【作者】 辛圣炜；

【导师】 田永君； 于栋利；

【作者基本信息】 燕山大学 ， 材料学， 2011， 博士

【摘要】 碱土金属的六硼化物是一类很重要的物质，它们具有高熔点、高强度和化学稳定性高的特点，其中许多还具有特殊的功能性，如：低的电子功函数、比电阻恒定、在一定温度范围内热膨胀值为零、不同类型的磁序以及高的中子吸收系数等。这些优越性能决定其在现代科学技术中有着广泛的应用。在我们的研究工作中，采用固相烧结法制备得到了CaB₆和SrB₆多晶样品。电阻率的测量表明CaB₆多晶体具有半金属的导电性质，霍尔系数的测量给出了CaB₆多晶体在室温下的载流子密度为6.82×10¹⁸electron cm-3。利用德拜和爱因斯坦组合的模型模拟了CaB₆的热容，与实验结果吻合得很好。采用高温高压法制备了CaB₆和SrB₆单晶，利用X射线衍射表征了样品的结构，表明晶体的空间群为Pm-3m，晶格常数分别为a=4.159（CaB₆）和4.1975（SrB₆）。利用扫描电子显微镜观察了晶体的形貌，合成的晶体主要有三种形状：棒状，片状和块状。基于实验结果分析了CaB₆单晶的生长机理。通过电阻率的测量表明CaB₆和SrB₆单晶具有半金属的导电性质。通过第一性原理计算，证实了CaB₆和SrB₆单晶具有半金属的导电性质并分析了其导电机理。霍尔系数的测量给出了CaB₆和SrB₆单晶在室温下的载流子密度为6.32×10¹⁸electron cm-3和7×10¹⁸electroncm-3。通过磁性测量表明CaB₆具有顺磁性，SrB₆具有抗磁性。利用德拜和爱因斯坦组合的模型模拟了CaB₆和SrB₆的热容，与实验结果吻合得很好。采用高温高压法制备了Mn掺杂CaB₆单晶。利用X射线衍射表征了样品的结构，与未掺杂的样品比较，发现随着Mn掺杂量的增加，晶格常数逐渐变小。通过磁电阻率的测量发现随着掺杂量的增加，导电性质由半金属转变为半导体。采用了四引线法测量了Mn掺杂CaB₆单晶的霍尔电阻率，并根据霍尔电阻率计算得到了Mn掺杂CaB₆单晶的霍尔系数。在2³00K的温度区间内，Mn掺杂CaB₆单晶的霍尔系数RH随着温度的降低而单调增大。负的霍尔系数表明Mn掺杂CaB₆单晶全部是N型半导体。通过磁性测量表明：同未掺杂的样品比较，掺杂之后的样品产生了弱铁磁性，随着Mn掺杂量的增加，样品的矫顽力和饱和磁矩逐渐变大。Mn掺杂CaB₆单晶的热容小于CaB₆，且随着温度的上升而单调增大；利用德拜和爱因斯坦组合的模型模拟了Mn掺杂CaB₆单晶的热容与温度的关系，与实验结果吻合得很好，拟合结果表明：随着Mn含量的增加，晶体的电子热容系数（γ）逐渐增大，德拜温度（TD）增加逐渐降低，爱因斯坦温度（T_E）几乎不变。采用高温高压法成功制备了Mg掺杂CaB₆单晶，样品的X射线衍射分析结果发现：随着Mg掺杂量的增加，晶体的晶格常数逐渐变小。电阻率测量表明：Mg掺杂CaB₆单晶在温度区间2²7K内具有半导体的导电性质，而在温度区间27³00K表现为半金属的导电性质，且随着Mg掺杂量的增加，电阻率逐渐增大。采用了四引线法测量了Ca₀.94Mg₀.06B₆单晶的霍尔电阻率，Ca₀.94Mg₀.06B₆单晶的霍尔电阻率随着温度的升高而增大，霍尔测量结果表明Ca₀.94Mg₀.06B₆内的多数载流子是电子。Mg掺杂CaB₆单晶的热容随着温度的上升而增大，但小于纯CaB₆晶体的热容。德拜和爱因斯坦组合模型计算的Mg掺杂CaB₆单晶热容随温度的变化规律与实验结果吻合，且随着Mg含量的增加，其电子热容系数（γ）和德拜温度（TD）逐渐增大，爱因斯坦温度（T_E）略有增加。更多还原

【Abstract】 Alkaline-earth metal hexaborides are a class of important materials. They are wellknown for high melting temperature, high hardness, and low coefficient of thermalconductivity. These excellent properties make them widely used in various fields.In our work, we adopt solid-state reaction method to prepare the CaB₆and SrB₆polycrystals. Resistivity measurements showed that the CaB₆polycrystals obtained have atypical semi-metal electron conductive behavior. The Hall-coefficient measurementsshowed that the majority carriers in the obtained CaB₆polycrystals are electrons withcarrier density of6.82×10¹⁸electron cm-3at room temperature. The Heat Capacity of CaB₆polycrystals is well described by Debye and Einstein combined Model.High quality CaB₆and SrB₆single crystals were successfully synthesized under highpressure and temperature. They possesses cubic structure with a=4.159and4.1975,which has been characterized by XRD. The morphology of single crystals was observedby FESEM. Three kinds of shapes were observed: rods, plate-like rectangular blocks, andcubes. Based on the results of the studies, we analyse the mechanism of crystal growth.Resistivity measurements show that the CaB₆and SrB₆single crystals have a typical semi-metal electron conductive behavior. The first principles calculations confirm theexperimentally observed conducting behavior of the CaB₆and SrB₆single crystals. TheHall measurements showed that the majority carriers in the obtained CaB₆and SrB₆singlecrystals are electrons. The carrier densities are6.32×10¹⁸electron cm-3and7×10¹⁸electron cm-3at room temperature, respectively. The magnetization measurement showsthat CaB₆is paramagnetic and SrB₆is diamagnetic. The Heat Capacities of CaB₆and SrB₆are well described by Debye and Einstein combined Model.Mn-doped CaB₆single crystals were successfully obtained by means of high pressureand temperature technique. The crystal structure has been characterized by XRD.Compared with the undoped sample, the structure parameters of Mn-doped CaB₆singlecrystals decrease with the increase of the doping concentration. Magnetoresistivitymeasurements show that the samples transform from semi-metal into semiconductor withthe increase of the doping concentration. We measured the sample Hall coefficient using a four-wire method. The results show that the Mn-doped CaB₆single crystals are N-typesemiconductor. The magnetization measurement shows that the Mn-doped CaB₆sampleshave a weak ferromagnetic. The coercivity and saturation magnetic moment increase withthe increases of the doping concentration. The Heat Capacity of Mn-doped CaB₆is welldescribed by Debye and Einstein combined Model. The coefficient of electronic heatcapacity for normal state increases gradually with increases of the doping concentration,and the Debye temperature decreases gradually. The Einstein temperature is nearlyunchanged with the change of the doping concentrationMg-doped CaB₆single crystals were successfully prepared under high pressure andtemperature. The crystal structure parameters decrease with the increase of the Mg dopingconcentration. Resistivity measurements show that the Mg-doped samples aresemiconductor in the2²7K temperature range and have typical semi-metal electronconductive behavior in the27³00K temperature range. The resistivity increases graduallywith the increases of the Mg doping concentration. The Hall coefficient measurementsshow that the majority carriers in the Ca0.93Mg0.07B₆single crystals obtained is electrons.By means of Debye and Einstein combined Model, we well described the Heat Capacitiesof Mg-doped CaB₆. The results show that the coefficient of electronic heat capacity andthe Debye temperature increases gradually with increases of the Mg doping concentration,and the Einstein temperature are nearly unchanged.更多还原