Xu Jianzhong, Liu Ying, Li Muzi, Sun Jianhong, Li Xiaohua, Bian Qian
(College of Chemistry and Environmental Science，Hebei University, Baoding 071002, China)
AbstractNegative thermal expansion solid solutions Zr1-XLa8-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 ZrW, Zr0.99La0.017.995 and Zr0.98La0.027.99 were -2.594×10-6-1,-2.309×10-6-1,and -2.298×10-6-1 in the temperature range from room temperature to 200ºC, -5.002×10-6-1,-4.810×10-6-1, and -4.730×10-6-1 from 200ºto 700ºrespectively. The phase transition temperature of the samples were between 150ºto 220ºC. There were slightly decrease in the thermal expansion property of the samples with the increase in substitution fraction X.
Materials that exhibit negative thermal expansion (NTE) [1-8] have been of considerable interest, since Sleight et al. reported ZrW 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 ZrW’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 Zr1-XLa8-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 Zr1-XLa8-X/2 (M was lighter rare earth ions). In this study, a new series of Zr1-XLa8-X/2 were synthesized successfully. Their structures and thermal expansion behaviors of Zr1-XLa8-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 Zr1-XLa8-X/2 solid solutions
Zr1-XLa8-X/2 solid solutions were synthesized by a solid-state reaction from the corresponding oxides ZrO, La and WO 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 Kradiation with a 2range 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. SiO 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
3.1 X-ray diffraction studies
Fig 1 XRD patterns of powders Zr1-XLa8-X/2（X=0.00-0.02）at room temperature
The XRD patterns of the Zr1-XLa8-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 ZrW crystal structure with a space group P23. 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 Zr1-XLa8-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 Zr1-XLa8-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 10m). The bonded block of Zr1-XLa8-X/2 (X=0.02) was dense, and the average particle size was much larger compared to Zr1-XLa8-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.
Fig 2 SEM image ofZr1-XLa8-X/2(X=0.00-0.02powders
(a) X=0.00 (b) X=0.01 (c) X=0.02
3.3 Thermal expansion studies
Fig 3 Linear thermal expansion % curvesof cubicZr1-XLa8-X/2（X=0.00-0.02）
Fig. 4 The average thermal expansion coefficients curvesof cubicZr1-XLa8-X/2（X=0.00-0.02）
Dilatometric curve of Zr1-XLa8-X/2 series from room temperature to 700ºwere 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 100ºboth in Fig.3 and Fig.4 mainly because of the density of sintering process. The average thermal expansion coefficients of Zr1-XLa8-X/2 series from X=0.00 to X=0.02 were -2.594×10-6-1,-2.309×10-6-1,and -2.298×10-6-1 in the temperature range from room temperature to 200ºC, -5.002×10-6-1,-4.810×10-6-1, and -4.730×10-6-1 from 200ºto 700ºrespectively. The thermal expansion property slightly decreased as the La3+ substitution degree increased. The thermal expansion coefficient of Zr1-XLa8-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 150ºto 200ºC, which corresponded to the phase transition of space group from P23 to Pa reported by Evans et al. The difference of phase transition temperature was suggested to be due to the difference of La3+ substitution degree.
Cubic Zr1-XLa8-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 Zr1-XLa8-X/2 specimens were attributed to the difference substitution degree of La3+