| Research Abstract |
The thermal decomposition of NH_4SCN was studies by TG-DTA analysis, and mass spectrometry. It occurred in the temperature range from 400 to 530 K. The decomposition products contained NH_3, CS_2, H_2S and HNCS gases. La_2O_3 and Gd_2O_3 were reacted with these gases at 1273 K for 28.8 ks in order to prepare rare-earth sulfides. The single phase powders of β-La_2S_3 and α-Gd_2S_3 with high purity were obtained.
We have developed a new strategy for the synthesis of the ternary rare-earth sulfides LnGdS_3 (Ln : Pr, Nd) and the quaternary rare-earth sulfide SmEuGdS_4. This strategy involves the use of a polymerized complex method and sulfurization. Multinary rare-earth oxycarbonate powders were first prepared from nitrates by the polymerized complex method. The oxycarbonates were then sulfurized with CS_2 gas to synthesize the multinary rare-earth sulfides. While the sulfurized LnGdS_3 powders were crystallized in the orthorhombic α-phase, the sulfurized SmEuGdS_4 powder was crystallized i
… More
n the cubic γ-phase. The α-LnGdS_3 transformed completely into γ-LnGdS_3 after heat treatment at 1723 K. The γ-LnGdS_3 and γ-SmEuGdS_4 powders were consolidated by pressure-assisted sintering to fabricate full-density compacts. While the NdGdS_3 sintered compact showed an n-type Seebeck coefficient and low electrical resistivity, the SmEuGdS_4 sintered compact showed a p-type Seebeck coefficient and high electrical resistivity. The thermal conductivity of NdGdS_3 ranged from 0.8 to 1.2 W/K m. The highest figure of merit (ZT=0.29) was obtained for NdGdS_3 at 950 K.
Cubic Th_3P_4-type γ-NdGdS_3 exhibits its highest ZT of 0.51 at 950 K in the NdGd_<1.0>2S_3 composition, in which the compound has an optimal carrier concentration. A further increase in ZT is expected from the grain size reduction of sintered γ-NdGd_<1.02>S_3 due to the scattering of phonons at grain boundaries. This work proposes a method for reducing the grain size of sintered γ-LnLn'S_3 compounds via phase transformation. Ln_2S_3 (Ln : La, Ce, Pr, Nd, Sm) transforms from the α phase (orthorhombic) to the γ phase (cubic ; Th_3P_4 type) via the βphase (tetrahedral). In contrast, Ln'_2S_3 (Ln' : Gd, Tb) directly transforms from the α phase to the γphase. It has been found that γ-LnLn'S_3 with high oxygen content transforms from the α phase to the γ phase with fine grains via the βphase, which is supposed to be Ln_5Ln'_5S_<14>O, during the sintering process. In previous studies, γ-NdGd_<1.02>S_3 has been synthesized from commercial Nd_2O_3 and Gd_2O_3 by sulfurization using CS_2 gas. In contrast, in the present study, LnLn'S_3 powder was prepared by the sulfurization of an oxycarbonate containing Ln and Ln', which was previously synthesized by a complex polymerization method, at 1273 K for 28.8 ks under CS_2 gas. After sulfurization, the powder was annealed under vacuum with an oxygen partial pressure less than 0.25×10^<-3> Pa at 1723-1773 K for 21.6-43.2 ks. The annealed powder was consolidated at 1723 K for 3.6 ks under vacuum by pulse electric current sintering at a pressure of 50 MPa. XRD revealed that after sulfurization, the powder was in the α phase. On the other hand, after annealing, the powder was in a single γ phase. In addition, the carbon and oxygen contents were observed to decrease and increase, respectively, with increasing annealing time. When the oxygen content was large, the linear shrinkage curve indicated the stagnation of the shrinkage during the sintering process. XRD measurements of the sintered samples before and after the stagnation of the shrinkage revealed the existence of the α phase before stagnation and both the α and β phases after the stagnation. In the samples with a single γphase that underwent such stagnation of shrinkage during the sintering process, a reduction in the grain size was observed. In contrast, the stagnation of the shrinkage was not observed during the sintering of the α phase powder (which contains less oxygen) without annealing after sulfurization, and hence the grain size was not reduced even if the powder consisted of the γ phase after sintering. In particular, in the cases of γ-NdGdS_3 and γ-PrGdS_3, the grain size was reduced significantly. While the respective grain sizes of γ-NdGdS_3 and γ-PrGdS_3 samples sintered without annealing were approximately 40 and 50 μm, they were approximately 5 and 6μm in the samples sintered after annealing. Less
|
