Relaxor behaviour of (Ba0·5Sr0·5)(Ti0·6Zr0·4)O3 ceramics

Ba_0·5Sr_0·5Ti_0·6Zr_0·4O₃ ceramic has been prepared through solid state reaction route. X-ray diffraction shows that the sample has cubic perovskite structure with space group Pm-3m at room temperature. Temperature dependent dielectric study of the ceramic has been investigated in the frequency range 50 Hz-1 MHz. The density of the sample is determined using Archimedes’ principle and is found to be ∼99% of X-ray density. The dielectric study revealed diffuse phase transition of second order. A broad dielectric anomaly coupled with the shift of dielectric maxima toward a higher temperature with increasing frequency indicates the relaxor-type behaviour in the ceramics. The index of relaxation (γ) and the broadening parameter (Δ) were estimated from a linear fit of the modified Curie-Weiss law. The value of γ ∼ 1·72 indicates the strong relaxor nature of the ceramic. A remarkably good fit to the Vogel-Fulcher relation further supports such a relaxor nature.     

Electrical conductivity and chemical stability of BaCe0·8 − x A x Gd0·2O3 − δ (A = In, Zr, Ta; x = 0, 0·1) ceramics

BaCe_0·8???x A _x Gd_0·2O_3???δ (A = In, Zr, Ta; x?=?0, 0·1) ceramics were synthesized by solid-state reaction method. Microstructure and electrical properties of BaCe_0·8???x A _x Gd_0·2O_3???δ ceramics were investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and complex impedance analysis at intermediate temperatures of 573–1073 K in different atmospheres. BaCe_0·8Gd_0·2O_3???δ , BaCe_0·7In_0·1Gd_0·2O_3???δ and BaCe_0·7Zr_0·1Gd_0·2O_3???δ ceramics exhibit a single cubic perovskite structure. BaCe_0·7In_0·1Gd_0·2O_3???δ ceramic has the highest conductivity of 4·6 × 10^???2 S·cm^???1 in air at 1073 K. BaCe_0·7In_0·1Gd_0·2O_3???δ and BaCe_0·7Zr_0·1Gd_0·2O_3???δ ceramics exhibit an excellent chemical stability against boiling water. Indium is a suitable doping element to promote the sintering densification and to enhance both electrical conductivity and chemical stability of Gd-doped BaCeO₃ at operating temperatures.     

In0.53Ga0.47As PIN光电探测器的温度特性分析

从理论和实验上分析了双异质结In0.53Ga0.47As PIN光电探测器在不同的反向偏置电压下暗电流在甚宽温度范围内的温度特性。结果表明：在反向偏置低压与高压段,产生一复合电流与隧道电流(缺陷隧道电流与带带间隧道电流)分别占主导地位,并呈现出相应的温度特性。还从理论与实验两方面探讨了噪声对探测器R0A的影响,结果表明：在低温段,产生一复合噪声起主要作用,在高温段,俄歇复合噪声起主要作用。     

In(Ga)As自组织量子点激光器获重大突破

Ｉｎ(Ｇａ)Ａｓ/ＧａＡｓ量子点体系因其独特、优越的光电性质,成为替代目前ＩｎＰ基材料,制备光纤通讯用13μｍ长波长激光器的热门材料之一,引起各国科学家的高度重视。许多科研小组已投入很多的财力和物力进行这种激光器的研制与开发,迄今已有美国德克萨斯大学、日本Ｎ?..     

Preparation and electrochemical properties of SrCe 0·4 Zr 0·4 Yb 0·2 O 2·9 electrolyte

The perovskite Yb-doped strontium cerate–zirconate material, SrCe_0·4Zr_0·4Yb_0·2O_2·9, was prepared by solid-state reaction and the structure was characterized by X-ray diffraction. The calcination process of the powder was investigated by thermogravimetric/differential thermal analysis (TG–DTA). The high temperature conductivities were measured by d.c. four-probe technique in the temperature range from 500 to 950°C in wet hydrogen and effect of temperature on conductivity was investigated. The conductivity increased with the elevation of temperature from 500 to 950°C. The highest conductivity of 4·4 × 10^???2 S· cm^???1 was observed for SrCe_0·4Zr_0·4Yb_0·2O_2·9 at 950°C. The current–voltage (I–V) and current–power (I–P) characteristics of the single cell (H₂, Pt/SrCe_0·4Zr_0·4Yb_0·2O_2·9/Pt, O₂) at temperature range from 600 to 850°C were tested. With the temperature increasing from 600 to 850°C, the open circuit voltage (OCV) decreased from 1·164 to 1·073 V and the ionic transfer number decreased from 0·996 to 0·946. At 850°C, the maximum power density of 25·2 mW· cm^???2 was observed.     

Spin canting phenomenon in cadmium doped cobalt ferrites, CoCd x Fe2−x O 4 ( x = 0·0, 0·2, 0·4, 0·6, 0·8 and 1·0), synthesized using sol–gel auto combustion method

Synthesis of non-collinear (spin canted) ferrites having the formula, CoCd _x Fe_2???x O ₄ (x?= 0·0, 0·2, 0·4, 0·6, 0·8 and 1·0), has been carried out using the sol–gel auto combustion method. The ferrite samples show an interesting magnetic transition from Neel to Yafet–Kittel configuration, as the Cd^2?+? concentration is increased beyond x?= 0·4. The FT–IR spectra confirm the formation of the metal oxide bond as they exhibit two frequency bands in the range of ~595 cm^???1 and ~450 cm^???1, corresponding to the tetrahedral and the octahedral stretching vibrations of the metal oxide, respectively. The structural evolutions of the nanophase investigated using powder X-ray diffraction (XRD) technique show that the average crystallite size is ~ 35 nm. The magnetic studies reveal that the saturation magnetization, M _s , increases up to x?= 0·4 and decreases when the value of x is >0·4. It is proposed that the incorporation of Cd^2?+? ion takes place into the tetrahedral sites and up to x?= 0·4, Neel’s model is followed. But for x?> 0·4, canting of spins occurs, as explained by Yafet–Kittel (Y–K) model. The d.c. resistivity decreases as a function of temperature, indicating semiconducting nature of the ferrites and the positive value of Seebeck coefficient establishes p-type conduction behaviour for all the ferrite samples.     

Magneto-dielectric effect in Pb(Zr0·52Ti0·48)O3 filled nanoporous Ni 0 ·5Zn 0 ·5Fe 2 O 4 composite

Nanoporous Ni_0·5Zn_0·5Fe₂O₄ particles of diameter, ~ 9·5 nm, were synthesized by citric acid assisted thermal decomposition in an autoclave. The BET surface area measured was 80 m² g^???1 and the average pore diameter was 2·5 nm. By soaking the particles in a suitable precursor solution and then subjecting them to a heat treatment at 923 K for 3 h, Pb(Zr_0·52Ti_0·48)O₃ was grown within the nanopores. X-ray and electron diffraction studies confirmed the presence of both these phases. The nanocomposites showed ferromagnetic behaviour over the temperature range 2–300 K. No ferroelectric hysteresis loop could be found which was consistent with the earlier theoretical prediction of loss of ferroelectricity below a critical thickness of 2·4 nm. Good magneto-dielectric response of the order of 7% at a magnetic field of 9 kOe was recorded for the present system. This is believed to arise due to a negative magnetostriction coefficient of Ni_0·5Zn_0·5Fe₂O₄ which exerted a compressive strain on Pb(Zr_0·52Ti_0·48)O₃ thereby lowering the tetragonality in its crystal structure.     

不同In组分对In_xGa_(1-x)As物理性质影响的研究

对InxGa1-xAs在x从0~1变化时的晶格常数、原子弛豫情况、分波态密度进行计算,结果表明当In组分增大时,晶格常数增大规律与Vegard定理吻合。体系弛豫后,Ga—As键长变小程度会随In组分增多而变大,In—As键长变小程度基本不随In组分变化;In—As—In键角会变小,当In组分超过0.625时,其变小程度非常微弱;Ga—As—Ga键角和Ga—As—In键角有不同程度地变大,且Ga—As—In键角的变化幅度在In组分为0.5时最严重;体系分波态密度的计算结果表明,In掺杂会严重影响GaAs材料的物理性质。     

Decarburization in 0·5Cr–0·5Mo–0·25V steel and its effects on creep and microstructure

Abstract

Many high–temperature creep tests are performed on low–chromium, ferritic steels in an uncontrolled atmosphere. Examination of creep rupture specimens of 0·5Cr–0·5Mo–0·25V steel tested in air has shown that decarburization accompanies oxidation and is an important factor in accelerating the failure of creep tests in air. Similarly, pre-aging in air reduces the creep life more than pre-aging in a capsule. There is also evidence that decarburization is accelerated during straining. Measurements of surface carbon contents in 10 mm thick blocks heat treated in air at 600–700°C have given an apparent activation energy for decarburization of about 250 kJ mol^?1, at least twice that for carbon diffusing in ferrite. However, this value is still below that for creep, so the influence of decarburization on creep life is expected to increase at lower temperatures. Structural observations are discussed in relation to loss of carbon and are related to creep behaviour. Secondary precipitation was observed after low-temperature treatments in aged encapsulated specimens, but not in specimens aged in air. This is attributed to the loss of carbon in the air aged specimens, which also showed a decrease in the M₃C content. The iron content of M₃C particles depends on carbon content as well as aging time.

MST/40     

Al0.15Ga0.85As／GaAs与In0.15Ga0.85As／GaAs甚长波QWIPs响应率的仿真对比

对In0.15Ga0.85As／GaAs和Al0.15Ga0.85As／GaAs两种甚长波段量子阱红外探测器（QwIP）响应率进行计算。采用物理模型与等效电路模型，结合Crosslight和Spice等软件详细表征了这两种QWIPs的吸收系数、暗电流、响应率、量子效率等物理特性。结果表明随外加偏压的升高QWIP的响应率增加，T=40K时，In0.15Ga0.85As／GaAs QwIP的响应率明显比Al0.15Ga0.85As／GaAs QWIP高出2倍以上，通过对量子效率的对比，使仿真结果得到验证。     

Wear perfornance of 0·6%Cr alloyed cast and heat treated steels with carbon contents of 0·7, 1·0, and 1·3%

Abstract

The effect of carbon content and heat treatment on the abrasive wear performance of three 0·6%Cr alloyed cast steels with carbon contents near the eutectoid composition (0·7, 1·0 and, 1·3%) were investigated using a pin on drum abrasion tester. Specimens were austenitised at 870,970, or 1070°C for 1 h and quenched. Additionally, a 1·3%C specimen was spheroidised, then austenitised at 870°C and quenched. The microstructure, which was dependent on heat treatment and carbon content, significantly influenced the wear rate. The wear rate is related non-linearly to hardness. The lowest wear rate was obtained for the 1·3%C alloy austenitised at 1070°C producing a quenched microstructure of 40% retained austenite and 60% martensite. This specimen also exhibited the lowest quenched hardness. However, for a practical industrial heat treatment, the 1·0 %C alloy austenitised at 970°C provides the best wear performance.

MST/3130     

Relaxation effect of pulse current on Ti50·0Ni50·0 structure during rolling

Abstract

Using transmission electron microscopy and differential scanning calorimetry, the microstructure and martensitic transformation of the Ti_50·0Ni_50·0 alloy after cold rolling and after rolling with a pulse current were compared. A structural relaxation effect was observed in the alloy during rolling with current. The effect is manifested in the more slow kinetics of the structure formation and less intensive suppression of martensitic transformations at rolling with current compared to cold rolling with some deformation degrees.     

Precipitation and age hardening behaviour of AI–0·2Ti–0·2Be alloy

Abstract

The effect of 0·2%Be on the aging behaviour and microstructure of Al–0·2% Ti alloy is investigated. It is shown that the addition of Be induces some age hardening in non age hardenable AI–0·2Ti alloy and significantly increases the precipitate density, indicating a Be enhanced nucleation for the precipitate phase.

MST/1250     

Electrical conduction and thermal properties of Bi-doped Pr0·7Sr0·3MnO3 manganite

Electrical and thermal properties of Bi-doped Pr _0·7 Sr _0·3 MnO ₃ (PSMO) compounds are reported here. It is observed that Bi-doped PSMO compounds follow variable range conduction mechanism. Specific heat, thermal conductivity and thermo-power measurements show that larger Bi-ion, in place of smaller Pr ion, enhances their transition temperatures (T _MI, T _C, T _CP). Thermo-power measurements further strengthen our conclusion for the presence of magnetic polaron, generated due to hybridization of Bi ³⁺ –6s ² and O ^2? –2p orbital and polaronic conduction mechanism.     

Study of dependence of magnetic properties on seeding temperature in fine particles of Cu0·25Co0·25Zn0·5Fe2O4

Using a combination of X-ray diffraction, room temperature/low temperature Mössbauer studies and d.c. magnetization, the structural and magnetic properties of nano-sized Cu_0·25Co_0·25Zn_0·5Fe₂O₄, prepared using the coprecipitation method at different seeding temperatures, have been studied. Formation of α-Fe₂O₃ in these samples is observed to be very sensitive to the seeding temperature and is totally suppressed at a seeding temperature of 333 K. These results are explained in terms of the rate of coprecipitation of the different components as a function of temperature. Since α-Fe₂O₃ is antiferromagnetic, the presence and quantity of this impurity is also observed to deteriorate the overall magnetic properties of the spinel phase.     

Statistical model for molten Fe1−yCryNx (0·4≤y≤0·9)

Abstract

Results of previous statistical thermodynamic analysis appeared to imply that N atoms in molten Fe_1?yCr_y (0·05<y<0·2) are trapped in octahedral voids (O-sites) surrounded by one Cr atom and five Fe atoms. However, previous work did not include development of a statistical model for molten Fe_1?yCr_yN_x-with higher Cr content. In the present work, available equilibrium nitrogen pressure–temperature–composition relationships for molten Fe_1?yCr_yN_x were analysed in the composition range 0·4≤y≤0·9. The results of the present analysis appeared to suggest that in this composition range O-sites surrounded by four Cr atoms and two Fe atoms provide the trapping sites for N atoms.

MST/721     

Effects of ferrite grain size and martensite volume fraction on dynamic deformation behaviour of 0·15C–2·0Mn–0·2Si dual phase steels

Abstract

Effects of ferrite grain size and martensite volume fraction on quasistatic and dynamic deformation behaviour of 0·15C–2·0Mn–0·2Si dual phase steels were investigated in this study. Dynamic torsional tests were conducted on six steel specimens that had different ferrite grain sizes and martensite volume fractions, using a torsional Kolsky bar, and then the test data were compared in terms of microstructures, tensile properties, fracture mode, and adiabatic shear band formation. Under dynamic torsional loading, maximum shear stress and fracture shear strain increased with decreasing ferrite grain size and increasing martensite volume fraction. Observation of the deformed area beneath the fracture surface after the dynamic torsional test indicated that adiabatic shear bands of 5 to 15 μm in width were formed along the shear stress direction, and that voids or microcracks initiated at ferrites or martensite/ferrite interfaces below the shear band. The width of the shear band decreased as the ferrite grain size increased or the martensite volume fraction decreased. These phenomena were then analysed by introducing concepts of theoretical critical shear strain.     

Structural Perfection of Four-Layer Ga x In1 – x As y P1 – y Laser Heterostructures

The structural perfection and coherent growth conditions of epitaxial layers in four-layer Ga _x In_{1 – x} As _y P_{1 – y} heterostructures on InP(001) substrates were studied by x-ray diffraction and topography. The composition of the active layer corresponded to a peak photoluminescence wavelength of 1.34 m. The coherent growth region was delineated, and the critical thickness of epitaxial layers was determined as a function of the interfacial lattice mismatch. The critical thickness determined experimentally exceeds the calculated value. It is shown that heterostructures up to 9 m in layer thickness, free of interfacial dislocations can be grown in a broad range of elastic strains in adjacent layers.     

Effect of cobalt substitution on magnetic and transport properties of Nd0·5Sr0·5Mn1?xCoxO3 (x = 0·1, 0·3 and 0·5)

The magnetic and transport properties of the compounds Nd_0·5Sr_0·5Mn_1-x_{{\rm 1}-{x}}Co_x_{{x}}O₃ (x = 0·1, 0·3 and 0·5), synthesized by citrate–gel route have been investigated. The spin transition in cobaltates at low temperatures affects the magnetic as well as transport properties. The irreversibility behaviour between the zero-field cooled (ZFC) and field cooled (FC) magnetization as a function of temperature becomes stronger with increasing Co content. This is understood on the basis of glassy behaviour, which becomes more robust with increasing Co substitution. The non-saturating M–H behaviour indicates strong magnetic inhomogeneities which may cause the magnetic phase separation at the nanoscopic length scale. The double exchange interaction is stronger between Mn^3 +–O^2 −–Mn^4 + as compared to Co^3 +–O^2 −–Co^4 + pairs. Co-substitution suppresses the double exchange which will lead to cluster/spin glass like behaviour as well as semiconducting features due to localization of charge carriers (mobile e_g{e}_{\rm g} electrons).     

Nitrates-melt synthesized LiNi0·8Co0·2O2 and its performance as cathode in Li-ion cells

Layered LiNi_0·8Co_0·2O₂ crystallizing in R $\bar 3$ m space group is synthesized by decomposing the constituent metal-nitrate precursors. Oxidizing nature of metal nitrates stabilizes nickel in +3 oxidation state, enabling a high degree of cation ordering in the layered LiNi_0·8Co_0·2O₂. The powder sample characterized by XRD Rietveld refinement reveals < 2% Li-Ni site exchange in the layers. Scanning electron microscopic studies on the as-synthesized LiNi_0·8Co_0·2O₂ sample reflect well defined particles of cubic morphology with particle size ranging between 200 and 250 nm. Cyclic voltammograms suggest that LiNi_0·8Co_0·2O₂ undergoes phase transformation on first charge with resultant phase being completely reversible in subsequent cycles. The first-charge-cycle phase transition is further supported by impedance spectroscopy that shows substantial reduction in resistance during initial de-intercalation. Galvanostatic charge-discharge cycles reflect a first-discharge capacity of 184 mAh g^?1 which is stabilized at 170 mAh g^?1 over 50 cycles.