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In the present work, with the aim of improving the local flux pinning at the unit cell level in the YBa 2 Cu 3 O 7-δ (YBCO) bulk superconductors, 20 wt% of nanoscale Sm 2 O 3 and micron sized (Nd, Sm, Gd) 2 BaCuO 5 secondary phase particles were added to YBCO and processed in oxygen controlled preform optimized infiltration growth process. Nano Dispersive Sol Casting method is employed to homogeneously distribute the nano Sm 2 O 3 particles of 30–50 nm without any agglomeration in the precursor powder. Microstructural investigations on doped samples show the chemical fluctuations as annuli cores in the 211 phase particles. The introduction of mixed rare earth elements at Y-site resulted in compositional fluctuations in the superconducting matrix. The associated lattice mismatch defects have provided flux pinning up to large magnetic fields. Magnetic field dependence of current density (J c (H)) at different temperatures revealed that the dominant pinning mechanism is caused by spatial variations of critical temperatures, due to the spatial fluctuations in the matrix composition. As the number of rare earth elements increased in the YBCO, the peak field position in the scaling of the normalized pinning force density (F p /F p max ) significantly gets shifted towards the higher fields. The curves of J c (H) and F p /F p max at different temperatures clearly indicate the LRE substitution for LRE' or Ba-sites for δT c pinning.

Controlling the microstructure of superconductors by incorporating the flux pinning centers and reducing the macro-defects to improve high field performance is the topic of recent research. In continuation, the preform optimized infiltration growth (POIG) processed YBa 2 Cu 3 O 7−δ (YBCO) system, Y-site substituted with three mixed RE (Nd, Sm, Gd) elements is investigated. 20 wt.% of (Nd, Sm, Gd) 2 BaCuO 5 were mixed with Y 2 BaCuO 5 and POIG processed in reduced oxygen atmosphere to obtain YNSG superconductor. No seed is employed for crystal growth; hence the processed samples are multi-grained. Microstructural and compositional investigations on YNSG revealed the presence of different phases in the matrix as well as in precipitates which are of the order of submicron to 4 μm. A large fraction of macro-defects (∼6% of porosity) was observed in the YNSG sample. For reducing the unwanted macro-defects and refine the non-superconducting precipitates, processed YNSG sample is pressed and resolidified (by infiltrating the liquid phases once again) in an argon atmosphere and the structural, microstructural, elemental and superconducting properties are compared with YNSG and undoped samples. Due to spatial scatter in superconducting critical temperatures, caused by the distribution of different REBCO unit cells in YBCO, superconducting transition curve is sharp in YNSG, whereas the resolidified sample showed the broad transition due to solidified liquid phases.

The effect of the addition of nanometric ceria (CeO2) on the physical properties of YBa2Cu3 O 7−δ superconductors is investigated. Preform optimized infiltration growth (POIG) process emerged as one of the advanced processing techniques to obtain the uniform distribution of fine Y2BaCuO5 (Y-211) particles of the order of ∼1 μ m. Nanoparticles of CeO2 were added to Y-211 in 0, 2, 5 and 10 wt% through a novel process called Nano Dispersive Sol Casting and subjected to POIG process. Nanometric CeO2 particles were distributed homogeneously throughout the Y-211 preform without any agglomeration. The micrographs of the final composites indicate refinement in Y-211 particle size and presence of defects in various length scales, starting from few nanometers to few microns. As the CeO2 content increased, the Y-211 particle size refined below 1 µm and, for 10 wt% of doping, even smaller than 0.5 µm. Elemental analysis confirmed the formation of different phases in the CeO2-added composites, due to which the superconducting transition is shown broadening. The sample with 10 wt% doping exhibited a stable J c curve to high applied magnetic fields due to the presence of multiple peak fields. The present work demonstrates the introduction of fine microstructural defects for pinning the flux lines at different applied magnetic fields in POIG-processed YBCO composites.

Silicon carbide particulate reinforced alumina matrix composites were fabricated using DIrected Metal OXidation (DIMOX) process. Continuous oxidation of an Al–Si–Mg–Zn alloy with appropriate dopants along with a preform of silicon carbide has led to the formation of alumina matrix surrounding silicon carbide particulates. SiC p /Al 2 O 3 ceramic matrix composites fabricated by the DIMOX process, possess enhanced mechanical properties such as flexural strength, fracture toughness and wear resistance, all at an affordable cost of fabrication. SiC p /Al 2 O 3 matrix composites were investigated for mechanical properties such as flexural strength, fracture toughness and hardness; the composite specimens were evaluated using standard procedures recommended by the ASTM. The SiC p /Al 2 O 3 ceramic matrix composites with SiC volume fractions from 0.35 to 0.43 were found to possess average bend strength in range 158–230 MPa and fracture toughness was found to be in range of 5.61–4.01 MPa√m. The specimen fractured under three-point loading as observed under scanning electron microscope was found to fail in brittle manner being the dominant mode. Further the composites were found to possess lower levels of porosity, among those prepared by DIMOX process.

Isothermal ac MH (magnetization-field) loops for varying field amplitudes were recorded at 77 K on YBa2(Cu1-xMnx)3O7-δ with x = 0, 0.010, 0.015, 0.020, 0.025, 0.035 and 0.050, YBa2(Cuo.o75Fe0.025)3O7-δ, YBa2(Cu0.075Ni0.025)3O7-δ and YBa2(Cu0.075Zn0.025)3O7-δ samples up to a maximum field amplitude of 80 Oe. Flat band susceptibility, ac losses and flux profiles were deduced from the ac MH loops. The undoped sample exhibited a minimum weak link ac loss and the 5.0% doped sample showed maximum weak link ac loss. Ni and Fe doped samples showed higher granular losses.Jcg estimated from the flux profiles decreases monotonically with increasing concentrations of Mn up to 2.5%.

Temperature variation of AC susceptibility has been measured on YBa 2( Cu 1 - x- Mn x)3 O 7-δ samples with x values 0, 0.01, 0.015, 0.02, 0.025, 0.035 and 0.05, YBa 2( Cu 0.075 Fe 0.025)3 O 7-δ and YBa 2( Cu 0.075 Ni 0.025)3 O 7-δ samples in the temperature range from 10 to 120 K. The weak link (inter-granular) and granular responses are distinctly pronounced at the studied fields of 0.2 to 30 Oe. The temperature variation of weak link critical current density Jcw(T) indicates the inter-granular coupling to be of superconductor-insulator-superconductor (SIS) type. The weak link AC loss calculated from the imaginary part of the AC susceptibility χ′′(T) is found to exhibit a power relation with temperature, with an average exponent value of 1.96. Field variation of AC susceptibility measured at 50 K and 77 K were well modelled with Kim's relation and the Jcw at 50 K and 77 K were estimated from the fit parameters.

Dc magnetization measurements were carried out on YBa2(Cu1-xMnx)3O7-δ samples with x taking values 0, 0.01, 0.015, 0.02, 0.025, 0.035 and 0.05, YBa2(Cu0.975Fe0.025)3O7-δ, YBa2(Cu0.975Ni0.025)3O7-δ and YBa2(Cu0.975Zn0.025)3O7-δ samples up to a maximum field of 5 T at temperatures 5, 50 and 77 K. The effect of Mn on the zero field critical current density, Jcg(0), estimated from the remnant magnetization, in general, is a monotonic suppression up to a concentration of 2.5% Mn and shows a saturation beyond that concentration. The field and temperature dependences of 2.5% Mn doped YBa2Cu3O7-δ samples were better than those of the Ni, Fe and Zn at the same doping level. Anomalous increases in Jcg(0), the indicative lower critical field, H*c1g, and the bulk flux pinning force, Fp, observed in the 2.0% Mn doped sample, at 50 and 77 K, have been attributed to the matching of the Abrikosov vortex density generated over these temperatures and the defect induced number of pinning sites due to the Mn doping.

The effect of lattice defects at the unit cell level created by the stress field when two rare earths with different ionic radii are mixed at the rare earth site, on the flux pinning and critical current density in Y1-xDyxBa2Cu3O7-δ thin films prepared by the pulsed laser deposition technique, was investigated using SQUID magnetometry at different temperatures for x = 0 to 1 in steps of 0.2. From the isothermal magnetic hysteresis recorded up to 5.5 T at 5, 35 and 77 K, the critical current density, pinning force density and irreversibility fields are estimated and their variation as a function of x analysed. An inverse relation between current density and irreversibility field is observed at all the temperatures except for the film having a 20% concentration of Dy in Y which also exhibits the highest critical current density among the films investigated, suggesting this to be the optimum composition for effective flux pinning from stress field induced lattice defects.

Through a measurement of flux profiles, the inductive method of Campbell has not only been used for determining the bulk critical current density of large sized samples but has also been extended to study the coupling strength of the domains in melt textured NdBa 2 Cu 3 O 7− δ superconductors. An analysis of the profiles measured for the slow (1°C/h) and fast cooled (40°C/h) samples through peritectic temperature, T p clearly showed that the domain boundaries in the fast cooled samples are decoupled in the presence of external dc fields. The observed features of flux profiles are correlated to the microstructural features of the samples. It is proposed that this technique can be used routinely to assess the quality of large size melt textured samples for high current applications like magnetic bearings or quasi permanent magnets.

Low and high field dependent microwave absorption is studied in various sintered and melt textured HTSs. The low field loss is found to originate from weak links and the high field loss is due to thermally assisted flux flow. Melt textured superconductors are found to be best suited for low field device applications due to the drastic reduction in low field induced loss when compared to sintered samples. Melt textured superconductors with 211 inclusions are found to show reduced high field losses due to pinning of field at the secondary defects created by 211 particulates. Various parameters of the critical state such as Jc and the pinning force constant, Kx, are determined from the microwave absorption study.

We report the effect of 250 MeV Ag17+ ion irradiation on DyBa2Cu3O7−δ thin films prepared by pulse laser deposition technique with fluences 5 × 1010, 1 × 1011, 5 × 1011 and 1 × 1012 ions/cm2. The SQUID measurements on irradiated films at 5, 35 and 77 K show that the film irradiated with a fluence of 1 × 1011 ions/cm2 shows a considerable improvement in critical current density, Jc, and pinning force density, Fp. The irreversibility field, Hirr, was estimated from the plot of normalized Fp vs the normalized field. A significant reduction in Hirr after irradiation and a dip in the plot of Hirr vs Bφ at a fluence of 5 × 1010 ions/cm2 were observed.

Superconducting samples of ${\mathrm{YBa}}_{2}({\mathrm{Cu}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}{)}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ with $x=0,$ 0.01, 0.015, 0.02, 0.025, 0.035, and 0.05 have been prepared and the temperature variation of the Seebeck coefficient, $S(T),$ has been measured over a temperature region of 77 to 250 K. The observed features of the results are analyzed in terms of the metallic diffusion model, the Nagaosa-Lee model, and a narrow band model. The physical parameters obtained are discussed.

We have melt processed NdBa2Cu3O -Nd4Ba2Cu2O10 (NdBCO) superconducting composites with various rates of cooling through their peritectic formation temperature. The microstructural studies reveal that the sample processed even with C/h rate of cooling is built up with aligned microstructure which is necessary for superior transport current and magnetic properties. In the case of YBa2Cu3O , C/h rate of cooling is necessary to get highly textured microstructure. Magnetic flux profile measurements reveal that even though the intradomain region is well textured and had good transport and magnetic properties, the interdomain region is relatively weak in the fast cooled sample in comparison with slow cooled ones but is much better than that in polycrystalline samples. The results suggest that, the NdBCO superconductor can be melt processed in short time with much better microstructural features in comparison with YBCO system.

NdBa 2 Cu 3 O 7− δ superconductor is processed with varying Nd 4 Ba 2 Cu 2 O 10 (Nd-422) content, thereby varying systematically its microstructure. The samples are then investigated by studying the flux penetration into the samples as a function of magnetic field using a flat-band lock-in technique. Unlike in sintered samples, the flux profiles in the melt-processed samples are found to be linear, whose slopes increase with increasing Nd-422 content. The slope of the profile gives a direct, model-independent measure of the critical current density in the materials, and the specimens measured can be large enough to include several millimeter-sized domains. The critical current densities thus obtained are compared with those obtained using SQUID magnetometer, wherein the samples have to be much smaller and mostly single domain. The extent of flux penetration, reversible penetration depth, and the slope changes in the flux profiles at high fields are correlated to the microstructural features of the melt-textured samples.

Temperature dependence of power absorption of HTSCs is found to follow the London two fluid model at both radio and microwave frequencies. The two fluid model is shown to account for the Josephson penetration depth at radio frequencies (MHz) and grain penetration depth at microwave frequencies in sintered HTSCs. The Josephson penetration depth determined is of the order of 100 μm. Also, it is discussed that reduced superelectron screening due to short mean free path in HTSC cannot account for the large penetration depth determined from the radio frequency absorption.

We have observed anomalous microwave absorption (MWA) representative of paramagnetic Meissner effect in GdBCO powder. The results are discussed in terms of π junction theory. The phase change in the MWA gives evidence for the change in behaviour of the π junctions from para to dia state. The intragrain weak links with high J c are likely to be forming π junctions in the sample. It is expressed that the high frequency absorption is a better tool than the magnetization measurements to investigate the π junctions. Thermally induced vortex–antivortex pair formation and the flux compression caused by an inhomogeneous superconducting transition are ruled out as the possible mechanisms of the PME, from the microwave absorption study.

The features of the flux profiles recorded for the sintered and press-sintered Bi{sub 1.2}Pb{sub 0.3}Sr{sub 1.5}Ca{sub 2}Cu{sub 3}O{sub y} (BSCCO), and sintered and melt-textured NdBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} (NdBCO) superconducting samples with widely different microstructures are analyzed. From the slope of the flux profiles (at H{sub ac}{r_arrow}0) recorded for various dc fields, the intergranular J{sub ci} (H{sub dc}) for the BSCCO samples is determined, whose field dependence is found to follow Kim{close_quote}s model. Using the fit parameters thus obtained, the flux profiles for different H{sub dc} could be simulated very well including features like nonlinearity at low fields, the presence of a peak, and the oscillatory behavior. The flux profile of the sintered NdBCO recorded with zero H{sub dc} could also be simulated well using Kim{close_quote}s critical-state model. An analysis of the intragranular flux profile determined from the measured profile at high fields suggests a wide distribution of H{sub g}{sup {asterisk}}, (the full penetration field of the grain) and thereby of the J{sub cg}. This result is in agreement with the broad transition observed in the ac susceptibility of this sample, attributed to the presence of Nd{sub 1+x}Ba{sub 2{minus}x}Cu{sub 3}O{sub 7{minus}{delta}} precipitates that modify the local J{sub cg} values drastically.more » All the sintered specimen show nearly 45{percent} of flux penetration (into the sample) at low enough fields (H{sub dc}{lt}40Oe), while the flux entry into the melt-textured NdBCO is {lt}3{percent}. At high dc fields (4{endash}8 KOe) the profile exhibits two distinct slopes that are associated with a flux entry initially into the microcracks and then into the grains, from the microstructural considerations. {copyright} {ital 1997} {ital The American Physical Society}« less

The temperature variation of resistivity of several Samples of 85 K and 110 K superconductors of BiSrCaCuO system prepaed under different conditions was analysed in terms of excess conductivity due to phase fluctuations in the paracoherence region. The average values of estimated excess conductivity critical exponent (γ) for the 85 K and 110 K superconductors are respectively 2.55 and 2.75. The paracoherence to coherence transition in a granular superconductor is compared to the paramagnetic to ferromagnetic transition of 3D XY ferromagnets. The γ values obtained for these superconducting samples differ from the predicted susceptibility critical exponent value for the pure 3D XY ferromagnets. However, the obtained γ values are comparable to the susceptibility critical exponent of the disordered 3D Heisenberg ferromagnet system.

Preform optimized infiltration growth process (POIGP) is employed to fabricate mixed rare earth (Y, Sm)Ba2Cu3O7−δ superconductors of high performance. POIGP creates wide range of pinning centres and also enables nano-particle additions to stable Y2BaCuO5 (Y-211) preforms. Nano-Sm2O3 particles (30–50 nm) were introduced in 0, 10, 20 and 30 wt.%, homogenously and without agglomeration, into the Y-211 preforms and the corresponding composites YBCO-Ar, YSm-10, YSm-20 and YSm-30 are fabricated. Though all the samples were processed in argon atmosphere to suppress the formation of solid solutions with Sm at Ba site, the effort was successful only in the YSm-20 sample, as seen from sharp diamagnetic transition and XRD studies. Magnetic property measurements were carried out on samples which showed strong (00l) texture, but were not necessarily single crystalline since no seed was used in the experiments. The calculated critical current densities (J c) are thus reflective of the values in the basal plane, in the most favourable direction. The final composites revealed YSm-20 to exhibit better J c with substantial flux pinning up to 9 T, at 77 K. Detailed examination of microstructures and field dependence of J c reveal that the addition of nano-particles is not always rewarding and can lead to reactive sintering of Y-211 and limit the extent of liquid phase inflow into preforms. This not only can lead to incomplete reaction leaving larger 211 particles in the matrix, but can also shift the composition of liquid phase such that it promotes the formation of undesired solid solutions in IG processed REBCO as seen in YSm-30. The effect of various factors like open porosity in the preform, the ease of liquid phase infiltration and the liquid phase composition on the residual 211 size in the composites, suppression of RE/Ba solid solutions, and the resultant J c versus H curves is discussed.

We report ferromagnetic resonance (FMR) measurements in rf sputter deposited Co100-xCux thin films with 27 < x < 100. Data were taken at 300 K at about 10, 22 and 36 GHz, with the field being rotated in the film plane and the plane perpendicular to it. It is found that, while STM suggests the presence of oblate spheroidal clusters, the FMR results are consistent with having a single magnetic entity, that is, a relatively complete alignment of these cluster axes parallel to the film normal.

We present a systematic study of the low (≲ 25 mT) field microwave magnetoabsorption in BSCCO as a function of temperature and magnetic history. It is shown that the virgin response (increase of absorption on first exposure to field) should be attributed to weak links while the hysteretic behavior happens because the weak links sense not only the applied field but also a gaussian distribution of random fields arising from flux trapped in the sample after it has been exposed to an external field.

ac-susceptibility (\ensuremath{\chi}',\ensuremath{\chi}'') measurements as a function of temperature and ac field amplitude have been carried out on cylindrical samples of the 110-K phase of the Bi-Sr-Ca-Cu-O system. The two single-phase samples used for studies were prepared under different experimental conditions with an aim to understand the regimes of validity of Bean's and Kim's critical-state models. Material-dependent parameters such as critical current density as a function of temperature and effective volume fraction of grains, were estimated using these models. Improvements in the critical-current density and intergrain coupling strength were found when a sample was subjected to intermediate cold pressing between sintering. Field variation of intergranular matrix susceptibility of the sintered sample could be fitted well to Kim's model whereas the data on the press sintered sample were in agreement with Bean's model.

We report the existence of a sub‐precipitate microstructure and the resulting fishtail effect in YBCO/Ag superconducting composites fabricated by Seeded Infiltration and Growth Processing. The SEM micrographs reveal sub‐precipitate microstructure in the form of precipitates of size less than 100 nm within the larger non‐superconducting Y‐211 precipitates that contributes to the enhancement of Jc in the form of secondary peak effect at lower fields.

The excess conductivity (∆σ) data on several samples of 85 K phase and 110 K phase of Bi-Sr-Ca-Cu-O system were fitted to a sum of direct (AL) and indirect (MT) contribution of fluctuation induced excess conductivity, in the mean field region. The samples of the 85 K phase are of compositions Bi2Sr2CaCu2Oy, Bi2Sr2CaCu2Ag0.8Oy, and Bi2Sr2.14Ca0.86Cu2Oy with Tc (zero) of 78.9 K, 86.5 K and 81.8 K respectively. The two samples of 110 K phase with composition Bi1.2Pb0.3Sr1.5Ca2Cu3Oy were prepared from nitrate and citrate routes respectively and have Tc (zero) of 105.3 K and 102.9 K. We find from our analysis relatively strong pair-breaking effects in both 85 K and 110 K phases, indicating the contribution of Maki-Thompson term to the excess conductivity to be rather insignificant. The average value estimated for the phase-breaking time for the 85 K and 110 K phases are (3.7±0.5)×10−16s and(11.3±1.6)×10−16s respectively, at 100 K.

The temperature variation of resistivity measured for the 110 K phase of the (Bi, Pb)-Sr-Ca-Cu-O system has been analysed to study the excess conductivity due to fluctuations in the mean field region. The resistivity data has been analysed using Aslamazov-Larkin (AL) as well as Lawrence-Doniach (LD) theories for the excess conductivity (Δσ), while the normal conductivity (σ n ) part was chosen to fit the usual linear equation (a+b T -1 as well as that of Veira and Vidal. We find from our analysis (both from AL and LD fits) that the fluctuations in the order parameter are two-dimensional in nature in the 110 K phase of the Bibased system in contrast to that of YBa 2 Cu 3 O 7 . The estimated value of coherence length ξ c (0) is 3.96 A choosing the interplanar spacing ( d ) to be the value obtained from the theoretical fit.

The 110 K superconductor of the (Bi,Pb)‐Sr‐Ca‐Cu‐O system is prepared in single‐phase form at the composition Bi1.6Pb0.4Sr2.01Ca2.71Cu3.93Oy. Disks of the material with favorable texture can be obtained in relatively short durations in an O2 poor atmosphere. The literature on the compositions of the 110 K superconductor is reviewed to reveal that the material can be synthesized in single‐phase form in a range of compositions, which is poorer simultaneously in Bi and Sr with respect to the nominal composition (Bi,Pb)2Sr2Ca2Cu3Oy, (2:2:2:3). At compositions close to 2:2:2:3, intermediate cold pressing steps between heat treatments are necessary to promote the formation of single‐phase material. We find that at compositions which are considerably poorer in Bi and Sr with respect to 2:2:2:3, no such steps are necessary. In this paper, the microstructural reasons for this difference in behavior are also investigated.

Field induced microwave absorption is recorded at low fields ( c ceramic superconductor (Bi,Pb)SrCaCuO, after cooling in zero field to 54,77 and 90 K at 9.9 GHz and to 77K at 36.4 GHz. The results are discussed in conjunction with the dc magnetization data obtained on the same sample. It is argued that the observed features do not follow the predictions of the flux flow model while they give qualitative evidence in support of the resistively shunted Josephson junction model.

The high-temperature superconductors YBa 2 Cu 3 O 7−δ and Bi 2 Sr 2 CaCu 2 O y , have been prepared by a new coprecipitation route using 8-hydroxy quinoline as the precipitating agent. The method is significant because it can be used for preparing the superconductors with predetermined low levels of impurity concentrations.

We demonstrate here the universal behavior of the fundamental intergranular matrix susceptibility ({chi}{sub {ital m}}) in terms of a single parameter ({delta}) in the 110 K phase of the Bi-Sr-Ca-Cu-O system, in the regime where flux profile is linear in the sample. Following the observation of universality of the third harmonic susceptibility {chi}{sub 3}({ital T},{ital H}{sub dc}){sub {ital H}}{sub ac} by Shatz {ital et} {ital al}. in Y-Ba-Cu-O, we have examined the field and temperature dependences of {chi}({ital H}{sub dc},{ital H}{sub ac}){sub {ital T}} and {chi}({ital T},{ital H}{sub dc}){sub {ital H}}{sub ac}, respectively, measured in Bi{sub 1.2}Pb{sub 0.3}Sr{sub 1.5}Ca{sub 2}Cu{sub 3}O{sub {ital y}} superconductor. Our analysis shows that the field dependence of {ital J}{sub {ital c}} is better described, for the present sample, by the power-law model and also demonstrates the universality of {chi}{sub {ital m}} in terms of {delta}.

Aluminum alloy matrix composites have emerged as candidate materials for electronic packaging applications in the field of aerospace semiconductor electronics. Composites prepared by the pressureless infiltration technique with high volume fractions in the range 0.41–0.70 were studied using ultrasonic velocity measurements. For different volume fractions of SiC, the longitudinal velocity and shear velocity were found to be in the range of 7600–9300 m s−1 and 4400–5500 m s−1, respectively. The elastic moduli of the composites were determined from ultrasonic velocities and were analysed as a function of the volume fraction of the reinforcement. The observed variation is discussed in the context of existing theoretical models for the effective elastic moduli of two-phase systems.

Aluminium matrix composites containing a high volume fraction of silicon carbide particulates were produced by the pressureless infiltration technique. The metal matrix composites thus obtained were characterized for phase composition, microstructure, density, porosity and coefficients of thermal expansion. All composites were studied in their as-prepared condition without any further heat treatment. Behaviour of bulk properties such as thermal expansion and density was studied as a function of the volume fraction of silicon carbide within the volume fraction range 0.41–0.67. The coefficients of thermal expansion of the composites were found to vary between 14 × 10−6 and 7 × 10−6 K−1 in a non-linear fashion as a function of the volume fraction of SiC. The variation in the density of the composites was found to be linear with volume fraction in the usual manner. The results obtained were compared with existing models on the behaviour of macroscopic properties of composite materials.

By 79,81Br and 127I NQR spectroscopy 4-(CH3)C6H4NH⊕ 3X⊖ , X = Br and I, and 4-(CH3)C6H4ND⊕ 3I⊖ were studied in the temperature range 77 ≦ TK ⪅ 340. The three compounds experience phase transitions, most likely of higher order. Two reversible transitions are observed for the para-toluidinium iodide. 7~i-n being in the temperature range 247 ⪅ T/K ⪅ 258. TII ⇌ III in the range 174 ⪅ T/K ⪅ 179. The deuterated compound shows slight shift of the transition temperatures compared to the protonated one. whereas the 127I-NQR resonance frequencies are considerably lowered as is the nuclear quadrupole coupling constant. The asymmetry parameter η, however, increases through deuteration of the NH3 group. The room temperature phase of the iodide crystallizes with an orthorhombic space group (Pmmn or Pmn21, Z = 2). Temperature dependence of the 79BrNQR spectrum of p-toluidinium bromide indicates a phase transition near T = 281 K.

Preliminary results of a study of the transformation behaviour, and those of microhardness measurements on quasi-crystalline Al-14 at.% Mn and Al-22 at.% Mn alloys are reported.

Synthesis of superconducting YBa 2 Cu 3 O 7−δ has been achieved by a rapid and energy saving process starting with nitrate precursors. The process involves the preparation of an intimate mixture of Y, Ba and Cu oxides by the combustion of a mixture of the corresponding metal nitrates with urea. The reaction takes place in just 5 min at 525 ° C. The oxide mixture is then heat treated for relatively short durations to obtain the superconducting material.

Recently, the technique of spin wave resonance has been used to measure the exchange coupling constants as a function of composition in several solid solutions of MnSb. From an examination of the results we show that in those alloys in which the linear chains of magnetic atoms are not disturbed, the exchange coupling constants become positive leading to ferromagnetic coupling when the distance between Mn atoms becomes equal to 2.83 A or more. This experimental value of the Mn-Mn distance for the onset of ferromagnetism is equal to the critical distance derived by Forrer after examining a large number of materials which owe their ferromagnetism predominantly to the direct exchange interaction.

Ferromagnetic resonance studies have been carried out in thin films of Mn1-xFexSb (0

We show in this paper that many of the difficulties, encountered in using the ferromagnetic MnSb films as storage media in memory devices employing the technique of laser Curie point writing and magneto-optic readout, can be overcome by forming solid solutions of MnSb with Fe, Co, In, and excess Mn, viz. Mn1-xFexSb, Mn1-xCo xSb, MnSb1-xInx and Mn 1+xSb (0�x�0.25 or 0.3). Spins that are in basal plane of MnSb at 300 K are found to reorient to be along the film normal, from ferromagnetic resonance work, beyond a critical composition (xc=0.16, 0.21, 0.16, and 0.14 for the systems containing Co, Fe, In, and excess Mn, respectively) ensuring a large readout efficiency. Moreover, the lowering of Curie temperature for higher compositions can enable us to work with low power lasers which help to achieve sharper writing without diffusion of heat into neighboring domains. In addition, the absence of any crystallographic phase transformation in the temperature range of operation (unlike in MnBi) makes the MnSb alloys potentially more attractive for use as recording media.

Details of a method which employs an rf oscillator to determine the Curie temperature and magnetization of ferromagnetic materials have been described. the results obtained on some intermetallic compounds viz. CrTe, Mn1.1Sb, Mn1.2Sb, Mn0.9Co0.1Sb and Mn0.9Fe0.1Sb are presented.