N-type Si-based typeⅠ clathrates with different Ga content were synthesized by combining the solid-state reaction method,melting method and spark plasma sintering(SPS) method.The effects of Ga composition on high temperature thermoelectric transport properties were investigated.The results show that at room temperature,the carrier concentration decreases,while the carrier mobility increases slightly with increasing Ga content.The Seebeck coefficient increases with increasing Ga content.Among all the samples,Ba7.93Ga17.13Si28.72 exhibits higher Seebeck coefficient than the others and reaches 135 μV·K-1 at 1000 K.The sample prepared by this method exhibits very high electrical conductivity,and reaches 1.95×105 S·m-1 for Ba8.01Ga16.61Si28.93 at room temperature.The thermal conductivity of all samples is almost temperature independent in the temperature range of 300-1000 K,indicating the behaviour of a typical metal.The maximum ZT value of 0.75 is obtained at 1000 K for the compound Ba7.93Ga17.13Si28.72.
We investigated the transport properties of isoelectronic substitution of Yb by Ca for Zintl phase YbCd2Sb2 below 300 K.The p-type Yb1-xCaxCd2Sb2(0.2≤x≤0.8) samples were synthesized via a solid-state reaction followed by suitable cooling,annealing,grinding,and spark plasma sintering(SPS) densification processes.For samples with x=0.2,0.4,0.5,0.6,0.8,the electrical conductivity,Seebeck coefficient,thermal conductivity,heat capacity and Hall effect measurements were carried out in the temperature range from 1.9 to 300 K.It was found that the Ca substitution effectively lowered the thermal conductivity at low temperature,while the electrical resistivity and the Seebeck coefficient in the compounds were found to increase with increasing Ca concentration.As a result,the dimensionless figure of merit ZT of 0.18 was attained at 300 K for sample with x=0.2.
Zintl phase compounds AM2Sb2(A=Ca,Sr,Ba,Eu,Yb;M=Zn,Cd)is a new class of promising thermoelectrics owing to their intrinsic features in electronic and crystal structure,such as a small or even disappeared band-gap,large density-of-states at the Fermi level,covalently bonded network of M-Sb,as well as the layered stacking by cations A2+and anionic slabs(M2Sb2)2–.In addition,the rich solid-state chemistry of Zintl phase allows structural modification and chemical substitution to adjust the fundamental transport parameters(carrier concentration,mobility,effective mass,electronic and lattice thermal conductivity)for improving the thermoelectric performance.In the present review,the recent advances in synthesis and thermoelectric characterization of title compounds AM2Sb2were presented,and the effects of alloying or substitution for sites A,M and Sb on the electrical and thermal transport were emphasized.The structural disorder yielded by the incorporation of multiple ions significantly increased the thermoelectric figure of merit mainly resulted from the reduction of thermal conductivity without disrupting the carrier transport region in substance.Therefore,alloying or substitution has been a feasible and common route utilized to enhance thermoelectric properties in these Zintl phase compounds,especially for YbZn0.4Cd1.6Sb2(ZT700 K=1.26),EuZn1.8Cd0.2Sb2(ZT650 K=1.06),and YbCd1.85Mn0.15Sb2(ZT650 K=1.14).
Ag and Cu filled Chevrel phase MxMo6Te8 (x=1.0, 2.0) samples were synthesized by direct solid state reaction and spark plasma sintering. The electrical and thermal properties were investigated in the temperature range of 300-800 K. The results show that both the electrical and thermal properties are affected by filler atoms. Although the electrical conductivity of MxMo6Te8 is slightly higher than that of state-of-the-art thermoelectric material, such as filled skutterudites, the absolute value of Seebeck coefficient is relatively low. Due to the phonon scattering by the filler atoms, the decrease of the thermal conductivity and the lattice thermal conductivity is obvious. As a result, the dimensionless figure of merit(ZT) is improved over the whole temperature region. The highest ZT value is 0.034 at 800 K for the AgMo6Te8 sample.
