Mg/Zn掺杂铜铁矿CuAl/CrO₂多晶的微结构和电热性能

【作者】 李毅；

【导师】 虞澜；

【作者基本信息】 昆明理工大学 ， 材料物理与化学， 2022， 硕士

【摘要】 铜基铜铁矿CuMO₂（M=Al、Cr、Fe、Ga等）族的晶体结构是由导电的Cu⁺层和绝缘的MO₆层沿c轴交替堆垛而成,直接带隙为3.0～3.5 e V,同时具有透明导电性。这种层状晶体结构,可以通过掺杂调控面内的载流子输运,产生大的热电输运各向异性,以及可能的“电子晶体-声子玻璃”的特征。本文制备了Mg/Zn掺杂的Cu AlO₂、CuCrO₂系列多晶,研究了微结构,电热性能及掺杂效应,讨论了各向异性、间隙氧,以及晶格热导率增大的机制。采用固相反应法,在空气中以1100℃/12 h+1150℃/12 h工艺烧结制备了Cu Al_1-xR_xO₂（R=Mg/Zn,x=0～0.04）系列多晶,研究了Mg/Zn掺杂对Cu AlO₂多晶的结构和性能的影响。Mg掺杂Cu AlO₂多晶在固溶度（x=0-0.02）范围内均为铜铁矿R3m单相,晶格常数和晶粒尺寸略微增大,密度提高,呈半导体热激活电输运行为;x=0.02多晶的室温电阻率下降20倍,热激活能明显减小（ρ～0.29Ω·m,E_a～0.218 e V）,空穴载流子浓度增加1个量级,载流子迁移率略增加。x=0.03、0.04时出现Cu O和尖晶石Mg Al₂O₄杂相。Cu Al_1-xMg_xO₂的晶格热导率占绝对优势,x=0.02多晶的室温热导率增大1倍（κ～13.07 W/m·K）,声速增大,Callaway模型模拟结果表明点缺陷-声子散射减弱,分析认为掺Mg形成强的Mg-O键,提高了晶体的弹性模量和声子频率,还阻隔减弱了本征点缺陷、Mg掺杂带来的质量波动和应变场波动对声子的散射。通过热分析和XPS初步研究了Cu AlO₂的本征点缺陷中间隙氧的形成。Zn掺杂Cu AlO₂多晶的固溶度为x～0.02,晶格常数a、c和晶粒尺寸略微增大,其电阻率减小2倍,热导率在450 K以上略微增大,是发生了双极扩散效应;x=0.03、0.04时出现绝缘Zn Al₂O₄尖晶石杂相。以1100℃/12 h+1100℃/12 h烧结制备了CuCr_1-xR_xO₂（R=Mg/Zn,x=0～0.05）多晶,研究了Mg/Zn掺杂对CuCrO₂多晶的结构和性能的影响。Mg掺杂量x=0～0.03时,多晶均为铜铁矿R3m单相,层状晶粒显著长大,致密度增大,c轴择优显著增强,x=0.03样品取向因子F_（00l）高达0.9023;所有样品呈现半导体的热激活电输运行为,x=0.03多晶相对未掺杂样品,室温电阻率下降了3个量级(ρ_300K～1.5×10^-3Ω·m),热激活能E_a由0.289 e V减小至0.039 e V,源于Mg²⁺取代Cr³⁺,在价带顶引入新的受主能级,使面内的空穴载流子浓度增加了3个量级,同时,迁移率以及ab面内电输运占比增加。CuCr_1-xMg_xO₂多晶的晶格热导率占绝对优势,载流子热导率的贡献略微增大;Mg掺杂使x=0～0.01多晶室温热导率提高50%,x=0.02～0.05下降,与室温下声速和弹性模量的规律基本一致;热导率-温度曲线的Callaway模型模拟表明,Mg掺杂使多晶声子-声子散射减弱,点缺陷-声子散射增强,其中Mg²⁺取代Cr³⁺,在[Cr O₆]八面体层引入更多的点缺陷和Cu⁺层引入更多的空穴载流子。x=0.02～0.05多晶的c轴择优很强(F_（00l）=0.8834～0.9023),使得热导率中沿c轴热传导的贡献显著增强,由于沿c轴热传导被[Cr（Mg）O₆]层阻隔,热导率下降。Zn掺杂CuCrO₂多晶的固溶度x～0.02,固溶掺杂使（00l）取向择优显著增大至F_（00l）～0.9505,而大的各项异性使得面内的输运分量增加,电阻率仅减小15倍,可能是Zn的固溶度太小,受主能级未向价带顶展宽;x=0.03～0.05时出现绝缘Zn Cr₂O₄尖晶石杂相,电阻率有增大。更多还原

【Abstract】 The crystal structure of the copper-based delafossite CuMO₂（M=Al,Cr,Fe,Ga）group is formed by alternately stacking of conductive Cu⁺layers and insulating MO₆layers along the c-axis,with a band gap of 3.0～3.5e V and transparent conductivity.This layered crystal structure can be modulated in in-plane carrier transport through doping,yields a large thermoelectric transport anisotropy and a possible feature of“electron crystal-phonon glass”.In this paper,Mg/Zn doped Cu AlO₂and CuCrO₂series polycrystals were prepared,the microstructure,electro-thermal properties and doping effects were investigated,then anisotropy and interstitial oxygen,as well as the mechanics for the increasing of lattice thermal conductivity were discussed.Cu Al_1-xR_xO₂（R=Mg/Zn,x=0～0.04）polycrystals were prepared by sintering at 1100℃/12h+1150℃/12 h in air by solid-phase reaction method.The effects of Mg/Zn doping on structures and properties of Cu AlO₂were investigated.When Mg-doped in x=0～0.02,the polycrystals are of a delafossite single-phase（R3m）,in which the lattice constant and grain size increase slightly,all showing thermally activated electrical transport behavior of semiconductor.For x=0.02 polycrystal,the room temperature resistivity decreased by 20 times,the thermal activation energy is significantly reduced（ρ～0.29Ω·m,E_a～0.218 e V）,the hole carrier concentration in-plane increased by 1 order of magnitude,and the carrier mobility slightly increased.The impurity phases of Cu O and spinel Mg Al₂O₄appear,when x=0.03 and 0.04.The lattice thermal conductivity in Cu Al_1-xMg_xO₂is absolutely dominant,the thermal conductivity of polycrystal（x=0.02）has doubled（κ～13.07 W/m·K）at room temperature,and the sound velocity increases.The simulation results of the Callaway model show that the point defect-phonon scattering is weakened by Mg-doping,it is discussed that Mg-doping forming strong Mg-O bonding,result in increasing the elastic modulus of crystals and the phonon frequency,which blocking of scatterings between point defects and phonon,the point defects including intrinsic point-defect,the mass fluctuation and strain field fluctuation caused by Mg doping.The formation of interstitial oxygen in Cu AlO₂is preliminarily investigated by thermal analysis and XPS.The solid solubility was about 0.02,of Zn doped Cu AlO₂polycrystals,while the lattice constants a,c and grain size were slightly increase,the resistivity decreases by 2 times,and the thermal conductivity increases slightly above 450 K,which is the result of bipolar diffusion effect;and the insulating Zn Al₂O₄spinel phase appeared when x=0.03 and 0.04.CuCr_1-xR_xO₂（R=Mg/Zn,x=0～0.05）polycrystals were prepared by sintering at 1100℃/12h+1100℃/12 h in the air,and the Mg/Zn doping effects were studied.For Mg doping in x=0～0.03,the polycrystals are of delafossite single-phase in R3m,the layered grains grow significantly in plane,the densities increase,and the c-axis preferences are significantly enhanced,up to 0.9023.All samples are in thermally activated electrical transport behavior of semiconductor,and x=0.03 sample compared to undoped sample,the room temperature resistivity drops by 3 orders of magnitude(ρ_300K～1.5×10^-3Ω·m),the thermally activated energy E_adecreased from 0.289 e V to 0.039 e V,due to the replacement of Cr³⁺by Mg²⁺,a new acceptor energy level is introduced at the top of the valence band,and the hole carrier concentration in-plane increases 3 orders of magnitude.Meanwhile,the mobility and the proportion of in-plane electrical transport both increase.The lattice thermal conductivities of CuCr_1-xMg_xO₂polycrystals are absolutely dominant,the contribution of carrier thermal conductivity slightly increased;Mg doping increases the thermal conductivity of polycrystals（x=0～0.01）by 50%,while x=0.02～0.05 decreases,which is basically consistent with behavior of the sound velocities and elasticity modulus at room temperature.The Callaway model simulation of thermal conductivity-temperature curves show that the phonon-phonon scattering is weakened and point defect-phonon scattering is enhanced by Mg doping,in which Mg²⁺replaces Cr³⁺,more point defects are introduced in the[Cr O₆]octahedral layer and more hole carriers are introduced in the Cu⁺layer.The c-axis orientations of polycrystals（x=0.02～0.05）are strongly enhanced(F_（00l）=0.8834～0.9023),which make the contributions of thermal conduction along c-axis are significantly enhanced.Since the thermal conduction along the c-axis is blocked by the[Cr（Mg）O₆]layer,the thermal conductivities decreases.The solid solubility of Zn-doped CuCrO₂polycrystal is about 0.02,the（00l）orientation factor up to F_（00l）～0.9505,while the large anisotropy makes the in-plane transport component increases,but the resistivity only decreases by 15 times,which may be related to the small solid solubility of Zn and the acceptor energy levels is non broadened toward to top of the valance.When x=0.03～0.05,the insulating impurity phase of Zn Cr₂O₄spinel appears and resistivity some raised.更多还原