热挤压工艺对Bi_(2)Te_(2.7)Se_(0.3)材料热电性能和力学性能的影响

Effect of Hot Extrusion Processes on the Thermoelectric and Mechanical Properties of Bi_(2)Te_(2.7)Se_(0.3) Materials

碲化铋(Bi_(2)Te_(3))基化合物作为室温附近性能最优异的热电材料,在热电制冷及发电技术中已获得广泛商业化应用。目前商业化生产的Bi_(2)Te_(3)基热电材料主要采用区熔法制备,虽然具有优异的热电性能,但材料容易解理,力学性能较差,无法满足微型热电器件的加工需求。本工作探索最佳热挤压工艺参数,采用热挤压技术在不同工艺条件下制备了n型Bi_(2)Te_(2.7)Se_(0.3)热电材料样品,并对样品进行物相分析、热电性能以及力学性能表征。结果表明,在确定的挤压比以及挤压速率下,提高挤压温度有利于减少样品裂纹,而过快的挤压速率则会导致样品断裂,最佳工艺参数为挤压温度400℃以及挤压速率0.05 mm/s。采用最佳工艺参数在不同工艺条件下制备的n型Bi_(2)Te_(2.7)Se_(0.3)样品中,通过放电等离子烧结结合热挤压所制备的样品热电性能最高,在400K时zT值可达到0.83,并且其力学性能显著提升,维氏硬度可达602.5 N/mm^(2),为区熔Bi_(2)Te_(2.7)Se_(0.3)材料的2倍。Bi_(2)Te_(2.7)Se_(0.3)材料的机械加工性能也随之提升,采用封装切割工艺可以加工出最小尺寸达到147μm的微型热电粒子,为后续Bi_(2)Te_(3)基微型热电器件的加工制备提供了材料基础。

Introduction Bi_(2)Te_(3)-based alloys are the most well-known thermoelectric materials for room-temperature applications.The existing Bi_(2)Te_(3)-based thermoelectric materials produced commercially are prepared by a zone melting method,resulting in highly oriented materials with high thermoelectric properties.However,the ingots produced by the zone melting method are prone to disintegration due to the weak van der Waals forces between the layers of Te atoms,leading to the poor mechanical properties and the proclivity for fracture during processing and practical applications.It is thus important to improve the mechanical properties and machinability of Bi_(2)Te_(3)-based materials with the excellent thermoelectric properties.This work was to prepare a n-type Bi_(2)Te_(2.7)Se_(0.3) thermoelectric material via spark plasma sintering combined with hot extrusion(SPS+HE).The parameters affacting hot extrusion process and preparation of n-type Bi_(2)Te_(2.7)Se_(0.3) materials under different process conditions were optimized,and the thermoelectric performance and mechanical properties were investigated.In addition,the machinability of the extruded Bi_(2)Te_(2.7)Se_(0.3) material was also analyzed.Methods The commercial zone-melting crystal rods were pulverized and sintered via spark plasma sintering(SPS)at 350℃and 20 MPa for 5 min.The ingot was then put into a hot extrusion mold,and treated at 390-550℃,extrusion speed of 0.05-0.50 mm/s,extrusion ratio of 9:1,and extrusion angle of 30°.Under the determined hot extrusion parameters,n-type Bi_(2)Te_(2.7)Se_(0.3) materials were prepared by different processes,i.e.,the raw materials were ground for 1 h and then sintered by SPS and subsequent hot extrusion(HE);the raw materials were melted at 700℃for 5 h and then cold-pressed(MT),and sintered by subsequent hot extrusion(HE);the raw materials were melted and cold-pressed,followed by annealing and hot extrusion(MT+AN+HE);and the raw materials were ground for 1.5 h and cold-pressed,followed by hot extrusion(BM+HE).The phase structure was analyzed by X-ray diffraction(XRD,D8 Advance,Bruker Co.Ltd.,USA)at room temperature.The microstructure and fracture morphology were determined by scanning electron microscopy(SEM,Supra 55,ZEISS Co.,Germany).The electrical conductivity and the Seebeck coefficient were measured by ZEM-3(ULVAC Co.Ltd.,Japan).The thermal diffusivity(λ)was measured by a laser flash method(LFA457,Netzsch Co.,Germany).The thermal conductivity was calculated according toκ=λρCp,where Cp was the heat capacity according to the Neumann-Kopp rule,and the densityρwas measured based on the Archimedes prinicple.Results and discussion The results show that n-type Bi_(2)Te_(2.7)Se_(0.3) thermoelectric materials can be prepared under optimum process conditions(i.e.,extrusion temperature of 400℃and extrusion speed of 0.05 mm/s).The XRD patterns of all the hot-extruded samples agree well with a rhombohedral structure(JCPDS 50-0954)of Bi_(2)Te_(2.7)Se_(0.3).No impurity peaks appear.The SEM images reveal that Bi,Te,and Se are homogeneously distributed in the materials without any impurity phases.The electrical conductivityσof all the samples decreases with increasing temperature,having a metallic conduction behavior.Theσof the MT+AN+HE sample is the maximum value of 1.4×105 S/m.All the samples have negative Seebeck coefficients S,indicating that electrons are the major charge carriers.The S of the SPS+HE sample is the maximum value of 191.0μV/K.At 300 K,the SPS+HE sample has the maximum power factor PF of 37.0μW/(cm·K^(2)).However,the PF of the material gradually decreases with the increase of temperature,which is consistent with the change of electrical conductivity and Seebeck coefficient due to the intrinsic excitation.The lattice thermal conductivity of MT+HE sample is the minimum value(i.e.,0.9 W·m^(-1)·K^(-1)).The maximum zT value reaches 0.83 at 400 K for SPS+HE sample and its Vickers hardness is 602.5 N/mm2, which is twice greater than that of the zone-melting sample. This is mainlydue to the dynamic recrystallization process of grain refinement during the hot extrusion process. The machinability of the extrudedBi_(2)Te_(2.7)Se_(0.3) material are improved. The thermoelectric micron-sized particles with a minimum size of 147 μm can be achieved,providing a promising material for the development of Bi_(2)Te_(3)-based thermoelectric micro-devices.Conclusions The optimal hot extrusion process parameters for n-type Bi_(2)Te_(2.7)Se_(0.3) thermoelectric materials were obtained (i.e.,extrusion ratio 9:1, extrusion angle 30°, extrusion temperature 400 K and extrusion speed 0.05 mm/s). A series of n-type Bi_(2)Te_(2.7)Se_(0.3)thermoelectric material samples were prepared via hot extrusion under the optimum process conditions. The Bi_(2)Te_(2.7)Se_(0.3) sampleprepared by SPS and sebsequent hot extrusion (SPS+HE) had the maximum zT value 0.83 at 400 K, and its mechanical propertieswere improved. The Vickers hardness of the SPS+HE sample could reach 602.5 N/mm2, which was twice greater than that of thezone-melting Bi_(2)Te_(2.7)Se_(0.3) material. In addition, the machining performance was also improved, and the thermoelectric micron-sizedparticles with the minimum size of 147 μm could be machined for the extruded Bi_(2)Te_(2.7)Se_(0.3) sample, providing a promising materialfor the Bi_(2)Te_(3)-based thermoelectric micro-devices.