摘要
熱電材料及裝置目前已被廣泛應用於熱發電器，感測器、熱電偶以及熱電製冷部分。其中陶瓷氧化物型熱電材料具有耐用、低成本以及對環境友善等優點。 La1.9Sr0.1CuO4-δ由於其具有氧缺陷之鈣鈦礦結構，及具有金屬般的電子電導，被歸類為具有潛力的熱電材料之一。
本研究檢討了ZrO2的添加對La1.9Sr0.1CuO4-δ塊材熱電性質的影響，研究中的La1.9Sr0.1CuO4-δ粉末是以固相反應法，在800℃ 2.5小時大氣的煆燒條件下製備而得。粉末以XRD繞射檢驗結果為單一相，而塊材則由粉末經950℃10小時大氣中燒結緻密塊材，而相對密度是隨著ZrO2添加量的增加而減少。

在熱電性質的探討部分，ZrO2的添加會使La1.9Sr0.1CuO4-δ塊材的電子電導以及熱導率降低，Seebeck係數提升。La1.9Sr0.1CuO4-δ塊材在1wt%ZrO2添加時有最高的熱電優值(ZT)，在300K時為0.027。

關鍵字:熱電材料、La1.9Sr0.1CuO4-δ、ZrO2、燒結、熱電優值

ABSTRACT
Effect of ZrO2 on sintering behavior and thermoelectric properties of La1.9Sr0.1CuO4-δ bulk were investigated in this study. The La1.9Sr0.1CuO4-d/xwt%ZrO2 (x=0-5) bulk materials were prepared using La1.9Sr0.1CuO4-d powders synthesized by solid state reaction in air.. The sintering behavior of La1.9Sr0.1CuO4-d was inhibited by the ZrO2. For the thermoelectric properties of The La1.9Sr0.1CuO4-d/xwt%ZrO2 (x=0-5) bulk, with the increase of ZrO2 amounts the electrical conductivity and thermal conductivity decreased, but the Seebeck coefficient increased. La1.9Sr0.1CuO4-d/1wt%ZrO2 bulk showed the highest ZT value of 0.027 at 300K.
Key words: Thermoelectric material, La1.9Sr0.1CuO4-d, ZrO2, sintering, figure of merit

INTRODUCTION

Thermoelectric materials have been widely investigated in power generator and thermoelectric refrigerator. The performance of thermoelectric material can be described by Figure of merit, ZT=S2σT/κ, where S, σ, T and κ are the Seebeck coefficient, electrical conductivity , absolute temperature and thermal conductivity, respectively. La1.9Sr0.1CuO4-d (LSCO) bulk has advantages of layer perovskite structure, and good electrical conductivity(1-3). However, the relatively low Seebeck coefficient and low thermal conductivity limit its application. Because ZrO2 shows wide band gap (~5eV),low electrical conductivity , thermal conductivity and stable in extremely high temperature ZrO2 was added as scattering source for increasing the Seebeck coefficient and decrease the thermal conductivity of La1.9Sr0.1CuO4-d.

EXPERIMENTAL
La1.9Sr0.1CuO4-d powders were prepared by solid-state reaction. Stoichiometric amounts of La2O3(99.9%), SrCO3(98%) and CuO(99%) powders were mixed ,and then calcined in air at 800℃ for2.5h to form La1.9Sr0.1CuO4-d powders. The powders were mixed and grinded with 0-5wt% ZrO2 (99%,monoclinic phase), powder into composite powder. The powder was cip-ed into pellets at 98 MPa for 1 minute and then sintered at 950℃ for 10h in air to form bulk. The XRD, SEM, thermoelectric properties analysis of the La1.9Sr0.1CuO4-δ/ZrO2 bulk began after sintering process.

RESULTS AND DISCUSSION

The XRD patterns with shifted and broaden peak and the lattice distortion were found in LSCO/ZrO2 bulk(Fig.1). Lattice parameter of a-axis and c-axis of LSCO/ZrO2 bulk increased with the increase of ZrO2 contents(Fig.2). It is resulted from the mismatch of thermal expansion coefficients between LSCO (1.1*10-4/ ℃) and ZrO2(8.9*10-6/ ℃), i.e. the thermal stress existed in the bulk. Another factor that induced these phenomena is the substitution of Zr4+ in Cu2+ site. The radius of Zr4+ ion (80pm) is slightly larger than Cu2+ ion (72pm), that the lattice parameter increased with the increase of ZrO2 contents, and induced the lower crystallinity.

The SEM images of LSCO/ZrO2 bulk show the relatively density and grain size decrease with the amount of ZrO2 .As the amount of ZrO2 increased more pores can limit atomic diffusion and grain boundary migration during sintering process, which lower the relatively density and grain size.

The Seebeck coefficient of LSCO/ZrO2 bulk increased with the increase of ZrO2 contents. It is due to the strongly scattering effect . The ZrO2 particles, grain boundaries and pores are the scattering source .The electrical conductivity decreased with the increase of ZrO2 contents ,It is because the substitution of Zr4+ in Cu2+ site will produce additional +2 charge, and the electrons will form to let the net charge equilibrium. The recombination of electrons and holes occurred which induced the decrease of the electrical conductivity. And that, the maximum power factor is at the 1 wt% ZrO2adding amount bulk, that is 4.786μW/κ2cm. The thermal conductivity decreased with the increasing ZrO2 amount, it is due to the thermal stress existed in LSCO/ZrO2bulk and the strongly scattering effect among the phonon and carriers. Finally, the maximum ZT value is about 0.027 for the LSCO/1wt%ZrO2 bulk in room temperature.

CONCLUSIONS

The LSCO/ZrO2 composite bulk was successfully prepared by solid-state reaction method. The lattice distortion and thermal strain existed in the bulk due to the mismatch of thermal expansion coefficients between LSCO and ZrO2. The substitution of Zr4+ in Cu2+ site induced the combination of electrons and holes and caused the decrease of electrical conductivity. Seebeck coefficient and ZT value increased up to 50% by strongly scattering effect of carriers and phonons due to the lattice distortion and smaller grain size. The maximum ZT value of LSCO/1wt%ZrO2 composite is 0.027 at 300K.

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