AgBi3S5三元硫化物的合成和输运性质.ppt

1、Synthesis and transport properties of AgBi3S5 ternary sulfide compound,前沿,实验,结果讨论,总结,Outline,什么是热电材料 热电材料（也称温差电材料，thermoelectric materials）是一种利用固体内部载流子运动，实现热能和电能直接相互转换的功能材料。, 什么是热电效应 热电效应是电流引起的可逆热效应和温差引起的电效应的总称，包括Seebeck效应、Peltier效应和Thomson效应。,热电材料的三个效应,Seeback效应两种不同金属接触时会产生接触电位差，形成回路时，两个接头的温度不同，是因为

2、两个接头的接触电位不同。 Peltier效应两个金属通过两个接点连成回路，并通以电流，会使得一头发冷一头发热。 Thomson效应基于前两个效应，还必须考虑单根金属线由于其两端温度差而产生的电动势。,Seebeck效应,Peltier效应,热电发电,热电制冷,热电材料的应用,热电器件方面研究较多的是热电发电机( TEG) 和温差制冷机。,热电性能评价,：seebeck系数 ：电导率（与载流子浓度和迁移率有关） ：热导率（包括声子热导和电子热导）,无论用于发电还是制冷, 热电材料的ZT值越高越好。从前面的公式可知, 材料要得到高的ZT值, 应具有高的Seebeck 系数、高的电导率和低的热导率,

3、 所以好的热电材料必须要像晶体那样导电, 同时又像玻璃那样导热，但在常规材料中是有困难的,因为三者耦合,都是自由电子(包括空穴)密度的函数, 材料的Seebeck 系数随载流子数量的增大而减小, 电导率和导热系数则随载流子数量的增大而增大。,目前提高热电材料热电性能的主要方法有以下几种： (1) 通过低维化改善热电材料的输运性能, 如将该材料做成量子阱超晶格、在微孔中平行生长量子线、量子点等； (2) 通过掺杂修饰材料的能带结构，使材料的带隙和费米能级附近的状态密度增大； (3) 通过梯度化扩大热电材料的使用温区，提高热电输出功率;,主要研究方法和手段,目前制备半导体热电材料的方法日趋成熟,主

4、要包括:熔体生长法、粉末冶金法、气相生长法( 包括物理气相沉积、化学气相沉积、分子束外延法等) 、化学法、电化学法、水热合成法、机械合金化法( MA 法) 、热压法、放电等离子烧结法等。,Experimental procedure,Raw materials: high-purity powders of Ag(99.9%),Bi(99.9%) and S(99.5%),Stainless steel vessel and balls were used, and the weight ratio of ball to powder was kept at 20:1.,The resulta

5、nt powders were then applied for SPS using a system (Sumitomo SPS1050, Japan) in a 20 mm graphite mould under an axial pressure of 40 MPa in vacuum.,disk-shaped bulk of 20 4 mm,673K,773K,873K下烧结五分钟 Bulk-673, Bulk-773, Bulk-873,the angular range of 10-100 with a step size of 0.02and speed for 10 per

6、minute,the 2 angular range of 10-95with a step size of 0.02 and speed for 1per minute,Metallographic phase was observed by using an Olympus PME-3 optical microscopy after grounding with SiC papers and polishing with diamond paste of 1.5 mm. The morphologies of fractographs and energy dispersive spec

7、trum (EDS) of bulks were investigated by using a field emission scanning electron microscopy (FESEM; SUPRA 55, Carl Zeiss, Nakano, Japan). The electrical transport properties were evaluated along the sample section perpendicular to the pressing direction of SPS. The electrical conductivity () and Se

8、ebeck coefficient () were measured at 323-573K in a helium atmosphere using a Seebeck coefficient/electric resistance measuring system (ZEM-2, Ulvac-Riko, Japan). A sample along the section parallel to the pressing direction of SPS was used to evaluate the thermal conductivity because of the limited

9、 sample size. The thermal conductivity () was calculated by the relationship of =DCpwith the specific heat Cp measured by a differential scanning calorimeter (SHIMAZDU DSC-60, Japan), density measured by the Archimedes method and the thermal diffusi-vity D measured by a laser flash method (NETZSCH L

10、aser Flash A-pparatus LFA427, Germany).,结果讨论,Fig. 1. XRD patterns of the powders subjected to MA at 425 rpm for 5, 10, 20, 30 and 40 h.,No ternary AgBi3S5 phase was detectable, indicating that the AgBi3S5 powder is unobtainable during MA process in the applied con-ditions.,Fig. 2. XRD patterns of th

11、e bulks obtained by applying SPS for 5 min at 673-873 K (a) and Rietveld refinement profile for the bulk sintered at 873 K (b, c).,The Bulk-673 still shows a similar XRD pattern to that of the powders, which has the compound character of Bi2S3 (PDF#17-0320) and AgBiS2 (PDF#21-1178). Compared with th

12、e pattern of the MA-treated powders, the diffraction peaks of the Bulk-673 become sharper with a narrowed full width at half maximum (FWHM), which reveals the grain growth and the improvement of the crystallinity by SPS.,In the Bulk-773,some new diffraction peaks such as at 13.14, 16.08, 16.46, 21.6

13、6, 22.88, 25.58, 26.24, 26.74, 27.80,30.08,32.32,40.76,41.24 appear as shown by arrows.These peaks match the pattern of the ternary AgBi3S5 although some peaks are overlapped with those (24.78,27.10,28.46,32.78, 33.82,36.34 and 39.84)of Bi2S3 phase and those (27.10, 31.30 and 45.02) of the ternary A

14、gBiS2 phase. Except the overlapped diffraction peaks,the intensity of diffraction peaks belonging to the Bi2S3 and AgBiS2 phases weaken obviously in the Bulk-773.,The XRD pattern of the Bulk-873 sho-ws a well matched pattern to the terna-ry AgBi3S5 phase, which suggests that a solid-state reaction w

15、as conducted to powders mixed with Bi2S3 and AgBiS2 during the sintering process by further raising temperature, as described in Eq: Bi2S3+ AgBiS2 AgBi3S5,Fig. 3. Differential thermal analysis curves of powders obtained by applying MA at 425 rpm for 20 h in which the pristine phase is pure Bi2S3 (do

16、t lines) and a mixture (solid lines) of Bi2S3 and AgBiS2.,The exothermic peak at about 790 K is considered to be attribu-ted to the solidestate reaction of Bi2S3 and AgBiS2 phase (Eq. (1). The lowered melting point of the milled Bi2S3 powder is considered to be due to the activated state with small

17、sizes (about 100 nm) and the more crystal defect.,The absent endothermic peak is due to fact that AgBi3S5 sample is not formed in the precursor powder but has undergone the phase transition via Eq. (1) during sintering.,Fig. 4. XRD patterns in selected 2q ranges around the diffraction peaks of (120)

18、 (a) and (220) (b)for Bi2S3, (112) (c) and (310) (311) (d) for AgBi3S5 for the bulks obtained by applying SPS for 5 min at 673-873 K.,The peak shift to low diffraction angle is due to the volatilization of sulfur in MA processes . According to the DTA result in Fig.3, raising sintering temperature e

19、xceed 712K should deepen the S volatilization and result in shifting the diffraction peak to lower diffraction angle. Hence, the peak shift to the high diffraction angle rather than low diffraction angle may due to the reduction of lattice caused by redistribution to atoms via phase transition (Eq.

20、(1) because of the coexisted three phases (Fig. 2(a). since the S volatilization occurs as raising temperature over 712K. Asimilar shift behavior of diffraction angle with decreasing sulfur content was also found previously in polycrystalline Bi2S3-x(x=0,0.05,0.1,0.15) prepared using the same MA and

21、 SPS processes. which reveal the presence of the Bi volatilization (Fig.3)and the regression of the remnant Bi and S close the stoichiometric ratio (Fig.5 (d) when the sintering temperature exceeds 826K.,Fig. 5. FESEM micrograph of the fractured surfaces (a-c) and optical micrographs (e-g) of bulks

22、obtained by applying SPS for 5 min at 673 K (a, e), 773 K (b, f) and 873 K (c, g) and EDS spectra (d) taken from a strip-like phase from the sample sintered at 773 K.,0.4 m,The measured porosity and density of the Bulk-673, Bulk-773 and Bulk-873 also rise from 5% to 11% and 22% and down from 6.6 to

23、6.3 and 5.9gcm-3.The loosen microstructure is considered to be due to the incre-ased volatilization of S and Bi with raising sintering temperature as co-nfirmed in Figs. 3 and 4.,a semiconductor-like behavior. a typical metal conducting behavior. the competitive action of metallic AgBi3S5 and semi-c

24、onductive Bi2S3.,The decreased values at high temperature ascribe dominantly to the metallic AgBi3S5 phase. The maximum value obtained for the Bulk-873 is 180 Scm-1at room tempera-ture.,It is well known that the value is inversely proportional to the porosity. However,the value of the Bulk-873 is st

25、ill higher than th-ose of the Bulk-673 and Bulk-773 regardless of the effect of the increased pores. Hence,further improved value could be expected by optimizing the microstructure.,The negative a values indicate that all the samples are n-type semiconductors. -ln n where n and are the carrier conce

26、ntration and scattering factor, respectively. In the case of the Bulk-673 which mainly consisted of semi-conductive Bi2S3, the decreased values with raising measurement temperature attributes to the increased n as-sociated with the enhanced thermal activat-ion. The intrinsic metal conducting behavio

27、r of AgBi3S5. The absolute values of the pre-sent AgBi3S5 bulk increase from 83 to 167 V K-1 with increasing temperature from 323 to 573 K.,-360,-153,83,-360,-153,83,The PF value of the present AgBi3S5 bulk material increases from 124 to 221Wm-1K-2 with raising temperature from 323 to 573 K, which a

28、re slightly lower than that (about 144-263Wm-1K-2)of the reported AgBi3S5materials at temperatures ranging from 300 to 575 K.,The gives more contribution to the PF than that of the .,Fig. 7. Temperature dependence of the thermal diffusivity D (a), specific heat Cp (b)the carrier thermal conductivity

29、 (c) and the ratios of lattice/(d) for the bulks obta-ined by applying SPS for 5 min at 673-873 K.,The D values for all the samples at 573 K present ascending trend with raising sintering temperature, in which the D values of the Bulk-873 decrease from 0.508 to 0.400 mm2 s-1 with raising temperature from 323 to 573 K.,The dominant mechanism is the phonon scattering. The k value of the present pure AgBi3S5 bulk material decreases from 0.70 to 0.64 W m-1K-1 with raising temperature from 323 to 573 K, which is the lowest value reporte