双层锰氧化物Nd1.2Sr1.8Mn2O7的相分离研究

通过固相反应法制备成了多晶样品Nd1.2Sr1.8Mn2O7,利用XRD和SQUID研究了样品的结构和磁性.研究结果表明,该样品的单相性很好,单相结构为Sr3Ti2O7四角形结构(14/mmm).磁性测量表明,温度高于居里温度(Tc=265K),样品呈现出顺磁性,随着温度的降低出现了顺磁-铁磁的转变,铁磁与反铁磁两相共存,温度低于50 K时铁磁逐渐消失,直到完全变为反铁磁.     

多层锰氧化物Ba_(6-x)Sr_xMn_5O_(16)(x=0,0.5,1.0,1.5)材料的制备和磁性研究

通过传统的固相反应法制备了多层钙钛矿锰氧化物Ba_(6-x)Sr_xMn_5O_(16)(x=0,0.5,1.0,1.5)系列单相样品,比较详细地分析了样品的结构和磁性变化规律。通过室温X射线衍射(XRD)测试,样品为纯的单相n=5的Ruddlesden-Popper型钙钛矿结构,具有四方结构的Cmca(No.64)空间群。采用超导量子磁强计(SQUID)对该系列样品的磁性进行了研究。通过采用超导量子磁强计(SQUID)测量样品Ba6Mn5O16的磁化强度随温度变化曲线得出:由于低温强的三维反铁磁耦合,当温度低于75 K时,零场冷却(ZFC)和带场冷却(FC)曲线几乎重合。当温度达到奈尔温度TN(≈150 K)时,三维的反铁磁耦合完全被破坏,曲线的特征有一个明显的大鼓包。在温度低于50 K时,观察到居里尾(Curie tail)。随着Sr离子替代Ba离子量的增加,TN逐渐降低。样品在TN附近有很宽的转化峰,很可能存在三维反铁磁耦合到二维反铁磁耦合再到顺磁的连续变化过程。     

钙钛矿锰氧化物Nd_(1.2-x)Tb_xSr_(1.8)Mn_2O_7(x=0,0.10,0.15)的类Griffiths相研究

采用固相反应法制备了Nd_(1.2-x)Tb_xSr_(1.8)Mn_2O_7(x=0,0.10,0.15)系列多晶样品,通过样品的XRD谱线,磁化强度随温度、外加磁场的变化曲线(M-T、M-H)等研究样品的晶体结构和磁性。研究结果表明,随掺杂量的增加,样品的结构没有发生明显变化,均为单相的Sr_3Ti_2O_7型四方结构,空间群为I4/mmm。在测量温度范围内,随温度的降低,三个样品分别先后经历了顺磁态、铁磁-顺磁共存态和反铁磁-铁磁共存态的转变。三样品体系中均存在类Griffiths相,在类Griffiths相温度(T_G≈275 K)以上,样品处于顺磁态;在类Griffiths相温区(θT275 K,其中θ为居里温度)范围内,样品处于铁磁-顺磁共存态;在奈尔温度(T_N)以下,样品铁磁性减弱,反铁磁性增强,铁磁和反铁磁性相互竞争导致类自旋玻璃行为出现。此外Tb元素的掺杂对系统中类Griffiths相温度基本没有影响,但对类Griffiths相温区有一定影响。     

钙钛矿锰氧化物La_(1.1)Dy_(0.1)Sr_(1.8)Mn_2O_7样品的磁性研究

采用传统固相反应法制备La1.1Dy0.1Sr1.8Mn2O7多晶样品,并通过测量样品的磁化强度随温度变化曲线(M-T)以及磁化强度随外加磁场的变化曲线(M-H)对样品的磁性进行了研究。结果表明,在整个温度测量范围内,随温度的降低,样品的磁性发生了多次转变,在类Griffiths相温度(TG≈350 K)以上,样品处于纯顺磁态;在三维铁磁有序温度T3D(≈187 K)至TG温度范围内,样品处于铁磁-顺磁共存态;在奈尔温度TN(≈165 K)至T3D温度范围内,样品的铁磁性随着温度的降低逐渐增强;在TN温度以下,样品的反铁磁性增强而铁磁性减弱,表现出了反铁磁和铁磁相互竞争的现象。另外,通过居里外斯拟合以及Griffiths相模型拟合,发现样品在TG以下存在类Griffiths相。     

Pr_(0.4)Ca_(0.6)Mn_(1-x)Cr_xO_3的磁性质及电荷有序相

用固相反应法制备了Pr0.4Ca0.6Mn1-xCrxO3(x=0,0.04,0.06,0.08,0.10,0.12),通过X射线衍射(XRD)图谱,磁化强度-温度(M-T)曲线、电子自旋共振(ESR)图谱,并用ESR作出归—化强度-温度(I/I300—T)曲线、谱线宽度-温度(△Bpp-T)曲线,研究Cr3+替代Mn3+对Pr0.4Ca0.6MnO3的磁性质及电荷有序相的影响。结果表明,母体样品Pr0.4Ca0.6MnO3的电荷有序转变温度Tco=267 K,205K~62 K温区是长程反铁磁序,50 K以下在反铁磁背景下出现少量铁磁成分;Cr替代量x=0.06时,电荷有序相已基本被破坏,随温度降低材料从顺磁向反铁磁转变,同时在反铁磁背底下存在铁成分;Cr替代量达到x=0.10时,电荷有序相完全被破坏,250 K以下是反铁磁与铁磁混合相,铁磁成分增多。用有磁性的且与Mn4+有相同电子结构(t32ge0g)的Cr3+替代Mn3+破坏电荷有序相的机制是:Cr3+替代Mn3+引起自旋序的改变从而引起电荷序的破坏,说明在CE型反铁磁体系中,自旋序与电荷序之间存在强耦合相互作用。     

双层钙钛矿锰氧化物Nd_(1.15)Tb_(0.05)Sr_(1.8)Mn_2O_7的磁性和电性研究

采用传统固相反应法制备双层锰氧化物Nd1.15Tb0.05Sr1.8Mn2O7多晶样品,通过测量样品的X射线衍射谱、磁化强度随温度变化曲线(M-T曲线)以及电阻率随温度变化曲线(ρ-T曲线),对其磁性和电输运性质进行了研究。结果表明,样品随温度的升高先后经历了四个磁转变点:奈尔温度点(TN≈50 K)、电荷有序温度点(TCO≈67 K)、居里温度点(TC≈107 K)和类Griffiths温度点(TG≈275 K)。在低温部分样品处于反铁磁-铁磁共存态,表现出了团簇自旋玻璃行为;在TC到TG温度范围内,样品处于铁磁-顺磁共存态,即存在类Griffiths相;在TG温度以上,样品处于纯顺磁态。其电输运性质表明,随温度的升高,样品的磁电阻率降低,没有发生金属-绝缘转变。通过对115 K~300 K温度范围内的ρ-T曲线拟合发现,样品在高温部分基本遵循三维变程跳跃的导电方式。     

钙钛矿锰氧化物Pr1-xCaxMnO3 (0.3≤x≤0.5)的磁跳变现象研究

通过固相反应法合成了Pr1-xCaxMnO3系列多晶样品(x＝0.3,0.4和0.5),并对样品的结构和磁性进行了研究.通过XRD检测结果发现样品均为单相,空间群为Pbnm.通过PPMS对样品在低温下的磁行为作了研究,对于Pr0.6Ca0.4MnO3样品,在温度为2K时,观察到了多级磁跳变现象.这是因为在2K下加场,样品长程磁有序消失,在某一临界场,反铁磁(antiferromagnetic AFM)区会突然全部转化为铁磁(ferromagnetic FM)区,磁化强度出现跳跃式变化.     

钙钛矿钴氧化物La_(0.5-x)Nd_xSr_(0.5)CoO_3(x=0,0.1,0.15)的磁性

采用传统的固相反应法制备了La_(0.5-x)Nd_xSr_(0.5)CoO_3(x=0,0.1,0.15)多晶样品,并通过磁化强度与温度的变化曲线(M-T曲线)和磁化强度与外场的变化曲线(M-H曲线)对其磁性进行了研究。研究结果表明,随着温度的降低三个样品的磁性都先后经历了顺磁、铁磁和反铁磁转变。掺杂量x的增加使得系统的居里外斯温度逐步上升(θ_(x=0)≈215 K;θ_(x=0.1)≈226 K;θ_(x=0.15)≈242 K),这表明Nd~(3+)掺杂使系统内部的铁磁耦合增强。通过对磁化率倒数与温度的变化曲线的拟合,发现三个样品都出现了向上背离居里外斯定律的现象,这种背离是由于系统内部反铁磁相互竞争所致。因此,三个样品均不具有类Griffiths相。另外,通过对高温部分的实验测量与理论计算发现,Co~(4+)的自旋态随着掺杂量x的增加逐渐由低自旋态向中间自旋态过渡。     

钙钛矿锰氧化物(La_(1-x)Gd_x)_(0.5)Sr_(0.5)MnO_3的电磁特性

通过磁化强度和电阻的测量对多晶样品(La1-xGdx)0.5Sr0.5MnO3(x=0,0.1,0.2,0.3,0.4)的电磁特性进行了系统的实验研究。当x≤0.2时,样品显示出铁磁金属态(FMM)到顺磁绝缘态(PM I)的转变;当0.3≤x≤0.4时,金属-绝缘体转变温度Tp消失,样品显示出反铁磁绝缘态(AFM I)到顺磁绝缘态(PM I)的转变。同时随着x的增加所有样品的居里温度Tc和金属-绝缘体转变温度Tp都降低,并且在居里温度附近观测到了磁电阻效应。     

Co替代Mn对Pr_(0.6)Ca_(0.4)MnO_3磁性质及电荷有序的影响

用固相反应法制备Pr_(0.6)Ca_(0.4)Mn_(1-x)CoxO_3(x=0,0.02,0.04,0.06)系列多晶样品。通过X射线衍射(XRD)图谱检测样品的结构,通过零场冷(ZFC)和加场冷(FC)的磁化强度-温度(M-T)曲线,电子自旋共振(ESR)图谱,研究Co替代Mn对Pr_(0.6)Ca_(0.4)Mn O_3磁性质及电荷有序相的影响。结果表明:母体样品的宏观磁性质随温度的变化而发生多重变化,在238 K出现电荷有序相,TCO=238 K,200~173 K温区的ZFC曲线出现一个平台,173 K为Neel温度,即T_N=173 K,在40 K附近ZFC曲线出现一个尖峰,样品出现自旋玻璃态,自旋玻璃态转变温度T_f=40 K。Co替代Mn对Pr_(0.6)Ca_(0.4)Mn O_3的磁性质及电荷有序相影响明显,所有掺Co样品都没有表现出电荷有序特征,随温度降低从顺磁向铁磁/反铁磁混合相转变。Co替代Mn对电荷有序的破坏非常容易,当Co替代量x=0.02时电荷有序相被完全融化。Co替代Mn对电荷有序相的破坏之所以容易,是因为用非Jahn-Teller(JT)离子Co~(3+)替代JT离子Mn~(3+),削弱了Jahn-Teller畸变,间接证明Jahn-Teller电-声子相互作用是产生电荷有序的主要原因。     

Correlation between the structure and internal friction of metallic glass Cu45Ti55

The structural change of metallic glass Cu₄₅Ti₅₅ from room temperature to 800 K is investigated by combining internal friction using the torsion penulum method with X-ray diffraction (XRD), differential scanning calorimetry (DSC), and transmission electron microscopy (TEM). The internal friction curve exhibits a point of inflection at about 640 K and an internal friction peak between 640 and 780 K, which corresponds to the precipitation of CuTi₂ (tetragonal) and a metastable phase I (tetragonal) and the formation of CuTi (tetragonal), respectively. The result shows that internal friction is closely related to the phase transition during crystallization, and there is much correspondence between internal friction and DSC in describing the crystallization behavior of metallic glasses.     

Gd_5Si_(1.8)Ge_(1.8)Sn_(0.4)合金的高温相变

为认识合金的高温相变,采用热分析技术对Gd5Si1.8Ge1.8Sn0.4合金可能的高温相变进行了定性研究,并利用高温XRD研究了合金在200～400℃之间的相结构。研究结果表明:升温过程中,Gd5Si1.8Ge1.8Sn0.4合金在200～300℃之间发生了由Gd5Si2Ge2-型单斜结构向Gd5Si4-型正交结构的转变。从而为探索材料方便快捷的制备工艺提供技术支持。     

Ferromagnetism of Ni-Doped ZnO Powders Prepared by Sol-Gel Method above Room Temperature

The magnetic and structural properties of 2% Ni-doped ZnO powers prepared by sol-gel method were studied. X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) of the sample do not show existence of any signal of second phase. The X-photoelectron spectroscopy (XPS) of the sample shows the chemical valence of Ni is 2 and the real Ni concentration is 2.19% determined by X-ray fluorescence spectra (XRFS). The magnetic property performed with superconducting quantum interference device (SQUID) shows that the sample exhibits ferromagnetism above room temperature and the Curie temperature (TC) is about 600 K.     

La_(0.8)Zn_(0.2)MnO_3固体电解质的制备及导电性能

以Mn(NO3)2、La(NO3)3.6H2O和Zn(NO3)2.6H2O为原料,通过溶胶-凝胶法制备单一钙钛矿结构La0.8Zn0.2MnO3(LZMO)。对合成后的LZMO凝胶进行自蔓延燃烧。XRD分析表明,高于873K煅烧后得到的LZMO,粉体形成了钙钛矿结构且没其他杂相。在673~973 K条件下,空气气氛中,用两端阻塞的交流阻抗方法研究了由1 073 K煅烧所得的钙钛矿材料LZMO离子导电性能,表明了该材料在中高温条件下已经具有了非常好的离子导电性能,673~973 K固体电解质的导电率为1.3×10-3~7.4×10-2Ω-1.m-1。应用Arrhenius公式对离子导电的活化能进行计算,求得离子导电活化能为70.17 kJ/mol。     

High temperature elastic properties of polycrystalline mar-m200 (a nickel base superalloy)

A sonic resonance technique has been used to measure the variation in Young’s modulus and shear modulus of MAR-M200 at elevated temperatures to 1125 K. The slopes of these moduli with temperature are found to change abruptly at 915 and 970 K, respectively. The reason for the above noted changes appears to be related to the elastic behavior of the γ phase of MAR-M200.     

AC susceptibility and FC-ZFC magnetic properties of SrTb_xFe_(12-x)O_(19) and SrTm_xFe_(12-x)O_(19) hexaferrites:a comparative study

In this comparative study,Tb and Tm substituted Sr-hexafe rrites(HFs) with the chemical compositions of SrTb_xFe_(12-x)O_(19) and SrTm_xFe_(12-x)O_(19)(x=0.00,0.02 and 0.04) were fabricated via citrate sol-gel approach.The AC susceptibility and FC-ZFC magnetization were investigated.The product structure and morphologies were also investigated via XRD,TEM and SEM along with EDX.Measurements of temperature dependence of magnetization M-T and AC magnetic susceptibility versus temperature were carried out.The various synthesized HFs displayed ferrimagnetic behavior within 10-325 K.At lower temperatures,super-spin glass-like behavior was noticed.Neel-Arrhenius and Vogel-Fulcher models were employed to explore the experimental data of AC susceptibility.     

High-Temperature Mechanical Behavior of Extruded Mg-Y-Zn Alloy Containing LPSO Phases

The high-temperature mechanical behavior of extruded Mg_97?3x Y_2x Zn _x (at. pct) alloys is evaluated from 473 K to 673 K (200 °C to 400 °C). The microstructure of the extruded alloys is characterized by Long Period Stacking Ordered structure (LPSO) elongated particles within the magnesium matrix. At low temperature and high strain rates, their creep behavior shows a high stress exponent (n = 11) and high activation energy. Alloys behave as a metal matrix composite where the magnesium matrix transfers part of its load to the LPSO phase. At high-temperature and/or low stresses, creep is controlled by nonbasal dislocation slip. At intermediate and high strain rates at 673 K (400 °C) and at intermediate strain rates between 623 K and 673 K (350 °C and 400 °C), the extruded alloys show superplastic deformation with elongations to failure higher than 200 pct. Cracking of coarse LPSO second-phase particles and their subsequent distribution in the magnesium matrix take place during superplastic deformation, preventing magnesium grain growth.     

Study on stability of RECo5 phases（RE=Dy,Gd）

The eutectoid decomposition reaction that occurred in RECo5 phases ( RE=Dy and Gd ) at low temperature was discussed and confirmed by X-ray powder diffraction (XRD), differential thermal analysis, and scanning electron microscopy. The decomposition temperature of the GdCo5 and DyCo5 were identified as 805.8 and 900℃, respectively. The GdCo5 and DyCo5 phases could not be found in the isothermal section of Gd-Dy-Co ternary system at 800 K. Reasons for the absence of the RE2Co7 phase in the XRD patterns were discussed in detail.     

Effect of Composition and Distribution of Phases after Aging on Stamp Ability for Aluminum Alloy D16 (AA2014) Sheets

The relevance of this research is connected to the increasing requirements for accuracy in stamped parts produced from aged aluminum alloys, and can also be applied for making layered composites. Indexed requirements can be provided by controlling the structure of sheet blanks, particularly by the phase composition and distribution behavior. Results of experimental research into the effect of aging modes on composition, dispersion behavior, and stamping number of sheet samples of the D16 aluminum alloy (AA2014) are presented. Heat treatment includes quenching from a temperature of 500°С into water of room temperature and further aging: natural aging for 7 days and artificial aging at temperatures of 50, 100, 150, and 200°С with a duration at each temperature of 15, 30, 60, 120, and 240 min. A method of estimating the quantity of the characteristics of dispersion phases is proposed for the microstructural picture. Stamp ability is evaluated using the stamping number, which is the ration between yield stress and tensile strength. It is found that increasing aging temperature and durance leads to the growth of the stamping number, which shows a low ability for sheet-stamping operations of alloy. Aging at 50°С did not lead to the sedimentation of dispersion phases for either optical metallography or scan electron microscopy. The inhomogeneity of phase dispersion inside the grain grows at the initial stages of aging, when durance is less than 1 h and temperature is 100, 150 and 200°С. Further increasing durance to 4 h leads to inhomogeneity decreasing. There is no correlation between the uniformity of phase dispersion and the stamping number. The chemical composition of phases plays the main role in stamping number, outside of phase-dispersion uniformity. The phase-composition changes depend on the mode of heat treatment: at an annealed and naturally aged state, the θ and S phase is sediment. After aging at a temperature lower 150°С after a short durance of less than 1 h, the θ, S and T phases are revealed; the θ phase appears after aging at temperatures higher than 150°C and long durance reaching 4 h.