溶胶-凝胶法制备La1-xSrxMnO3及其电磁性能研究

采用溶胶凝胶法,制备了La_1-xSr_xMnO₃(LSMO)纳米微粉。探究了Sr^₂₊的掺杂量对LSMO晶体结构、磁学性质、电磁特性和微波吸收性能的影响。结果表明,随Sr^₂₊含量的升高,样品的晶格常数和Mn-O-Mn键角增大,平均晶粒尺寸逐渐下降,样品出现从反铁磁性向铁磁性的转变,复介电常数呈先增大后减小的趋势。在2~18GHz内,x=0的样品在厚度为2mm时有最佳吸波效果,反射率小于-10dB对应的有效吸波频段为12.5~18GHz;Sr^₂₊的掺杂可使吸波频段有效的向低频移动,在X波段内,x=0.2的样品在厚度为2.3mm时的有效带宽达2.6GHz,证明LSMO是一种性能优异的介电损耗型吸波材料。     

La1-xKxMnO3的微波吸收特性

用溶胶-凝胶法制备La1-xKxMnO3粉晶,用X射线衍射仪和扫描电镜表征样品的晶体结构和微观形貌,用微波矢量网络分析仪测试了该样品在2~18 GHz微波频率范围的复介电常数和复磁导率,并计算损耗角正切及微波反射率,分析K掺杂量和样品厚度对体系微波吸收性能的影响及微波损耗机制。结果表明:晶体结构为钙钛矿型,颗粒形貌为不规则椭球状或短棒状;当样品厚度为2.40 mm、x=0.3时,吸收峰值为27.1 dB,10 dB以上有效吸收频带宽度达10.6 GHz。纳米La1-xKxMnO3兼具介电损耗和磁损耗,介电损耗相对较强。磁损耗因子和介电损耗因子随微波频率的变化相反,是基体中铁磁与反铁磁团簇在微波电磁场作用下相互转变引起。     

KS-B高吸收高发射率无机热控涂层低轨环境效应

目的 研究低轨环境对KS-B高吸收高发射无机热控涂层(简称KS-B涂层)性能的影响。方法 对KS-B涂层分别进行总剂量为3.9×10²² atoms/cm²的原子氧辐照、剂量为5000 ESH的真空-紫外辐照、总注量为9.30×10¹¹ p/cm²的真空-质子及总注量为1.108×10¹⁴ e/cm²的真空-电子综合辐照试验,分析空间模拟辐照环境中KS-B涂层太阳吸收比(αS)、半球发射率(εH)、表面形貌、表面组分含量、质量损失率等性能的变化规律,研究KS-B涂层的耐空间环境稳定性。结果 原子氧暴露后,KS-B涂层太阳吸收比增加0.003,半球发射率增加0.004;原子氧辐照后,KS-B涂层表面形貌出现了一定程度的糙化,表面Si、O元素含量下降。初始暴露时,KS-B涂层质量损失率逐渐提高,最终质量损失率为0.96%。真空-紫外辐照后,KS-B涂层太阳吸收比增加0.003,半球发射率无变化。真空-质子及真空-电子辐照前后,KS-B涂层太阳吸收比增加0.001,半球发射率无变化。结论 经历原子氧、真空-紫外、真空-质子及真空-电子模拟辐照后,KS-B涂层的太阳吸收比及半球发射率变化较小,具有较好的耐空间辐照性能,可以满足空间站等低轨航天器的长寿命服役需求。     

等离子喷涂(La0.4Sm0.5Yb0.1)2Zr2O7涂层的热性能研究

本文采用大气等离子喷涂制备了(La0.4Sm0.5Yb0.1)2Zr2O7涂层,并在相同的条件下制备了Sm2Zr2O7涂层作为对比。分别用扫描电子显微镜和X射线衍射仪分析 LSYZO涂层的微观形貌和相结构,并对两种涂层的热导率进行测定。结果表明,等离子喷涂后LSYZO涂层的相结构未发生变化。LSYZO涂层的结合强度为22 MPa,与SZO涂层相当。LSYZO涂层热震40次后发生失效,其热导率明显低于SZO涂层,这主要与LSYZO陶瓷自身的复杂结构有关。     

La2/3Ca1/3MnO3∶Ag0.4与La2/3Ba1/3MnO3∶Ag0.4的制备及其性能研究

采用化学共沉淀法合成制备了Ag掺杂摩尔百分比为0.4的La2/3Ca1/3MnO3∶Ag0.4 (LCMO∶Ag0.4)、La2/3Ba1/3MnO3∶Ag0.4(LBMO∶Ag0.4)多晶复合材料,通过XRD和R-T分别对两种材料的结构和性能进行测试分析.实验结果表明:所得样品均为正交晶系钙钛矿结构;LCMO∶Ag0.4的金属-绝缘转变温度(Tp=280.2 K)比LBMO∶Ag0.4要高(Tp=256.8 K);LCMO:Ag0.4样品的电阻值比LBMO∶Ag0.4要小两个数量级;LCMO∶Ag0.4的TCR最大值为28％,要远远高于LBMO∶Ag0.4的TCR最大值(0.5％).     

热喷涂纳米结构La_2Zr_2O_7(LZ)/8YSZ双陶瓷热障涂层

为了满足发动机以及涡轮机越来越高的性能要求,双陶瓷热障涂层逐渐取代单陶瓷层的8YSZ涂层,成为可以长期使用温度高于1 200℃的新型陶瓷涂层。采用等离子喷涂方法制备了La2Zr2O7(LZ)/8YSZ双陶瓷热障涂层,并同时制备了微米结构和纳米结构的单层8YSZ陶瓷涂层作为对比。通过X射线衍射仪和扫描电子显微镜研究了粉体喂料和涂层的组织结构。采用对偶拉伸试验法、水淬方法和日本工业标准等研究了涂层的结合强度、隔热效果、热震行为以及高温抗氧化行为。结果表明,与单层的8YSZ陶瓷层相比,双陶瓷型n-LZ/8YSZ涂层的隔热效果提高了35%,热震次数增加了一倍,热氧化失效时间延长了100多小时,具有较佳的隔热效果、抗热震性能以及抗高温氧化性能。     

Ni2Cr(BO3)O2涂层的制备及其红外发射性能

通过喷雾造粒和高温焙烧制备Ni_2Cr(BO_3)O_2粉末后利用等离子喷涂得到一种高红外发射涂层,并研究了该种涂层的红外发射性能。SEM观察涂层的表面、断面形貌,发现涂层与基体结合紧密、无脱落;XRD对焙烧后的粉末物相组成进行了表征,主要以Ni_2Cr(BO_3)O_2为主。对涂层红外波段发射率的测试表明,在0.76~2.5μm波段的发射率为0.896、2.5~14μm波段发射率为0.925,具有优异的红外发射性能。Ni_2Cr(BO_3)O_2晶胞内的畸变、非对称性以及电子转移跃迁是导致Ni_2Cr(BO_3)O_2这种材料具有高红外发射率的主要原因。Ni_2Cr(BO_3)O_2涂层能够经受37次"900℃~水冷"热震循环。该种涂层由于其高红外发射性能、优异的耐热震性能和热稳定性能而具有较高的实用价值。     

柠檬酸法制备热障涂层用La2Ce2O7粉体

以ZrO(NO3)2·2H2O、Ce(NO3)·6H2O和La2O3为原料,采用柠檬酸法制备了La2Ce2O7粉体,研究了pH值、乙二醇及柠檬酸用量、煅烧温度等工艺参数对La2Ce2O7粉体的影响。结果表明,采用该方法能够制备纯净的La2Ce2O7粉体,最佳实验条件是pH值为5,乙二醇、柠檬酸与金属离子的摩尔比分别是1.8和2,煅烧温度为400℃。     

Critical behavior of La0.87K0.13MnO3 manganite

The heat capacity of La_0.87K_0.13MnO₃ manganite is measured in temperature intervals from 80 to 350 K. A nature of the ferromagnetic phase transition and critical properties of the heat capacity near Curie temperature are studied. The principles of change in universal critical parameters near a phase transition point are determined. The critical exponents and heat capacity amplitude are calculated with account of correction to the scaling α = −0.13, А⁺/А⁻ = 1.178, and correspond to the critical behavior of 3D Heisenberg model. It is found a critical exponent of the correlation radius υ = 0.71, which is also conform to the Heisenberg universality class.     

The high temperature phase transitions of orthorhombic La0.95MnO3

The high temperature phase transitions of orthorhombic La_0.95MnO₃ from room temperature to 1273 K were studied using high temperature XRD technique in air. Two-phase transitions were found in temperature ranges, 373–473 K and 973–1273 K, respectively. The crystal symmetry increases from orthorhombic to rhombohedral then cubic, while the second transition was not completed in the observation of the present study. The first transition could be due to the depression of Jahn–Teller distortion, while the second one could be caused by the ionic oscillation at high temperature. The lattice thermal expansion coefficients of rhombohedral La_0.95MnO₃ from 473 to 973 K were also calculated using lattice parameters.     

Gd~(3+)/Ga~(3+)共掺杂石榴石型Yb_3Al_5O_(12)材料的制备及热物理性能

采用柠檬酸溶胶-凝胶法制备了Gd~(3+)/Ga~(3+)双掺杂的石榴石型Yb_(3-x)Gd_xAl_(5-x)Ga_xO_(12)固溶体陶瓷材料(x=0,0.1,0.2,0.3,0.4,0.5;YGAGO)。结果表明:Gd~(3+)/Ga~(3+)共掺杂的YGAGO保持了石榴石相结构;随着Gd~(3+)/Ga~(3+)掺杂量的增加,固溶体样品的衍射峰位整体向低角度偏移;SEM形貌表明Yb_3Al_5O_(12)材料的微观形貌由纳米级晶粒和高密度晶界构成;大离子Gd~(3+)与Ga~(3+)分别对Yb_3Al_5O_(12)晶体结构中Yb~(3+)与Al~(3+)晶格位的取代,不仅因引入的点缺陷明显降低了材料的热导率,同时还造成石榴石型Yb_3Al_5O_(12)晶体结构的松弛,利于热膨胀系数的提高。随着Gd~(3+)/Ga~(3+)掺杂量的增加,晶体中点缺陷浓度不断升高,声子平均自由程不断减小,使得YGAGO的热导率在x=0.5时达到最低值(λ=1.67W/(m·K),T=1273 K),热膨胀系数达到最高值(α_1=11.71×10~(-6)K~(-1),T=1273 K)。     

Magnetic held dependence of elastic modulus and resistance in La2/3Ca1/3MnO3

We have measured the magnetic field dependence of the anelastic modulus and the electric resistance of a ceramic sample of the magnetoresistant perovskite La_2/3Ca_1/3MnO₃ around the metal–insulator transition by the vibrating reed technique (2.5 KHz). Previous work [1] showed that the modulus becomes higher while the internal friction has a peak at the transition temperature (T_c=262 K). In this work, improvements made on the equipment allowed us to measure at constant deformations (<10⁻⁵) and magnetic fields up to 4500 Gauss. We made isothermal measurements of internal friction, modulus and resistance as a function of an applied magnetic field. We found that most of the changes induced by the magnetic field take place in a few degrees (almost 5 K) near the transition temperature where the changes in resistance are more important. Up to the highest magnetic field applied, we found 3% maximum variations of the modulus and no hysteresis while cycling the magnetic field. We suppose that the experiment is placed in the linear response of the inverse Wiedemann effect [2], due to the small deformations used, and that the ferromagnetic domain structure is responsible for the observed effects. Some additional measurements are needed (magnetic hysteresis loops) to be compared with our results.     

Elastic behavior of La0.67Ca0.33MnO3:ZrO2 composites

A series of Colossal Magneto Resistance materials, with compositional formula (1 − x) La_0.67Ca_0.33MnO₃ + xZrO₂ (where x = 0%, 10%, 20%, 40%, 60%, 80%) were prepared by sol–gel technique. When characterized structurally by X-ray diffraction they are found to have cubic structure. After measuring their bulk densities, the ultra sonic longitudinal (V_l) and shear velocities (V_s) were measured at room temperature using the pulse transmission technique. Using the ultrasonic data, the values of Young's and rigidity moduli along with Poisson's ratio and Debye temperatures have been calculated. As the materials are porous, zero porous elastic moduli have also been arrived at using a well-known model. The observed variation of elastic moduli with varying ZrO₂ concentration has been explained qualitatively.     

不同Ce掺杂比例对La1-xCexMnO3薄膜输运性质及机理研究

本文中研究了电子型掺杂钙钛矿薄膜La_1-xCe_xMnO₃的输运性质和外场作用下的输运机理。研究表明,La_1-xCe_xMnO₃薄膜呈现出典型的金属-绝缘体转变,且与Ce的掺杂浓度相关。电阻温度曲线表明,在低温时,电子-电子散射和磁畴对电子的散射是电阻形成的主要原因,而在高温下小极化子的跳跃机制起主要贡献。通过激光照射样品表面,发现光场诱导金属绝缘体转变温度向着低温区间偏移,该现象产生的原因在于La_1-xCe_xMnO₃薄膜内部铁磁相与顺磁相的共存,此外,高能量的激光对样品的电阻变化影响更明显。进一步研究表明,Ce的掺杂浓度将会通过金属-绝缘相变对La_1-xCe_xMnO₃的磁电阻效应产生显著的调制作用。本文将为新型能量转换器件的开发与应用提供新的研究思路。     

(Gd0.8Ca0.2)2Ce2O6.6陶瓷的制备及热导率

以Gd2O3、CeO2和CaO为原料,采用固相反应法制备(Gd0.8Ca0.2)2Ce2O6.6陶瓷材料。用X射线衍射(XRD)技术和扫描电镜(SEM)研究样品的相组成和微观组织,用激光脉冲法测试样品的热扩散系数。结果表明,所制备的样品具有单一的萤石结构,组织致密且晶界清晰,其在800℃的热导率仅为YSZ陶瓷的89%,较低热导率表明该材料有潜力用作新型热障涂层用陶瓷材料。     

Size effects and interactions in La0.7Ca0.3MnO3 nanoparticles

La_0.7Ca_0.3MnO₃ (LCMO) nanoparticles with average diameter of 16-73 nm were prepared by reactive milling and thermal processing methods. Interaction and size effects on the magnetic properties of the LCMO nanoparticle samples were investigated. Phenomena related to the interparticle interaction, such as an un-overlapping of the M(H_ext,T)/M_S vs. H_ext/T scaling plots and a Curie-Weiss rather than Curie law behavior of the dc susceptibility at high temperatures were analyzed. The magnetization curves of interacting nanoparticles were well described by using the mean-field approximation. The dependence of the blocking temperature T_B on the strength of the interactions, magnetic anisotropy, as well as the thermal dependence of magnetization deviates from the expected Bloch law was also estimated.     

Effects of ferrites on magnetic transport properties in La2/3Sr1/3MnO3/ferrites composites

Effects of soft-magnetic MnZn ferrite (Mn0.5Zn0.5Fe2O4, MZF) and hard-magnetic Ba ferrite (BaO·6Fe2O3, BaM) on the structure and magnetic transport properties of [La2/3Sr1/3MnO3] (LSMO)/(x) [ferrites] (ferrites=MZF, BaM) composites have been investigated. It was found that the inclusion of MZF phase reduces magnetization and ferromagnetic-paramagnetic transition temperature (Tc) of the composites. With increasing the content of the dopants, the high-temperature magnetoresistance (MR) decreases, whereas low-...     

铝合金表面ZnO/Y2O3/Al2O3微弧氧化涂层的制备及其热控性能研究

目的 改善铝合金表面热控性能。方法 通过微弧氧化技术,采用六水合硝酸钇(Y(NO3)3.6H2O)为稀土改性剂,在铝合金表面原位构筑ZnO/Y2O3/Al2O3微弧氧化涂层。利用X射线衍射仪、X射线光电子能谱仪和扫描电子显微镜,分析涂层晶体结构、化学组成和表面形貌;利用红外发射率仪和紫外/可见/近红外分光光度计分别测量涂层的发射率与太阳吸收率,进而探究涂层热控性能及其影响因素。结果 实验表明,钇源的用量能够显著影响涂层结构和热控性能,当钇源质量浓度为6 g/L时,所得ZnO/Y2O3/Al2O3涂层的红外发射率为0.859,太阳吸收率为0.405,具有最低吸辐比0.471,显示出优异的热控性能。结论 原位构筑稀土元素Y改性的微弧氧化涂层可有效提高铝合金表面的热控性能。这得益于微弧氧化过程中所生成的Y2O3,它可有效调控涂层的形貌、粗糙度和厚度。此外,由于Y2O3具有较大的禁带宽度,可降低微弧氧化涂层中ZnO在高能紫外光波段的吸收。     

Studies on La0.67Ca0.33MnO3-SrTiO3 composites using two-phase model

In this study we report the structural, electrical and magnetic properties of (1 − x)La_0.67Ca_0.33MnO₃ (LCMO)-(x)SrTiO₃(STO) composites. For this series we have observed a minute change in ferromagnetic (FM)-paramagnetic (PM) transition temperature with STO addition in LCMO matrix; however a reasonable change is observed in metal-insulator transition temperature, along with the occurrence of percolation threshold for x = 0.30 sample. Overall pattern for temperature dependence of resistivity for this series has been best-fitted using the formula 1/ρ = (1 − f)/ρ_PM + (f/ρ_FM), whereρ_PM and ρ_FM are the resistivities of the PM and FM contents in the sample and f is the volume fraction of FM phase in the sample. Investigations on magnetoresistance (MR) using magnetic field up to 3 T show enhancement of extrinsic MR in the composite samples which can be viewed in the light of spin polarized tunneling.     

微弧氧化技术在热控涂层中的应用

热控涂层在航空、航天及其他许多领域有着广泛的应用。介绍了热控涂层的工作原理和热控涂层类型,着重论述了微弧氧化技术在钛、镁、铝合金表面制备热控涂层的研究现状,目前高吸收发射比的钛、镁、铝合金热控涂层的最大吸收率分别可以达到0.96、0.94、0.90,最大发射率分别可以达到0.95、0.87、0.90;低吸收发射比的钛、镁、镁锂合金涂层的吸收率分别可以达到0.237、0.35、0.33,发射率分别可以达到0.99、0.88、0.85。此外,还分析了涂层的组成结构和形貌,以及微弧氧化工艺条件对热控性能的影响。微弧氧化热控涂层的组成结构和形貌特征可以通过电解液配方和工艺参数调整来进行调控。适当延长反应时间、增加电流密度,涂层厚度增加、粗糙度变大,高吸收发射比涂层的吸收率和发射率升高,低吸收发射比涂层的吸收率降低、发射率升高。电解液中添加阴、阳离子或纳米/微米颗粒,或调控不同组分在微弧氧化涂层中的分布,对于改善涂层的吸收率和发射率具有重要作用。最后,从微弧氧化热控涂层的综合性能、实际应用环境、复合技术应用以及开发微弧氧化智能热控涂层四个方面进行了展望。