceramics, which may delay dimension changes in the process of heating and cooling. The existence of different dimension lags in Fig. 5 reconfirms the influence of pores on dimension changes.
Conclusion In summary, a series of Zr1-xYbxWMoO8-x/2 (x = 0, 0.04) ceramics were fabricated by in situ synthesis from trigonal polymorphous precursors under different conditions. The study on sintering process demonstrates that temperature has little influence on densification of ZrWMoO8, but improves that of Zr0.96Yb0.04WMoO7.98 markedly. Besides, the substitution of Zr4? with Yb3? improves the densification of Zr0.96Yb0.04WMoO7.98 obviously. And, dense NTE Zr1-xYbxWMoO8-x/2 (x = 0, 0.04) ceramics can be obtained by sintering corresponding precursor pellets for 30 min at temperatures insuring the phase transition. The CTEs of Zr1-xYbxWMoO8-x/2 (x = 0, 0.04) ceramics are -4.3 9 10-6 °C-1 (x = 0) and -4.7 9 10-6 °C-1 (x = 0.04) from room temperature to 600 °C. This work will provide important information in fabricating dense NTE ZrWMoO8 ceramics. Acknowledgements The research was supported by the Natural Science Foundation of China (Grants No. 20471010). Ruiqi Zhao also thanks the Doctoral Foundation of Henan Polytechnic University (Grants No. B2009-90) and the Key Technology Foundation of Henan Province (Grants No. 092102210363).
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