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.     

Influence of temperature on structural and magnetic properties of Co0·5Mn0·5Fe2O4 ferrites

Co_0·5Mn_0·5Fe₂O₄ ferrites have been synthesized using a single-step sol-gel auto-combustion method in which the metal nitrate (MN)-to-citric acid (CA) ratio was adjusted to 0.5: 1 and pH to 7, respectively. The structural and magnetic properties of as-burnt and annealed samples were studied as a function of temperature. The inverse spinel structure was confirmed by X-ray diffraction (XRD) and crystallite size was estimated by the most intense peak (311) using Scherrer’s formula. Contrary to earlier studies reported in the literature, both as-burnt and annealed samples exhibit crystalline behaviour. Room temperature magnetic properties were studied using vibrating sample magnetometer (VSM) with field strengths up to ±10 kOe. Lattice constant and crystallite size increased as the annealing temperature was increased. However, the coercivity (H _c) initially increased and then decreased with the increase of crystallite size. The variation in coercivity is ascribed to the transition from a multi-domain to a single-domain configuration.     

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.     

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.     

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     

Ion-Selective Properties of KxV2O5·nH2O

Inorganic Materials - Compounds with the general formula $${{{\text{K}}}_{x}}{\text{V}}_{x}^{{{\text{4}} + }}{\text{V}}_{{{\text{2}} - x}}^{{{\text{5}} + }}{{{\text{O}}}_{{\text{5}}}}{\kern 1pt}...     

Magnetodielectric effect in Ni0∙5Zn0∙5Fe2O4–BaTiO3 nanocomposites

Composites comprising of nanoparticles of Ni_0?5Zn_0?5Fe₂O₄ (NZF) and BaTiO₃ (BT), respectively were synthesized by a chemical method. The particles had diameters in the range of 15–31 nm. NZF was prepared by a coprecipitation technique. This was soaked in a sol containing BT. Compositions synthesized were xNZF-(1 – x) BT, where x = 0?7, 0?5 and 0?3, respectively. The composites showed ferromagnetic hysteresis loops due to NZF phase. The analysis of coercivity variation as a function of temperature gave blocking temperatures in the range of 306–384 K depending on the diameter of the ferrite nanoparticles. This implied that superparamagnetic interactions are above these temperatures. The nanocomposites also exhibited ferroelectric behaviour arising due to the presence of BT. The remanent polarization of the samples was small. This was adduced to the nanosize of BT. The specimens showed magneto-dielectric (MD) effect in the magnetic field range 0–0?7 Tesla. The MD parameter measured at the maximum magnetic field was around 2%. This was one order of magnitude higher than that reported so far in similar composite systems. This was explained on the basis of a two-phase inhomogeneous medium model with an interface between them, the phases possessing drastically different electrical conductivities.     

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.     

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.     

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).     

Ce取代对纳米晶Ni0．5Zn0．5Fe2O4电磁性能的影响

以丙烯酰胺为聚合单体,N,N-亚甲基双丙烯酰胺为网络剂,采用聚丙烯酰胺凝胶法制备了尖晶石型Ni0.5Zn0.5CexFe2-xO4(x=0, 0.05)纳米晶.采用X射线,FT-IR,TEM和波导等方法对产物进行了表征.X射线结果表明,当煅烧温度为600℃时,形成纯相的尖晶石型Ni0.5Zn0.5CexFe2-xO4(x=0,0.05);由透射电镜照片可知Ni0.5Zn0.5Fe2O4平均粒径约为30nm;纳米晶体在8.2～12.4GHz的测试频率范围内具有介电损耗(ε")和磁损耗(μ"),Ni0.5Zn0.5Ceo.05Fe1.95O4的ε" 和μ"均高于Ni0.5Zn0.5Fe2O4,Ni0.5Zn0.5Ce0.05Fe1.95O4的ε"和μ"的最大值分别为0.93和0.15.     

Y2O3掺杂（Bi0．5Na0．5）0．94Ba0．06TiO3无压电陶瓷的研究铅

采用固相合成法制备了Y2O3掺杂（Bi0．5Na0．5）0．94Ba0．06TiO3无铅压电陶瓷。研究了Y2O3掺杂对（Bi0．5Na0．5）0．94Ba0．06TiO3陶瓷晶体结构、介电与压电性能的影响。XRD分析表明,在所研究的组成范围内陶瓷均能够形成纯钙钛矿固溶体。介电常数-温度曲线显示陶瓷具有弛豫铁电体特征,陶瓷的弛豫特征随掺杂的增加更为明显。在Y2O3掺杂量为0．5％时陶瓷的压电常数d33分别为137pC／N,为所研究组成中的最大值,掺杂量为0．1％时,机电耦合系数kp与kt最大值为0．30,0．47。     

Effect of beryllium and calcium on aging behaviour of Al–0·75Mg–0·5Si alloy

Abstract

The effect of microadditions of Be and Ca on the aging behaviour of Al–0·75Mg–0·5Si alloy is investigated. It is shown that the addition of 0·1%Be significantly increases the hardening rate and the maximum hardness level attainable when the alloy is aged at various temperatures from room temperature to 300°C, while the addition of 0·2%Ca decreases both the hardening rate and the maximum hardness level attainable. Optical and scanning electron microscopical observations show a significantly higher precipitate density for the Be containing alloy and a slightly lower precipitate density for the Ca containing alloy when compared with the base Al–Mg–Si alloy. The results are consistent with an earlier kinetic study that indicated a Be enhanced nucleation rate for precipitation in the same alloy.

MST/936     

Electrical properties of Sr x Ba1 − x Fe 0·6 Sn 0·4 O3 − ε NTC thermistors

Polycrystalline thermistor ceramics with the stoichiometric formula, Sr _x Ba_1???x Fe_0·6Sn_0·4O_3???ε (0·2 ≤ x ≤ 0·8), had been prepared by a standard solid state reaction route. X-ray diffraction analysis indicates that the ceramic system remains cubic perovskite structure with a very small lattice change. The electrical properties of Sr _x Ba_1???x Fe_0·6Sn_0·4O_3???ε thermistors were studied using a digital thermometer over a wide range of 298–400 K. The thermistors showed an excellent negative temperature coefficient (NTC) thermistor behaviour. The values of resistivity at 298 K (ρ ₂₉₈), thermistor constant (B _298/358) and activation energy (E _a) of the Sr _x Ba_1???x Fe_0·6Sn_0·4O_3???ε thermistors, decreasing with the increase of Sr content, were in the range of 0·37–11·0 kΩ ·cm, 2466–3703 K and 0·212–0·319 eV, respectively. For the thermistors with the compositions x?= 0·2 and 0·4, the fitting equivalent circuit was composed of three RCPE elements corresponding to grain, grain boundary and ceramic–electrode interface, respectively. From the impedance plots, it was found that the fitting data showed good agreement with the experimental data. The extracted grain boundary resistance exhibited a NTC thermistor behaviour.     

Relaxor behaviour and dielectric properties of BiFeO3 doped Ba(Zr0·1Ti0·9)O3 ceramics

Ba_1???x Bi _x (Ti_0·9Zr_0·1)_1???x Fe _x O₃ ( x = 0–0·075) ceramics are prepared using a conventional solid state reaction method. X-ray diffraction shows the presence of a single phase. Addition of Bi^3?+? and Fe^3?+? strongly influences the crystal structure and dielectric properties of the ceramics. The evolution from a normal ferroelectric to a relaxor ferroelectric is emphasized. Ba_0·99Bi_0·01(Ti_0·9Zr_0·1)_0·99Fe_0·01O₃ ceramic shows a relaxor behaviour at room temperature with Δ T _m =12 K. P– E hysteresis loop of the composition, x = 0·007, shows a remanent polarization ( P _r ) of 0·5 μC/cm² with a coercive field ( E _C ) of 2 kV/cm. Raman spectra of all compounds are performed and correlated well with the X-ray diffraction and dielectric measurement results.     

In situ templating synthesis of conic Ba0·5Sr0·5Co0·8Fe0·2O3?δ perovskite at elevated temperature

Conic Ba_0·5Sr_0·5Co_0·8Fe_0·2O_3−δ (BSCF) functional composite oxide was synthesized via a simple in situ templating process. The treatment of the solid precursor with concentrated nitric acid resulted in the mismatch of ionic radius at A-site and B-site of the ABO₃ perovskite, due to the oxidation of cobalt/iron ions, and the formation of Ba_0·5Sr_0·5(NO₃)₂ solid solution. Therefore, instead of the direct formation of BSCF oxide, an intermediate phase of Ba_0·5Sr_0·5CoO₃ (BSC) in hexagonal lattice structure and with conic particle shape was preferentially formed during calcination at low temperature. BSCF perovskite was then produced by the in situ templating of BSC with iron diffusing into the BSC lattice during calcination at high temperature. Well-crystallized BSCF particles in conic shape were obtained by the calcination of the nitric acid treated precursor at 900°C.     

The role of fluorine in the devitrification of SiO2·Al2O3·P2O5·CaO·CaF2 glasses

A series of eight glasses based on a glass system with the generic composition 1.5(5–Z)SiO₂·(5–Z) Al₂O₃·1.5P₂O₅·(5–Z)CaO·ZCaF₂ were studied where Z = 2, 1.75, 1.5, 1.25, 1, 0.5, 0.25, 0. These glasses were characterised using differential scanning calorimetry (DSC) and x-ray powder diffraction (XRD). Glasses with high fluorine contents were found to crystallise readily to fluorapatite via a homogeneous nucleation route probably involving prior amorphous phase separation. These results are explained in terms of an approach which views glasses as being inorganic polymers where the presence of fluorine disrupts the glass network and thereby reduces the energy barrier to homogeneous nucleation and crystallisation of fluorapatite.     

Na0．5Bi0．5TiO3-K0．5Bi0．5TiO3无陶瓷的制备与性能铅压电

采用传统的干压成型法制备了Na0．5Bi0．5TiO3-K0．5Bi0．5TiO3无铅压电陶瓷，研究了不同K0.5Bi0.5TiO3含量对Na0．5Bi0．5TiO3-K0．5Bi0．5TiO3陶瓷的微观结构与电性能的影响规律。结果表明，Na0．5Bi0．5TiO3-K0．5Bi0．5TiO3无铅压电陶瓷随K0.5Bi0.5TiO3含量增加，晶格常数增大，密度减小，晶粒尺寸减小，压电常数先增大后减小，介电常数增大，介电损耗增加，机械品质因数下降，而居里温度不断升高，在200℃附近存在由铁电相向反铁电相转变的一个相变点，组分为0．84 Na0．5Bi0．5TiO3-K0．5Bi0．5TiO3的陶瓷位于准同型相界附近，具有最佳的压电性能。     

Synthesis, microstructure and thermal expansion studies on Ca0·5+x/2Sr0·5+x/2Zr4P6−2x Si2x O24 system prepared by co-precipitation method

We report on the synthesis, microstructure and thermal expansion studies on Ca_0·5?+?x/2Sr_0·5?+?x/2Zr₄P_6???2x Si_2x O₂₄ (x = 0·00 to 1·00) system which belongs to NZP family of low thermal expansion ceramics. The ceramics synthesized by co-precipitation method at lower calcination and the sintering temperatures were in pure NZP phase up to x = 0·37. For x ≥ 0·5, in addition to NZP phase, ZrSiO₄ and Ca₂P₂O₇ form as secondary phases after sintering. The bulk thermal expansion behaviour of the members of this system was studied from 30 to 850 °C. The thermal expansion coefficient increases from a negative value to a positive value with the silicon substitution in place of phosphorous and a near zero thermal expansion was observed at x = 0·75. The amount of hysteresis between heating and cooling curves increases progressively from x = 0·00 to 0·37 and then decreases for x > 0·37. The results were analysed on the basis of formation of the silicon based glassy phase and increase in thermal expansion anisotropy with silicon substitution.     

Hydrothermal synthesis of mixed phosphates of neodymium and alkaline metals (Me2O·Nd2O3·4P2O5)

The phase formation in the system Me₂O-Nd₂O₃-P₂O₅-H₂O (where Me = Li, Na, K, Rb and Cs) has been studied under hydrothermal conditions in the temperature and pressure range 300 to 700 C and 0.5 to 600 atm, respectively, using vitreous carbon glass liners. A composition diagram showing the possible fields of crystallization of different phases under equilibrium conditions is given. AB-diagrams of fields of crystallization of different phases in the system investigated are given. These diagrams are in correspondence with the theoretical composition diagram. The advantages and disadvantages of different methods employed in the growth of MeNd-phosphates are discussed. The crystals obtained were characterized by various methods.