粗晶和纳米晶Sm_3Co合金的制备及其性能研究

本文针对Sm3Co粗晶和纳米晶合金材料的制备和基础性能进行了研究.采用磁悬浮熔炼技术多次精炼制备出Sm3Co粗晶合金.以此为母材,利用高能球磨非晶化和放电等离子烧结致密化并同步晶化的技术路线,制备出平均晶粒尺寸为8 nm的超细纳米晶Sm3Co合金块体材料.构建了Sm3Co纳米晶合金的晶体结构模型,并结合其显微组织的表征,分析了Sm3Co纳米晶合金的磁性能和力学性能,并与粗晶合金进行了比较粗晶Sm3Co合金不具有硬磁特性,而同种成分的纳米晶合金则表现出一定的硬磁特性.纳米晶Sm3Co合金的显微硬度和弹性模量分别达到4.87 GPa和63.7 GPa,比粗晶合金增大约8.7%和13.3%.本文研究结果为Sm-Co体系合金的基础性能及其纳米尺度效应提供了系统的参考依据.     

Spin-polarized tunneling in nanostructured La2/3Sr1/3MnO3

We present magnetic and transport properties of nanocrystalline La_2/3Sr_1/3MnO₃ powders prepared by a gel-combustion method using citric acid as the fuel. The coercive magnetic field H_c is significantly different to the field H_c^* for which the magnetoresistance (MR) is maximum. The MR at low fields (LFMR) exhibits a power-law dependence with magnetization, MR∝Mⁿ, with n=2.5–3.3 for temperatures ranging from 5 to 200 K. The results are discussed in terms of a distribution of particle size in our sample.     

Magnetic resonance in crystalline La0.9Ca0.1MnO3: Comparative study of bulk and nanometer-sized samples

X-band electron magnetic resonance (EMR) measurements were done at 115?T?600 K on bulk and nanometer size-grain powder single-crystalline samples of La_0.9Ca_0.1MnO₃, in order to study an impact of structural inhomogeneity on magnetic ordering. For the nano-crystal sample, two superimposed EMR lines are observed below 240 K, while for bulk-crystal one, a second line emerges in narrow temperature interval below 130 K. Temperature dependences of resonance field and line width of the main and the secondary line are drastically different. EMR data and complementary magnetic measurements of bulk-crystal sample reveal mixed-magnetic phase, which agrees with the published phase diagram of bulk La_1−xCa_xMnO₃. In a marked contrast, the same analysis for nano-crystal sample shows two phases one of which is definitely ferromagnetic (FM) and other is likely such, or super paramagnetic. The data obtained are interpreted in terms of very different magnetic ground states in the two samples, that is attributed to different randomness of the indirect FM exchange interactions mediated by bound holes.     

氧离子导体La1.9Y0.1Mo2O9细晶粒陶瓷的制备和电学性质研究

采用溶胶凝胶法合成的La_1.9Y_0.1Mo₂O₉纳米晶粉体, 结合微波烧结技术制备出不同晶粒度的La_1.9Y_0.1Mo₂O₉块体样品. 利用X射线衍射仪(XRD)、高分辨透射显微镜(HRTEM)、场扫描显微镜(SEM)对粉体及陶瓷块体的物相、 形貌进行了表征, 利用交流阻抗谱仪测试了样品不同温度下的电导率. 实验结果表明, 掺Y的La_1.9Y_0.1Mo₂O₉能将高温立方β 相稳定到室温; 块体样品致密均匀, 平均晶粒度范围在60 nm–4 μm之间; 致密度高的样品表现出高的电导率, 其中900 ℃烧结样品的电导率600 ℃时高达0.026 S/cm, 比固相反应法制备的La_1.9Y_0.1Mo₂O₉样品高出约1倍. 总结认为样品的致密性对电导率影响较大, 是通过影响晶界电导率来影响总电导率的, 样品的晶粒度(在60 nm–4 μm范围内)对电导率的影响还不能确定. 关键词： 氧离子导体 1.9Y_0.1Mo₂O₉')" href="#">La_1.9Y_0.1Mo₂O₉ 细晶粒陶瓷 微波烧结     

Magnetic structure and properties of a bulk Fe72Al5P10Ga2C6B4Si1 alloy in the amorphous and nanocrystalline states

The structure forming under controlled crystallization of a bulk Fe₇₂Al₅P₁₀Ga₂C₆B₄Si₁ amorphous alloy has been studied using differential scanning calorimetry, transmission electron microscopy, and x-ray diffraction. Crystallization of the alloy was established to result in the formation of a nanocrystalline structure consisting of three phases. The domain structure and magnetic properties of amorphous and nanocrystalline samples were investigated using the magnetooptic indicating film technique (MOIF) and a vibrating-sample magnetometer. The coercive force and the saturation magnetization of the amorphous samples were found to be 1 Oe and 130 emu/g, respectively. It was shown that the formation of the nanocrystalline structure entails a dramatic decrease in domain size (down to 1–4 μm) as compared to an amorphous sample (∼1 mm). Simultaneously, a decrease in the saturation magnetization and a strong increase in the coercive force of the samples were observed. __________ Translated from Fizika Tverdogo Tela, Vol. 46, No. 5, 2004, pp. 858–863. Original Russian Text Copyright ? 2004 by Abrosimova, Aronin, Kabanov, Matveev, Molokanov.     

钙钛矿型锰氧化物(La$lt;sub$gt;0.8$lt;/sub$gt;Eu$lt;sub$gt;0.2$lt;/sub$gt;)$lt;sub$gt;4/3$lt;/sub$gt;Sr$lt;sub$gt;5/3$lt;/sub$gt;Mn$lt;sub$gt;2$lt;/sub$gt;O$lt;sub$gt;7$lt;/sub$gt;的磁性和电性研究

采用传统固相反应法制备钙钛矿型锰氧化物 (La_0.8Eu_0.2)_4/3Sr_5/3Mn₂O₇多晶样品, X-射线衍射分析表明, 样品(La_0.8Eu_0.2)_4/3Sr_5/3Mn₂O₇结构呈现良好的单相. 通过磁化强度随温度的变化曲线(M-T)、不同温度下磁化强度随磁场的变化曲线(M-H)和电子自旋共振谱发现: 在300 K以下, 随着温度的降低, 样品先后经历了二维短程铁磁有序转变 (T_C^2D ≈ 282 K)、三维长程铁磁有序转变(T_C^3D ≈ 259 K)、奈尔转变(T_N ≈ 208K)和电荷有序转变(T_CO ≈ 35 K); 样品 (La_0.8Eu_0.2)_4/3Sr_5/3Mn₂O₇在T_N以下, 主要处于反铁磁态; 在T_C^3D达到370 K时, 样品处于铁磁-顺磁共存态, 在370 K以上时样品进入顺磁态. 此外, 分析电阻率随温度的变化曲线(ρ-T)得到: 样品在金属-绝缘转变温度(T_P ≈ 80 K)附近出现最大磁电阻值, 其位置远离T_C^3D, 表现出非本征磁电阻现象, 其磁电阻值约为61%. 在T_CO以下, 电阻率出现明显增长, 这是由于温度下降使原本在高温部分巡游的e_g电子开始自发局域化增强所致. 通过对 (La_0.8Eu_0.2)_4/3Sr_5/3Mn₂O₇的ρ-T 曲线拟合, 发现样品在高温部分的导电方式基本遵循小极化子的导电方式. 关键词： 磁性 电性 金属-绝缘转变温度 电子自旋共振     

Tb掺杂双层锰氧化物La_(4/3)Sr_(5/3)Mn_2O_7的磁熵变和电输运性质

研究了Tb掺杂对双层锰氧化物La_(4/3)Sr_(5/3)Mn_2O_7磁熵变和电输运性质的影响.样品采用传统固相反应法制备,两样品的名义组分可以表示为(La_(1-x)Tb_x)_(4/3)Sr_(5/3)Mn_2O_7(x=0,0.025),磁场为7 T时的最大磁熵变?S_M分别为-4.60 J/(kg·K)和-4.18 J/(kg·K).比较后发现,Tb元素的掺杂使得最大磁熵变值减小,但同时增大了相对制冷温区.电性测量结果表明,x=0.025的样品在高温区的导电机制可以用小极化子模型解释,与母体三维变程跳跃模型不同;当温度降低至三维长程铁磁有序温度(T_c~(3D))附近时,掺杂样品发生金属绝缘相变;掺杂后样品在T_c~(3D)附近,磁电阻取得极大值(约为56%),表明是本征磁电阻效应.     

Ni2O3掺杂对新固相源顶部籽晶熔渗生长法制备单畴GdBCO超导块材超导性能的影响

本文通过在新固相源中添加Ni₂O₃的方法, 采用顶部籽晶熔渗生长工艺(TSIG)制备出组分为(1-x) (Gd₂O₃+1.2BaCuO₂)+x Ni₂O₃、直径为20 mm的单畴GdBCO 超导块材(其中x = 0, 0.02, 0.06, 0.10, 0.14, 0.18, 0.30, 0.50 wt%), 并研究了Ni₂O₃的掺杂量x对样品的表面生长形貌、微观结构、临界温度T_c、磁悬浮力以及俘获磁通密度的影响. 研究结果表明, 当Ni₂O₃的掺杂量x在0–0.50 wt%的范围内时, 均可制备出单畴性良好的样品, 且Ni₂O₃的掺杂对样品中Gd211粒子的分布和粒径没有明显的影响. 在Ni₂O₃的掺杂量x从0增加到0.50 wt%的过程中, 样品的临界温度T_c呈现下降的趋势, 从x=0时的92.5 K下降到x=0.50 wt%时的86.5 K, 这是由于Ni^{3 +}替代GdBCO晶体中Cu^{2 +}所致; 样品磁悬浮力和俘获磁通密度均呈现先增大后减小的变化规律, x=0.14 wt%时, 磁悬浮力达到最大值34.2 N, x=0.10 wt%时, 俘获磁通密度达到最大值0.354 T. 样品磁悬浮力和俘获磁通密度的变化规律与Ni₂O₃的掺杂量x有密切关系, 只有当掺杂量x合适时, Ni³⁺对Cu^{2 +}的替代既不会造成T_c的明显下降, 但又能产生适量的Ni^{3 +}/Cu²⁺ 晶格畸变, 从而达到提高样品磁通钉扎能力和超导性能的效果.     

Dilatometry of the high-temperature proton conductor Ba3Ca1.18Nb1.82O9 − δ

Firstly, the production of solid bulk specimens of the proton conductor Ba₃Ca_1.18Nb_1.82O_{9 − gd} (BCN18) of known water content up to [H^.] = n_H/n_Ba = 0.16 is described. Secondly, measuring the length change of such samples versus water content [H^.] demonstrated that the sample lengths increased linearly with a slope of (Δl/l)/[H^.] = (2.13 ±0.07) × 10⁻². Thirdly, the density of bulk samples was found to decrease linearly with water content [H^.]. This decrease was in good agreement with the above values derived from the length change. Fourthly, high temperature dilatometry showed that samples reach the thermodynamic solubility values in water vapor atmospheres only at temperatures above 700 °C. Two time scales were observed for the time-dependence of the elongation upon exposure to water vapor. A fast process occurred within minutes above 700 °C, a slow one took several hours. The fast one was identified as the chemical diffusion of the diffusion pair H^. and V_o^. which is required for the water uptake and loss of BCN18. The chemical diffusivity of water is described by the parameters D₀ = (0.022 ± 0.002) cm²/s and ΔH* = (0.79 ± 0.05) eV.     

晶粒细化对MgB_2超导临界电流密度的作用

在多晶系统的MgB2超导体中存在晶粒间较小的整体电流和晶粒内大的局域电流.用改变升温速率的方法制备了不同晶粒大小和晶粒连接性的MgB2样品,并对它们的晶粒大小进行了统计.采用一种测量超导临界电流密度的Campbell法,分别测量和计算得到了它们的整体电流和局域电流密度.研究表明:长时间的烧结造成晶粒变大,材料有较大的整体临界电流密度,而短时间烧结的样品则相反;同时发现晶粒细化只提高了样品的局域临界电流密度,而且样品内部缺陷、杂质及晶界等因素是影响MgB2超导体整体电流传输的主要因素.     

钙钛矿锰氧化物(La_(1-x)Eu_x)_(4/3)Sr_(5/3)Mn_2O_7(x=0,0.15)的磁性和电性研究

通过传统固相反应法制备了钙钛矿锰氧化物(La1-xEux)4/3Sr5/3Mn2O7(x=0,0.15)多晶样品,并且对其磁性和电性进行了研究.磁性测量表明:随着温度的降低,样品经历了一个复杂的转变过程,在温度为T*时经历二维短程铁磁有序转变,在温度为TC时进入三维长程铁磁态.随着Eu的掺杂,T*和TC减小,并且样品(La0.85Eu0.15)4/3Sr5/3Mn2O7在低温区表现出自旋玻璃行为.电性质测量表明:在母体La4/3Sr5/3Mn2O7中La位掺杂Eu后电阻率明显变大,金属绝缘转变温度TMI降低,磁电阻峰值增大.这些影响归因于较小的Eu3+离子替代La3+离子导致平均离子半径减小,晶格发生畸变.此外,较小的Eu3+离子优先占据层间岩盐层的R-site,使La3+,Sr3+,Eu3+离子在(La0.85Eu0.15)4/3Sr5/3Mn2O7中的分布更加有序,所以x=0.15的样品的ρ-T曲线只有一个峰.     

钙钛矿锰氧化物(La1-xGdx)4/3Sr5/3Mn2O7 (x=0, 0.025) 磁性和输运性质研究

采用传统的高温固相烧结法制备了双层钙钛矿锰氧化物(La_1-xGd_x)_4/3Sr_5/3Mn₂ O₇ (x=0, 0.025)多晶样品. 通过X射线衍射仪研究发现样品为Sr₃Ti₂O₇型四方结构, 空间群为I4/mmm; 磁性测量表明, Gd³⁺掺杂后的样品(La_0.975Gd_0.025)_4/3Sr_5/3Mn₂O₇的三维磁有序转变温度(T_C1^3D)、磁化强度(M)均降低, 这是由于Gd³⁺的掺杂引起晶格的畸变, 从而使得晶格常数发生改变, 减弱了铁磁耦合而导致的; 通过电子自旋共振谱测量发现, 在T_C^3D<T<300 K温度范围内, 两样品在顺磁的基体上均有短程的铁磁团簇存在, 出现了相分离现象. 电性测量表明: 两样品分别在T_C1^3D (La_4/3Sr_5/3Mn₂O₇ 样品的三维磁有序转变温度, T_C0^3D)<T<300 K温度范围内均以三维变程跳跃的方式导电, 分析得出Gd³⁺的掺杂使得载流子局域长度的减小. 这表明载流子需要吸收更多的能量才能克服晶格的束缚进行跳跃, 因此(La_0.975Gd_0.025)_4/3Sr_5/3Mn₂ O₇ 样品的电阻较高.     

Intrinsic magnetic properties of In2O3 and transition metal-doped-In2O3

This paper reports on the influence of the sintering temperature and atmosphere and transition-metal doping on the magnetic properties of nanocrystalline and bulk In₂O₃. Undoped nanocrystalline In₂O₃ is diamagnetic whatever the sintering temperature and atmosphere. All single-phase transition-metal-doped In₂O₃ samples are paramagnetic, with a paramagnetic effective moment originating from weakly interacting transition metal ions. No trace of ferromagnetism has been detected even with samples sintered under argon, except extrinsic ferromagnetism for samples with magnetic dopant concentrations exceeding the solubility limit.     

Harmonic generation in TlBa2Ca3Cu4Oy superconductor

We have studied harmonic generation in the magnetic response of bulk samples of the ceramic superconductor TlBa₂Ca₃Cu₄O_y. Dependence of the odd and even harmonics of magnetization on the DC magnetic field, amplitude of the AC magnetic field and temperature was investigate experimentally. We have used a critical state model recently developed by Müller et al. to calculate the full harmonic response. At fixed temperature, the dependence of the odd order harmonic response on the amplitude of the AC magnetic field H_m shows an unambiguous crossover from an H²_m behaviour (Bean-like) at small amplitudes to an H³_m behaviour for larger H_m, in quantitative agreement with the modified critical state model.     

Effect of thermal treatment on the microstructure and magnetic properties of a bulk amorphous Fe72Al5P10Ga2C6B4Si1 alloy

The structure and chemical composition of the phases that form in the controlled crystallization of a bulk amorphous Fe₇₂Al₅P₁₀Ga₂C₆B₄Si₁ alloy are studied by differential scanning calorimetry, transmission electron microscopy, and x-ray diffraction. It was established that, when the alloy is annealed at a temperature above the glass transition point, a nanocrystalline structure consisting of three phases arises. The magnetic properties of amorphous and nanocrystalline samples were studied with a vibrating-sample magnetometer. The coercive force and saturation magnetization of amorphous samples were found to be 1 Oe and 130 emu/g, respectively. The structure and chemical composition of the forming phases and their correlation with the magnetic properties of the samples were determined. __________ Translated from Fizika Tverdogo Tela, Vol. 46, No. 12, 2004, pp. 2158–2163. Original Russian Text Copyright ? 2004 by Abrosimova, Aronin, Kabanov, Matveev, Molokanov, Rybchenko.     

The flux jumps in high TcBi1.7Pb0.3Sr2Ca2Cu3Oy bulk superconductor

There was giant flux jumps in high Tc Bi_1.7Pb_0.3Sr₂Ca₂Cu₃O_y bulk superconductor. The relaxation time, τ, decreased with both the increase of magnetic field and the rise of temperature. The maximum τ was about 40 min.. The average-dM/dt increased with both the increase of magnetic field and the rise of temperature. The minimum average-dM/dt was about 4.1×10⁻²G/min.. The flux jump weakened with time. It was dependent on the decrease of gradient of magnetic flux density dn/dx in the sample.     

预退火时间对Fe_(80.8)B_(10)P_8Cu_(1.2)非晶合金微结构及磁性能的影响

研究了预退火时间对Fe_(80.8)B_(10)P_8Cu_(1.2)非晶合金微结构及磁性能的影响.穆斯堡尔谱研究表明:在660 K的预退火温度下,随着预退火时间的增加,Fe原子不断富集,非晶基体中的类Fe_3B化学短程有序结构向类Fe B结构转变,并且非晶基体中Fe第一近邻壳层中Cu原子的逐渐脱离以及Fe-P配位键数量的明显减少可间接表征CuP团簇的形成过程.同时,本研究通过调节预退火时间来调控非晶基体中CuP团簇和Fe团簇的数量,促进后续退火晶化过程中α-Fe纳米晶相的析出,并细化纳米晶尺寸,从而获得综合磁性能更加优异的非晶/纳米晶软磁合金.     

Magnetocaloric and magnetoresistance properties of La2/3Sr1/3Mn1−xCoxO3 compounds

Structural, magnetic, magnetoresistance and magnetocaloric studies on La_2/3Sr_1/3Mn_1−xCo_xO₃ compounds were reported. The samples were prepared by the conventional ceramic method. X-ray analysis showed the presence of one phase only, in all studied samples. From electrical resistance measurements it was found that the samples show large negative magnetoresistance behavior. The magnetic measurements were performed in a large temperature range, 4.2–750 K and external magnetic fields up to 5 T. The adiabatic magnetic entropy changes, |ΔS|, were determined from magnetization data. Large magnetocaloric effect (MCE) has been obtained in all studied samples.     

高压烧结法制备Bi2Te3纳米晶块体热电性能的研究

用高压烧结法对水热法制备的Bi₂Te₃纳米线及纳米颗粒粉体进行了压制成型, 并与真空热压法制备的样品进行了形貌和热电性能的比较. 研究表明, 高压烧结样品内的晶粒尺寸明显小于热压样品. 热电性能的研究表明, 高压烧结样品的电阻率、赛贝克系数和热导率均优于真空热压样品. 由纳米线粉体高压烧结的样品其热电优值ZT 在室温时达到了0.5, 高于真空热压样品的值, 表明高压烧结是热电材料纳米粉体成型的一种有效方法.     

Annealing dependence of magnetic properties in nanostructured Sm0.5Y0.5Co5

Nanocrystalline Sm_0.5Y_0.5Co₅ powders with high coercivity H_C and enhanced remanence M_r were prepared by mechanical milling and subsequent annealing. Annealing temperatures T ranging from 973 to 1173 K, and times t ranging from 1 to 5 min were used. X-ray diffraction (XRD) and DC-magnetization measurements were carried out to study the microstructure and magnetic properties of these samples. XRD patterns demonstrate that the average grain size D of the nanocrystalline powders depends on the annealing temperature T and time t: D ranges from 11 nm (for T=973 K and t=1 min) to 93 nm (for T=1173 K and t=5 min). Magnetic measurements performed at room temperature indicate high coercivity values (H_C>955 kA/m), and enhanced remanence (M_r/M_max>0.5) for all samples. A strong annealing-induced grain size dependence of these magnetic properties was found.