Zr_1-XLa_XW₂O_8-X/2 (X=0.00-0.02)：solid solutions of negative thermal expansion-synthesis, characterization and property

Xu Jianzhong, Liu Ying, Li Muzi, Sun Jianhong, Li Xiaohua, Bian Qian
(College of Chemistry and Environmental Science，Hebei University, Baoding 071002, China)

Abstract Negative thermal expansion solid solutions Zr_1-XLa_XW₂O_8-X/2 (X=0.00-0.02) powders were synthesized using solid state reaction with Zirconium dioxide, Lanthanum oxide and Tungsten trioxide as raw materials. The structure and morphology of the resulting powders were characterized by powder X-ray diffraction (XRD) and scanning electron microscope (SEM) respectively. The thermal expansion coefficient of the samples were measured by dilatometer at room temperature to 700ºC. The results showed that samples were single cubic phase. The average thermal expansion coefficients of ZrW₂O₈, Zr_0.99La_0.01W₂O_7.995 and Zr_0.98La_0.02W₂O_7.99 were -2.594×10^-6K^-1,-2.309×10^-6K^-1,and -2.298×10^-6K^-1 in the temperature range from room temperature to 200ºC, -5.002×10^-6K^-1,-4.810×10^-6K^-1, and -4.730×10^-6K^-1 from 200ºC to 700ºC respectively. The phase transition temperature of the samples were between 150ºC to 220ºC. There were slightly decrease in the thermal expansion property of the samples with the increase in substitution fraction X.
Keywords negative thermal expansion; Zr_1-XLa_XW₂O_8-X/2 (X=0.00-0.02); solid state synthesis

1. INTRODUCTION
Materials that exhibit negative thermal expansion (NTE) ^[1-8] have been of considerable interest, since Sleight et al. reported ZrW₂O₈ shows NTE in 1996. Such NTE materials could be mixed with others to form various composites with controlled thermal expansion, which can include low or even zero thermal expansion. Applications range from the high-tech to the more mundane and include electronic components, printed circuits boards, optical substrate, low temperature thermocouples. Due to ZrW₂O₈'s relatively large isotropic, unusual negative thermal expansion over a wide range of temperature from 10 to 1050k , and Zr cation position could be partly substituted by various cations, the thermal expansion coefficients could be adjusted to a desired value. Synthesis and properties of negative thermal expansion materials  Zr_1-XLa_XW₂O_8-X/2 substituted for Zr(IV) sites by the heavy rare earth ions have been recently reported^[9-12]while no paper has been written on the preparation of  Zr_1-XLa_XW₂O_8-X/2 (M was lighter rare earth ions). In this study, a new series of  Zr_1-XLa_XW₂O_8-X/2 were synthesized successfully. Their structures and thermal expansion behaviors of   Zr_1-XLa_XW₂O_8-X/2 (X=0.00-0.02) were investigated by means of  X-ray powder diffraction, scanning electron microscope, and dilatometer.

2. EXPERIMETAL SECTION
2.1 Preparation of Zr_1-XLa_XW₂O_8-X/2 solid solutions
Zr_1-XLa_XW₂O_8-X/2 solid solutions were synthesized by a solid-state reaction from the corresponding oxides ZrO₂, La₂O₃ and WO₃ with high purity (AR: 99.9%). Appropriate amounts of the reactants were intimately mixed by grinding using an agate mortar and pestle. This mixture was heated at 873K for 3h,reground, and heated a second time for 5h at 1173K. The last heat treatment was performed at 1423k for 2 hours. Then quenched in cold water.
2.2 Experimental techniques
Room temperature X-ray diffraction data were collected on Y-2000 using Cu Ka radiation with a 2q range from 10 to 50° by a step scanning method. XRD patterns were indexed using Powder X software.The microstructure of the specimen was observed by scanning electron microscope (SEM) using KYKY-2800B. The thermal expansion of the samples were measured by dilatometer using PCY from room temperature to 973K. The shape of the specimen was a rectangular bars of 3.5 mm×7 mm×50mm. SiO₂ was acted as a reference material and push rod. The measurement was carried out at the rate of 10K/min in the open air.

3. RESULTS AND DISCUSSION
<sup>3.1 X-ray diffraction studies

Fig 1</sup> XRD patterns of powders Zr_1-XLa_XW₂O_8-X/2（X=0.00-0.02）at room temperature

The XRD patterns of the Zr_1-XLa_XW₂O_8-X/2 with different La content at room temperature were presented in Fig. 1.
    ^{It can be seen that all samples studies showed the similar XRD patterns. This was mainly because of the very limited content of La substituted for Zr. We indexed these XRD patterns using Powder X software with a cubic unit cell corresponding to the ZrW₂O₈ crystal structure with a space group P2₁3. The lattice parameter was 0.915 nm at RT which was in good agreement with the results in precious studies, and on the other hand proved that Zr_1-XLa_XW₂O_8-X/2(X=0.00-0.02) solid solutions have been successfully synthesized.
3.2 SEM studies        
Scanning electron microscopic studies were performed on the fractured surface of Zr_1-XLa_XW₂O_8-X/2 (X=0.00-0.02), and presented in Fig. 2(a), 2(b), and 2(c), respectively. As seen from the micrograph, the samples consisted of small particles(about 10} mm). The bonded block of Zr_1-XLa_XW₂O_8-X/2 (X=0.02) was dense, and the average particle size was much larger compared to Zr_1-XLa_XW₂O_8-X/2(X=0.00-0.01), the last two specimens were of porosity. The degree of substitution by La³⁺ may influence the density of the composites.
　
                            (a)                                                                    (b)

          (c)

^{Fig 2 SEM image of Zr_1-XLa_XW₂O_8-X/2(X=0.00-0.02) powders
(a) X=0.00 (b) X=0.01 (c) X=0.02}

3.3 Thermal expansion studies

<sup>Fig 3 Linear thermal expansion % curves of cubic Zr_1-XLa_XW₂O_8-X/2（X=0.00-0.02）

Fig. 4 The average thermal expansion coefficients curves of cubic Zr_1-XLa_XW₂O_8-X/2（X=0.00-0.02）</sup>

    Dilatometric curve of Zr_1-XLa_XW₂O_8-X/2 series from room temperature to 700ºC were shown in Fig.3. It was seen from Fig.3, that all specimens showed negative thermal expansion curves regradless of substitution fraction X. The anomaly of the curve below 100ºC both in Fig.3 and Fig.4 mainly because of the density of sintering process. The average thermal expansion coefficients of Zr_1-XLa_XW₂O_8-X/2 series from X=0.00 to X=0.02 were -2.594×10^-6K^-1,-2.309×10^-6K^-1,and -2.298×10^-6K^-1 in the temperature range from room temperature to 200ºC, -5.002×10^-6K^-1,-4.810×10^-6K^-1, and -4.730×10^-6K^-1 from 200ºC to 700ºC respectively. The thermal expansion property slightly decreased as the La3+ substitution degree increased. The thermal expansion coefficient of Zr_1-XLa_XW₂O_8-X/2（X=0.00）was higher than the results reported in precious studies, the main reason was the synthesis method and the temperature range of measurement. It was also shown in Fig.4 that each expansion curve of the samples has a shift at the temperature range from 150ºC to 200ºC, which corresponded to the phase transition of space group from P2₁3 to Pa₃ reported by Evans et al^[3]. The difference of phase transition temperature was suggested to be due to the difference of La³⁺ substitution degree.

4. CONCLUSION
Cubic Zr_1-XLa_XW₂O_8-X/2 (X=0.00-0.02) sold solutions can be synthesized by solid state route. The thermal expansion behavior of the series have been studied by dilatometry. A difference in phase trasition temperature range and thermal expansion observed in Zr_1-XLa_XW₂O_8-X/2 specimens were attributed to the difference substitution degree of La³⁺.

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负热膨胀材料Zr1-XLaXW2O8-X/2(X=0.00-0.02)的合成、表征及性质研究
徐建中 刘 颖 李木子 孙建红 李晓花 边倩
(河北大学 化学与环境科学学院，河北 保定 071002)
摘要 采用固相反应法，以ZrO₂、La₂O₃和WO₃为原料制备了复合负热膨胀材料Zr_1-XLa_XW₂O_8-X/2(X=0.00-0.02)粉体。通过X射线粉末衍射、扫描电子显微镜分别对粉体进行物相分析和形貌观测，用热膨胀仪测定了样品从室温到700℃的膨胀行为。结果表明合成产物为立方结构，室温到200℃复合负热膨胀材料Zr_1-XLa_XW₂O_8-X/2 （X=0.00、0.01、0.02）的平均线性热膨胀系数分别为-2.594×10^-6K-1、-2.309×10^-6K-1、-2.298×10^-6K^-1，200℃到700℃的平均线性热膨胀系数分别为-5.002×10^-6K^-1、-4.810×10^-6K^-1、-4.730×10^-6K^-1，在150℃到220℃,Zr_1-XLa_XW₂O_8-X/2（X=0.00、0.01、0.02）发生相变且其负热膨胀性能随X值增加而略有减弱。
关键词  负热膨胀；Zr1-XLaXW2O8-X/2；固相合成