Zn掺杂n型笼合物Ba8Ga16-2xZnxGe30+x的热电传输特性

用高温熔融结合放电等离子烧结法制备了Zn掺杂单相n型Ba₈Ga_16-2xZn_xGe_30+x笼合物,探索了Zn对Ga的取代对其热电传输特性的影响规律.研究结果表明,n型Ba₈Ga_16-2xZn_xGe_30+x化合物的电导率随着x的增加逐渐增 关键词： 热电传输性能 n型笼合物 框架取代     

Zn掺杂p型Ba8Ga16ZnxGe30－x笼合物的合成及热电性能

用熔融法结合放电等离子烧结方法合成了Zn掺杂单相p型Ge基Ⅰ型笼合物Ba₈Ga₁₆Zn_xGe_30-x(x=3, 4, 5, 6)，探索Zn取代Ge对其热电性能的影响规律，结果表明：所制备的Ba₈Ga₁₆Zn_xGe_30-x化合物为p型传导，随Zn取代量x关键词： p型笼合物 合成 热电性能     

Cd掺杂p型Ge基Ba8Ga16CdxGe30-x Ⅰ型笼合物的结构及热电特性

以Cd作为掺杂元素,用熔融法结合放电等离子体烧结（SPS）技术制备了具有不同Cd含量的Ba₈Ga₁₆Cd_xGe_30-x（x=0.95, 1.00, 1.05, 1.10） Ⅰ型笼合物,研究了Cd掺杂对其结构及热电性能的影响.Rietveld结构解析表明所制备的Ba₈Ga₁₆Cd_xGe_{30- 关键词： p型笼合物 结构 热电性能}     

熔体旋甩工艺对Zn掺杂Ⅰ-型Ba8Ga12Zn2Ge32笼合物微结构及热电性能的影响

采用新颖的熔体旋甩(MS)结合放电等离子烧结(SPS)技术制备了单相Zn掺杂的Ⅰ-型Ba₈Ga₁₂Zn₂Ge₃₂笼合物,研究了熔体旋甩工艺对其微结构以及热电性能的影响. 结果表明,MS得到的薄带自由面主要由300nm—1μm的小立方体单晶组成,薄带经SPS烧结后得到了具有大量层状精细结构的致密块体. 与熔融+SPS工艺制备的试样相比,熔融+MS+SPS制备的Ba₈Ga₁₂Zn 关键词： 熔体旋甩 Ⅰ-型笼合物 热电性能     

Yb/Sr双原子复合填充的I-型YbxSr8－xGa16Ge30笼合物的合成及热电性能

用熔融法结合放电等离子烧结（SPS）合成了Yb/Sr双原子复合填充的n型Yb_xSr_8-xGa₁₆Ge₃₀（x=0,05,10,15）笼合物,研究了双原子复合填充及Yb填充量x对Yb_xSr_8-xGa₁₆Ge₃₀笼合物热电传输特性的影响规律.结果表 关键词： Ⅰ-型笼合物 双原子填充 热电性能     

I型锗基笼合物Ba8Ga16-xSbxGe30的合成及热电性能

用高温熔融结合放电等离子烧结（SPS）方法合成了Sb掺杂的单相n型Ba8Ga16-xSbxGe30化合物，探索了Sb对Ga的取代对其热电性能的影响规律．研究结果表明：随着Sb取代分数x的增加，Seebeck系数逐渐降低，Seebeck系数峰值对应的温度向低温方向偏移．电导率随着x的增加先增大后减小，当x=2时达到最大值．Sb取代Ga后对化合物的热性能有较大影响，其热导率和晶格热导率都有不同程度的降低．在所有n型Ba8Ga16-xSbxGe30化合物中，Ba8Ga14Sb2Ge30化合物的ZT值最大，在950K左右其最大ZY值达1．1．     

Ge掺杂n型Sn基Ⅷ型单晶笼合物的制备及热电传输特性

采用Sn自熔剂法制备了具有n型传导的Ⅷ型Ba₈Ga_16-xGe_xSn₃₀ （0 ≤ x ≤ 1.0）单晶笼合物,并对其结构和热电特性进行研究. 研究结果表明：Ge在单晶中的实际含量较少,随着掺杂量的增加样品的晶格常数略有减小,Ge掺杂后样品的载流子浓度较掺杂前低,迁移率增加;所有样品的Seebeck系数均为负值,且绝对值较未掺杂样品低,但Ge掺杂后样品的电导率提高了62%;x=0.5的样品在500 K附近取得最大ZT值1.25. 关键词： Ⅷ型笼合物 n型传导 热电性能     

Ge掺杂n型Sn基VIII型单晶笼合物的制备及热电传输特性

采用Sn自熔剂法制备了具有n型传导的VIII型Ba8Ga16 xGexSn30(0 x 1.0)单晶笼合物,并对其结构和热电特性进行研究.研究结果表明:Ge在单晶中的实际含量较少,随着掺杂量的增加样品的晶格常数略有减小,Ge掺杂后样品的载流子浓度较掺杂前低,迁移率增加;所有样品的Seebeck系数均为负值,且绝对值较未掺杂样品低,但Ge掺杂后样品的电导率提高了62%;x=0.5的样品在500 K附近取得最大ZT值1.25.     

Zn掺杂n型笼合物Ba8Ga16-2xZnxGe30+x的热电传输特性

用高温熔融结合放电等离子烧结法制备了Zn掺杂单相n型Ba8Ga16-2xZnxGe30 x笼合物,探索了Zn对Ga的取代对其热电传输特性的影响规律.研究结果表明,n型Ba8Ga16-2xZnxGe30 x化合物的电导率随着x的增加逐渐增大,Seebeck系数随着x的增加而逐渐减小.当Zn完全取代Ga时,Ba8Zn8Ge38化合物的电导率反而急剧下降,Seebeck系数显著增大.Ba8Ga16-2xZnxGe30 x化合物的载流子迁移率随着温度的升高而降低,当Zn掺杂后,化合物的载流子迁移率有一定的增加,随着x的增加而逐渐增大.Ba8Ga16-2xZnxGe30 x化合物的热导率和晶格热导率变化规律类似,随着x的增加先减小后增大.在所有n型Ba8Ga16-2xZnxGe30 x笼合物中,Ba8Ga8Zn4Ge34化合物的ZT值最大,在1000 K时其最大ZT值达0.85.     

Zn掺杂P型Ba8Ga16ZnxGe30-x笼合物的合成及热电性能

用熔融法结合放电等离子烧结方法合成了Zn掺杂单相p型Ge基Ⅰ型笼合物Ba8Ga16ZnxGe30-x(x=3,4,5,6),探索Zn取代Ge对其热电性能的影响规律,结果表明:所制备的Ba8Ga16ZnxGe30-x化合物为p型传导,随Zn取代量x的增加,化合物室温载流子浓度Np逐渐增加,室温载流子迁移率μH和电导率逐渐降低.在所有试样中,Ba8Ga16Zn3Ge27化合物的Seebeck系数α在300—870K内始终最大,温度为300K时Seebeck系数为234μV/K,在700K附近达295μV/K.化合物的热导率随Zn取代量x的增加而降低.Ba8Ga16Zn3Ge27化合物在806K最大ZT值达0.38.     

Structural and electronic properties of type-I and type-VIII Ba8Ga16Sn30 clathrates under compression

The total energy and electronic structures for type-I (β phase) and type-VIII (α phase) Ba₈Ga₁₆Sn₃₀ clathrates under hydrostatic pressure have been investigated using density functional theory (DFT) calculations. It was found that the type-VIII phase is more stable than the type-I one at ambient conditions and that β→α phase transition can not occur under hydrostatic pressure. The band structures show that the type-I and type-VIII Ba₈Ga₁₆Sn₃₀ are indirect semiconductors with band gaps of 0.24 eV and 0.19 eV, respectively. The results suggested that type-I clathrate Ba₈Ga₁₆Sn₃₀ has a larger value of the thermoelectric (TE) power than that of type-VIII clathrate. We found that pressure tuning changes the k-point of conduction band minimum (CBM) in the Brillouin zone for β-phase, but it is not the case for α-phase. Furthermore, the results show that the pressure can change the interaction between guest atoms and the host lattice, and consequently results in the decrease of the band gap of β-phase and the increase of the band gap of α-phase, indicating that the pressure effect can play an important role in the magnitude of the TE power.     

Epitaxial growth of Ba8Ga16Ge30 clathrate film on Si substrate by RF helicon magnetron sputtering with evaluation on thermoelectric properties

Epitaxial Ba₈Ga₁₆Ge₃₀ clathrate thin films were successfully grown on Si substrate by using helicon magnetron sputtering. The (1 0 0) lattice of Ba₈Ga₁₆Ge₃₀ was identified grown on four Si(2 0 0) lattices in small mismatch (0.1%). Both the color of samples and XRD results suggest 600 °C is the optimal substrate temperature for the growth of high quality Ba-Ga-Ge clathrate film on Si substrates. High Seebeck coefficients and electrical resistivities for the deposited clathrate thin films in comparison with those of bulk are obtained. The high crystal quality and thermionic effects in heterostructures may contribute to the larger Seebeck coefficients, while the increasing of interface scattering for electrons probably is the reason for large electrical resistivities. Although the thermoelectric (TE) results are not ideal as designed, our results are significant due to the first successful work on epitaxial growth of Ba₈Ga₁₆Ge₃₀ clathrate thin films on Si substrate with large Seebeck coefficient.     

Dielectric behavior of hexaferrites BaCo2−xZnxFe16O27

The dielectric constant (′) and dielectric loss (tan δ) for hexaferrites BaCo_2−xZn_xFe₁₆O₂₇ have been studied as a function of frequency (f), temperature (T) and composition (x). The experimental results indicate that ′ and tan δ above the relaxation frequency only decrease as the frequency increases and as the temperature decreases. Tan δ shows the dielectric relaxation at certain critical frequencies which rise as temperature increases. The activation energy for the dielectric relaxation (E_D), ′, and tan δ are found to be minimum for x = 0.8.     

Normal-state transport properties of Ba1−xKxBiO3 crystals

The normal-state transport properties of Ba_1−xK_xBiO₃ crystals with a wide range of potassium compositions (0≤x≤0.62) were studied. Although the host material BaBiO₃ has a monoclinic structure, the system changes from a monoclinic to an orthorhombic structure with a small doping of potassium (0≤x<0.35) and behaves similar to a doped semiconductor, without exhibiting superconductivity. In the composition range, holes are majority carriers in the transport phenomena. When x exceeds a critical value (0.35), the system goes into a cubic superconducting phase with a single metallic band. The vicinity of the critical composition transport phenomena is easy to understand assuming the existence of two conducting channels that are made up of metallic and semiconducting phases. Maximum T_c exceeding 30 K was observed at x0.4, where carrier density was at its maximum. Overdoping with potassium suppresses superconductivity. In the metallic composition of x>0.45, transport seems to correlate with the phonon mode with an energy distribution of 15–43 meV.     

Structure and low-temperature properties of Ba8Ni6Ge40

Structural, magnetic, heat capacity, electrical and thermal transport properties are reported on polycrystalline Ba₈Ni₆Ge₄₀. Ba₈Ni₆Ge₄₀ crystallizes in a cubic type I clathrate structure with unit cell a=10.5179 (4) Å. It is diamagnetic with susceptibility χ_dia=−1.71×10^-6 emu/g Oe. An Einstein temperature 75 K and a Debye temperature 307 K are estimated from heat capacity data. It exhibits n-type conducting behavior below 300 K. It shows high Seebeck coefficients (−111×10^-6 V/K), low thermal conductivity (2.25 W/K m), and low electrical resistivity (8.8 mΩ cm) at 300 K.     

Unconventional magnetic and transport properties in Gd1−xLaxMn2Ge2 with two-dimensional arrangement of Mn atoms

In this paper, we present our recent experimental results of magnetic and transport properties of Gd_1−xLa_xMn₂Ge₂ intermetallic compounds with the ThCr₂Si₂-type layered structure. The results obtained indicate that, in GdMn₂Ge₂, a first-order transition from a collinear antiferromagnetic to a collinear ferrimagnetic state appears with decreasing temperature at T_t3, below the Néel temperature T_N. In Gd_1−xLa_xMn₂Ge₂ compounds with x=0.05 and 0.075, after ordering ferrimagnetically at T_t1, two kinds of first-order transitions from a canted ferrimagnetic to a non-collinear antiferromagnetic state and from a non-collinear antiferromagnetic to a reentrant canted ferrimagnetic state occur at T_t2 and T_t3. In Gd_0.925La_0.075Mn₂Ge₂, a field-induced metamagnetic transition from non-collinear antiferromagnetism to canted ferrimagnetism occurs at relatively low fields, accompanied by fractal like multi-step transitions, the so called “devil's stair-case”. Furthermore, a negative giant magnetoresistance (GMR) effect (Δρ/ρ15%) was observed at the field-induced metamagnetic transition. The mechanism of this negative GMR was clarified by comprehensive measurements of the resistivity on single crystals Gd_0.925La_0.075Mn₂Ge₂ and TbMn₂Ge₂. With further increasing x, only canted ferrimagnetism appears with a compensation temperature for 0.10<x<0.40, whereas no compensation behavior appears for x>0.50. The phase diagram obtained indicates that the overall magnetism is controlled by the Mn–Mn intralayer distance in the tetragonal c-plane, reflecting the two-dimensional arrangement of Mn atoms.     

Oxide-ion conductivity of (Ba1−xLax)2In2O5+x system based on brownmillerite structure

(Ba_1−xLa_x)₂In₂O_5+x, whose end member is Ba₂In₂O₅, is an oxygen-deficient perovskite oxide showing high oxide-ion conductivity. In order to clarify the reason why the high oxide ion conductivity appeared in this system, the electrical conductivity was measured as a function of temperature and La content. With an increasing La content, the discontinuous jump of ion conductivity in the Arrhenius plot, which is related to the disordering of the oxygen vacancies, disappeared for the sample with x0.2. Above x=0.12, the ion conductivity linearly increased with La content, while the activation energy remained constant with respect to the La content. Moreover, the conductivity for x=0.6 was 0.042 (S/cm) at 1073 K, which exceeded that of 8 mol% yttria-stabilized zirconia. The higher oxide-ion conductivity of this system could be dominated by the amount of mobile oxygen ions.     

Magnetic properties of NdCo2Ge2, ErCo2Ge2 and PrFe2Ge2 compounds

Magnetometric and neutron diffraction studies of polycrystalline NdCo₂GE₂, ErCo₂Ge₂ and PrFe₂Ge₂ compounds were carried out in the temperature range between 4.2 and 300 K. All samples are antiferromagnetic with Néel temperature 26.5, ～ 4.2 and 13 K, respectively. The RECo₂Ge₂ compounds have collinear antiferromagnetic order of +?+? type. For PrFe₂Ge₂ a sinusoidal magnetic structure is observed. Magnetic moment is localized on RE atoms only and is equal to that of RE³⁺ free ion value. In ErCo₂Ge₂ the magnetic moment of Er atoms is perpendicular to the c-axis, whereas for remaining compounds it is parallel to the c-axis.