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In the present paper an attempt has been made to understand the growth mechanism of SiGe alloy nanowires synthesized in H-mode cylindrical microwave cavity resonator at 900 °C in a short time of nearly 5 min. The role of ambience, microwave H-field, duration of interaction with microwaves and germanium concentration in the starting precursor have been investigated. It is believed that H-field of microwaves efficiently interacts with the starting precursor powder in air ambience and results in superheating/supersaturation, which gives rise to the formation of innumerable critical sized nuclei via heterogeneous nucleation, leading to one dimensional growth of cylindrical nanowires.

Measurement of electrical and magnetic parameters of thin/thick films at microwave frequencies has always been challenging. This is because the measurement of permittivity and permeability of the films in GHz range is not as straight forward as of the bulk samples. The present paper reviews some of the available techniques with emphasis on the efforts carried out in authors’ laboratory in this direction during past 10–15 years.

Silicon germanium alloy has successfully been synthesized in a 2.45 GHz, single mode microwave H field at 900 °C in 5 min in the Microwave Laboratory. The phase formation occurs at a temperature which is lower by 400 °C than its equilibrium temperature. In the present paper an attempt has been made to understand the mechanism of rapid alloying of Si and Ge at a much lower temperature in microwave H field. The shape and size of starting particles play a significant role in microwave absorption. It is proposed that the interaction of H field at 2.45 GHz first induces B-field in the irregular shaped precursor powder particles of different sizes; the gradient in induced magnetic flux generates circulating electric field. Consequently, dipolar losses are induced by local electric field in the pellet, which generate heat and culminate into rapid alloying of Si and Ge (nonferric diamagnetic elements). Furthermore, magnetic reorientation of diamagnetic fine particles by H-field at 2.45 GHz assists in an efficient heating of the Si and Ge powders and results in alloy formation at a much lower temperature.

NiCuZn ferrite nanopowders and polycrystalline bulk samples were prepared by wet chemical (citrate) method followed by sintering at temperatures lower than those for conventional solid-state ceramic route. The citrate method results in nano-powders. The addition of copper promotes sintering, and results in grain-growth during sintering, even at a low temperature. Composite thick films of desired thickness were prepared by die casting the ferrite mixed with a polymer in different weight ratios. The dielectric properties of the polycrystalline NiCuZn ferrite and their composite thick films were investigated at radio- and X-band microwave frequencies. Dielectric and magnetic losses are estimated to be appreciably low, even at microwave frequencies.

A simple theoretical model is developed for studying the magnetic behavior of films at microwave frequencies, and verified experimentally on the ferrite-polymer thick-film samples. The measured values of permeability by the proposed model/technique are found to be in good agreement with the values obtained by the standard cavity perturbation technique.

In the present work, silicon nanowires have been grown in a TE011 single-mode cylindrical microwave resonant cavity operating at 2.45 GHz. The microstructural, structural, elemental, and room temperature photoluminiscence studies have been carried out of the grown nanowires. The wires are encapsulated by an ultrathin layer of silicon oxynitride. The observed strong visile photoluminescenc is ascribed to ballistic transport and radiative recombination (at Si/SiON interface) of the carriers generated due to light absorption.

The ZnTi substituted barium hexaferrite polycrystalline samples have been synthesized by solid-state reaction method. The complex permittivity and permeability measurements are carried out on the BaFe12–2xZnxTixO19 (0.2 ≤ x ≤ 0.6) samples in the microwave frequency range of 8–13 GHz by cavity perturbation technique. The observed abnormal dielectric behavior, i.e., an increasing tendency of dielectric constant with frequency has been attributed to the mixed conduction. The effect of ZnTi substitution on the permittivity and dielectric loss has been investigated in the frequency range of 1 kHz–1 MHz as well. The increase in concentration of ZnTi dopants monotonically increases both the permittivity and the permeability in the investigated frequency ranges, which can be attributed to increased polarizability and unpinning of the domain walls, respectively.

Silicon and germanium alloys have been formed for the first time by heating powdered mixtures of 75%Si and 25%Ge in just 2 min in the H field in a single mode 2˙45 GHz microwave cavity. As is well known, alloy formation in conventionally heated silicon–germanium mixtures is difficult, even when processed for much longer time durations at the same temperature. Further, any oxide contaminants, if present in the starting material or formed during heating, somehow, appear on the surface of the specimen after microwave treatment.

Cobalt-substituted lithium ferrites (with general composition Li0.5–x /2CoxFe2.5–x /2O4, where x = 0.0, 0.2, 0.4, 0.5 and 0.6) were prepared at lower processing temperatures (∼600 °C) by the citrate route. The single-phase spinel structure was confirmed by X-ray diffraction. The average particle size of calcined powders calculated by the Scherrer formula is estimated to be 8–90 nm, whereas an average particle size of ∼20 nm is observed from TEM for Co concentration x = 0.5. Average grain sizes from SEM micrographs of pellets sintered at 1000 °C (1 h) are observed to be 0.5–1 μm, much smaller than the size reported for the standard ceramic method (2.1–6.8 μm). The experimental density is observed to be 3.59–4.47 gm/cm3, which is greater than 85% of the densities evaluated from XRD. Compared with the standard ceramic method, lower dielectric constant (10–103) and higher dc resistivity (105–1010 Ω cm) is observed for Li–Co ferrites prepared by the citrate route. Improved magnetic properties, such as higher saturation magnetization (38–79 emu g–1) and Curie temperature (535 to 620 °C) are also investigated for the citrate-route-processed samples. These results demonstrate promising features of Li–Co ferrites in microwave applications. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We measured dielectric, magnetic, and microwave properties of Co 2U hexaferrite (Ba4Co2Fe36O 60) polycrystalline bulk and composite thick film samples, and studied the effect of annealing temperature on phase formation and microstructure. We synthesized the bulk samples from a precursor prepared by the citrate method. The values of dielectric constant at radio frequencies (50 Hz-1 MHz) of the bulk Co2U12 sample sintered at 1200degC are much higher, and the resistivities are lower, compared to M-type barium hexaferrites. Coercivity is also low, having a value of 5 G for the Co2U12 sample, whereas the saturation magnetization value is 59 emu/g, which is comparable to that of M-type hexaferrites. We also measured the complex permittivity and permeability for Co2U12 samples at microwave frequencies and found the values high compared to M-type barium hexaferrite at these frequencies. Thick composite films were prepared from a ferrite-polymer mix, and all the above properties were studied for these films. We observed that these thick films have lower values of dielectric and magnetic parameters both at low and microwave frequencies. We measured microwave-absorbing properties for the ferrite-polymer sample (ferrite to polymer ratio 70/30) which showed a maximum reflection loss of -37.5 dB at 11.5 GHz for the 3.5-mm-thick sample

NiCuZn ferrite has recently attracted a lot of attention for its application in high frequency (up to a few GHz) multilayer chip inductors (MLCIs) and for other microwave devices owing to their favorable electromagnetic properties and low densification temperature. In order to study the effect of substitution of cations by cobalt in small concentration on the dielectric and magnetic properties at low and high frequencies, bulk polycrystalline ferrite samples of starting composition (Ni0.2Cu0.2Zn0.6)1 − x Co x Fe2O4, having x = 0, 0.01, 0.03 and 0.05, were prepared by citrate precursor method. Pure spinel (cubic) ferrite formation was confirmed by powder X-ray diffraction technique. Complex permittivity and permeability were measured at microwave frequencies (X-band) using the cavity perturbation method, which is a non-contact method. The values of real part of permittivity (e ′) vary in the range 7–9.6 and of the imaginary part (e ″) vary from 0.020–0.120, whereas real part of the permeability (μ′) lies in the range 2.6–14.0 and the imaginary part of permeability (μ″) varies from 0.5–6.0. It is observed that there is an increase in μ′ and decrease in the magnetic loss (tan δ μ) on increasing the cobalt concentration from x = 0 to x = 0.05. The variation of these parameters, both with frequency in X-band and with the cobalt concentration, is discussed in this paper.

Analytical expressions are derived for electromagnetic mode density in 1D periodic dielectric structures consisting of dielectric slabs of different dielectric constants. Firstly, the Maxwell's equation is solved for a dielectric scatterer block that contains two dielectric slabs of different dielectric constants, and then it is solved for a dielectric scatterer block that contains three dielectric slabs of different dielectric constants. Finally, a generalized expression for the dielectric scatterer block, containing n dielectric slabs of different dielectric constants, is written. The dependence of the photonic band gap upon the dielectric contrast, dimensions of the unit cell, and number of periods is investigated. The various properties of mode density at the band edge are discussed for both the quarter-wave stack as well as the generalized case. © 2004 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2005.

We present our experimental findings on microstrip-type metallic photonic band gap (PBG) periodic structures prepared using a commercial aluminum foil. Effect of change in geometry of the periodic dielectric structure, namely, period, rectangular hole length, and defect on PBG, is studied. It is found that as the rectangular-hole-cut dimension decreases in terms of length, the lower edge of the band gap decreases toward the lower frequency side of the microwave region. It is also observed that as the defect length increases, the band gap edge again shifts toward the lower frequency side. These findings of band gap with air as substrate confirm the role of wave impedance in purely metallic periodic structures. © 2002 Wiley Periodicals, Inc. Microwave Opt Technol Lett 35: 483–486, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.10644

Ferroelectric thin films of barium titanate were fabricated by sol–gel technique on platinum substrates. The processing temperature was 700 °C. The films obtained with a thickness of 1.5 μm were dense, transparent, and showed ferroelectricity. Scanning electron microscopy and x-ray diffraction were used for studying the surface morphology and crystallographic structure of the film. Films in the metal–ferroelectric–metal configuration (MFM) were used for the electrical measurements. Dielectric constant and loss tangent were found to be 430 and 0.015, respectively, at 10 kHz under ambient conditions. The e′(T) curve shows broad peak centered around 120 °C as in the case of diffuse phase transition. The ac conductivity is proportional to ω0.9 in the low frequency region and ω1.8 in the high frequency region. The dc conductivity versus temperature curve showed a change in the slope around 125 °C, corresponding to the phase transition. To study the low temperature phase transitions, dielectric parameters on th...

In the present work, Si0.95Ge0.05 alloy nanowires were synthesised in a single-mode resonant cavity operated in H 011 mode in ambient at 2.45 GHz in a short duration (5 min) and at a temperature (900 ◦ C) lower than the equilibrium alloy-phase formation temperature. The photoluminescence (PL) studies were carried out at room temperature. The intense PL peaks, in the energy range of 2.9–3.6 eV, can be ascribed to radiative recombination of the generated carriers. The low temperature (100–250 K) dc electrical-transport is investigated; and the activation energy is estimated to be lower by an order than that of polycrystalline samples. The lower activation energy is possibly due to electronic/structural defects introduced by the non-equilibrium growth process and high aspect ratio of the grown nanostructure. With these improved characteristics, the alloy nanowires can be exploited for device fabrication.

Barium titanate sol was prepared using barium ethyl hexanoate and titanium isopropoxide. The sol was then spin coated on p-type single crystal silicon (100) wafers, stainless steel and fused silica substrates and annealed to give polycrystalline, transparent, and crack-free films. The surface morphology and structural properties of the films were studied using scanning electron microscopy and X-ray diffraction respectively. Crystalline phase could form only at an annealing temperature of 650°C and above. The effect of post deposition annealing on the optical and structural properties as well as on the band gap were analysed. Transmission spectra were recorded and from this, refractive index, extinction coefficient and thickness were calculated for films on fused silica annealed at different temperatures. The dispersion curve for the refractive index n of 650°C annealed film is fairly flat beyond 450 nm and rises sharply towards the shorter wavelength region, showing the typical shape of a dispersion curve near an interband transition. The present study indicates the validity of the DiDomenico model for the interband transition with a single electronic oscillator. The refractive indices lie in the range 1.75–2.5 for films annealed in the range 300–750°C. The electrical measurements were conducted on metal-ferroelectric-semiconductor (MFS) and metal-ferroelectric-metal (MFM) capacitors. The typical measured small signal dielectric constant was 66 and 140 at 1 MHz for the MFS and MFM capacitors respectively. Debye type dispersion was observed for films on stainless steel substrates with an activation energy of about 0.34 eV. The low field ac conduction was found primarily due to hopping of electrons through the trap centres. The I-V characteristics of the MFS capacitor were found to be ohmic at low fields and space charge limited at high fields. I-V characteristics of the MFM capacitor showed a strange behaviour, linear dependence of current on voltage up to 106 V/m and V5/2 dependence beyond 106 V/m.

Dielectric thin films now have a wider range of applications in high frequency micro and digital circuits. Therefore, their dielectric investigation in gigahertz region are of much technical consequence. The paper reviews different experimental techniques available for thin film characterization at microwave frequencies. An extended cavity perturbation technique is also discussed.

BaTiO3 thin films were prepared on single crystal silicon (1 0 0) and platinum substrates by sol-gel technique. Amorphous films with thickness uniformity were obtained by spinning the solution at 3000 rpm for 30 s and by post-deposition annealing at 400°C. The films exhibited good dielectric and insulating properties. The dielectric constant and dissipation factor at a frequency of 100 kHz were 17 and 0.20, respectively, for 1400 A thick film on platinum substrate (MIM). The corresponding values were 16 and 0.015 for films on Si (MIS). Dielectric properties were also studied as functions of frequency and voltage. The C-V curve for MIS structure exhibited a hysteresis. The density of interface states recharged during the bias cycle in hysteresis measurement was estimated to be of the order of 2.10 × 1011 cm−2 and total oxide charge density was about 4.28 × 1011 cm−2. I-V measurements were performed on films of different thicknesses. The leakage current densities at 5 V for the films having thicknesses 1400 and 2800 A were 0.86 and 0.11 μA/cm2 respectively. The conduction mechanism is found to be Poole-Frenkel and Schottky mechanisms at low and high fields, respectively.

Strontium titanate sol was prepared using strontium ethyl haxanoate and titanium isopropoxide. The sol was then spin coated on fused silica, p-type single-crystal silicon wafers (100) and stainless-steel substrates and annealed to give polycrystalline, transparent, and crack-free films. The surface morphology and structural properties of the films were studied using scanning electron microscopy and x-ray diffraction, respectively, and differential thermal analysis was used to observe structural transition. The dielectric measurements were conducted on films with metal–insulator–metal and metal–insulator–semiconductor configurations. Capacitance–voltage (C–V) measurements were carried out and the effect of the annealing temperature was studied. The dielectric constant and loss tangent at 1 MHz at room temperature were found to be 105 and 0.02, respectively, for 1.1 μm thick films. These measurements were also carried out at low temperatures down to 20 K. There are indications for a phase transition from a ...

Electrical conduction in transition metal oxide (TMO) glasses is generally explained on the basis of the small polaron theory. Present results on CuO doped sodium borate glasses seem to be inexplicable on the basis of the above model. Experimental data on dc and ac conductivities of CuO doped sodium borate glasses at different frequencies are presented. It is found that these results can be explained in terms of the concept of small ionic polarons developed for superionic conductors.

A sensing probe has been introduced to profile the magnetic fields in a 2.45GHz, high-power single-mode microwave cavity. The treatment of a ferrite sample inside the cavity has established that, during the heating process, there is an unusual concentration of a magnetic field around the sample. During field heating, intense magnetic fields have been monitored, and are analogous to the redistribution of an electric field reported by Riedel and Svoboda [Proceedings of the Eighth Ampere Conference and Microwave Heating (2001)] in their simulation studies on microwave sintering.

Polymer ferroelectric films were prepared by dispersing TGS powder in polystyrene. Permittivity versus temperature plots exhibit a peak at around the transition temperature of TGS. The peak gets suppressed with increasing frequency and the trend continues up to microwave frequencies. We observed that the dielectric constant of films at microwave frequency stays close to the dielectric constant of polystyrene. The domain structure of TGS crystals essentially seems to affect their dielectric properties. Single domain films exhibit a higher spontaneous polarization and a higher coercive field.

The goal of this work was to understand the correlation between microscopic material parameters and the dielectric function of candidate materials for applications in the microwave frequency range. The structure and dielectric properties of Ba2+(Y3+ 1/2Ta5+ 1/2)O3 (BYT), a typical representative of the Ba(B3+ 1/2B5+ 1/2)O3 complex perovskite family, has been investigated from 102 to 1014 Hz and from 20 to 600 K. At T c =253±1 K, BYT undergoes an equitranslational improper ferroelastic, second‐order phase transition, characterized by the tilting of the oxygen octahedra. The space group symmetry changes from Fm3m, in the high temperature phase, to I4/m below T c . The existence of an intermediate temperature region (T c −40

The complex dielectric permittivity of pure and doped Triglycene Sulphate (TGS) crystals have been studied at microwave frequency(10GHz). Measurements have been carried out from room temperature to about 60°C, a temperature well above the curie temperature(49.1°C) for TGS. The results of measurements of the real and imaginary parts of the permittivity have been compared with those reported by earlier workers. Our results do not fully agree with the results of earlier workers. The behavior of our pure TGS crystal comes close to the behaviour of TGS crystals reported by Hill and Ichiki and is similar to the behavior of Cu++ doped TGS crystal studied by Bolko et.al. and the behavior of our urea doped TGS crystal is similar to the behavior of pure TGS crystal as reported by Pereverzeva and others.

Ferroelectric hysteresis studies have been carried out on the pellets of Pb0.6Bi1.4Sr2Ca2Cu3O10 superconductor selected out of sixteen samples prepared. The results suggest that the sample behaved like a normal dielectric ceramic between room temperature and 211 K. Below 211 K, its behaviour has been interpreted as that of a relaxor ferroelectric. At 118 K, the (2223) phase is known to become superconducting. At this temperature, a very thin hysteresis loop was observed which has been attributed to low temperature superconducting (2212) phase present in the material. This thin loop also disappeared at 50 K. The studies show that even for high-Tc superconductors, Matthias's suggestion that superconductivity and ferroelectricity are mutually exclusive is valid.

Samples of Bi2-xPbxSr2Ca2Cu3O10 superconductor were prepared with different Pb concentrations, each concentration being subjected to different sintering times. This was done to investigate the corresponding effect on the microstructure and hence the superconducting behaviour. The samples were subjected to XRD, scanning electron microscopy (SEM), dc resistance and microwave surface resistance measurements. XRD measurements show the complex variation of percentage volume of high-Tc phase and evolution of peaks of other impurity phases with change in fabrication parameters. Addition of Pb and prolonged sintering upto a certain limit are found to be effective in the synthesis of the high-Tc phase. SEM photographs revealed the surface structure and grain size. Proper choice of Pb and sintering time was possible from the characterization results. DC resistance of the samples was determined by four-probe technique and microwave surface resistance was determined by cavity perturbation technique. Characte...

The X-ray diffraction (XRD), ac susceptibility, dc resistivity, and scanning electron microscopy (SEM) measurements on Bi 2−x Pb x Sr 2 Ca 2 Cu 3 O 10+δ (x = 0.0, 0.2, 0.4, and 0.6) superconductors have been correlated with the Raman scattering studies. Remarkable increases in the transition temperature and percentage volume of high T c phase are found till x = 0.4. Raman spectra also reveal dramatic changes at x = 0.4 in the phonon modes at 625 and 460 cm −1 associated with oxygen vibrations in BiO plane and apical oxygen of CuO 5 pyramid, respectively. The observed changes have been attributed to the phenomenon of oxygen redistribution among various layers. Optimum solubility limit of Pb substitution is found to be x = 0.4.

Dielectric properties of single crystals of LaGaO3 have been measured at low frequencies as well as in the microwave region over a wide temperature range. Measurements performed on two crystal orientations, viz. (001) and (110), show dielectric anomalies at a transition near 145 °C. Dielectric anisotropy below, but not above, 145 °C confirm the previously reported orthorhombic symmetry at room temperature and rhombohedral symmetry above 145 °C. Domain wall motion which arises as a result of a phase transition has been observed around 145 °C.

Because of the combination of very low losses, rather high dielectric permittivity and high temperature stability of the resonance frequency (zf), the non-ferroelectric complex perovskites are being used as microwave resonators. The parameters controlling the relevant properties have not been studied in details and no general guidelines exist with regards to the synthesis of materials with pre-defined properties although experimental correlation between one-phonon absorption range and mw and correlation between structural details such as the degree of B-site cations disorder and the dielectric properties have been considered in the past.(3)

Organic photovoltaic solar cells based on metal-dye or sensitized dye-SnO2 junctions are formed. The electrical and photovoltaic characteristics of these Schottky junctions have been studied on two pure and sensitized dyes (Eriochrome Blue Black B and Rodamine B). From the photovoltaic action spectra the active region responsible for electric power generation was found to be confined to the SnO2-dye interface. The effect of sensitization, electrode material and intensity on photovoltaic and electrical parameters has also been discussed in detail. Finally theC-V characteristics are discussed in detail.

High permittivity and varying degrees of losses in diffused phase ferroelectrics negate the use of conventional techniques for their study at microwave frequencies. Therefore, frequency difference and quarter wave plate matching techniques have been employed. Results are presented on lead-magnesium niobate-lead titanate (PMN-PT) solid solutions of several compositions. It has been suggested that partial charge compensation at the grain boundaries on solid dilution may play a major role on the dielectric properties.

The variation of magnetic permeability of substrate on Photonic Band Gap (PBG) has been studied for microstrip type periodic metallic structure and the experimental findings will be presented and discussed. Periodic structure was carved out in the metallic foil of 18 micron thickness and was put on the composite of magnetic and dielectric substrate. As the dielectric constant of the substrate affects the band gap for the photons in the microwave region, the combined effect of magneto-dielectric substrate have been studied here for different combination of ferrite materials with different composition and different sintering temperature. The substrate of Ni-Zn ferrite was prepared on the Perspex sheet of desired dimensions. The behavior of variation of band gap was also been studied for the air as dielectric material of the substrate. We found a well defined PBG and the band gap increases and transmission loss decreases in the microwave region with appropriate combination of magnetic and dielectric substrate. Thus it could be concluded that the dielectric constant viz. a viz. magnetic permeability plays an important role in the formation of broad band photonic materials for the microwave applications such as filters, antennas, frequency selective surfaces etc. Further work is going on to fabricate the patch antenna on the PBG embedded ground plane.

There has been some emphasis recently on the optical properties of glassy and amorphous semiconductors doped with small amounts of rare earth ions [1–8]. These glasses are potential materials for lasing action. This arises from the fact that the electrons responsible for the spectral and magnetic properties of rare earth ions are 4f electrons, which are very effectively shielded from interaction with external forces by the overlaying 5S and 5P shells. Therefore their energies and the energies of the f–f transitions are not likely to be significantly affected by external fields. Some features of the band structure of glasses can be understood by an analysis of the optical absorption edge. The electronic states of the glasses interact with the incoming radiation and give rise to optical absorption. Doping of V2O5–P2O5 glasses by different rare earth oxides does not show any significant change in optical ‘‘band gap’’ Eopt (it varies from 2.20 eV to 2.37 eV only) [7]. This slight difference in calculated Eopt values for different dopants is a consequence of small differences in the observed position and slope of the absorption edge. A survey of the literature indicates that rare earth oxide glasses have not been studied for their optical band gap and related properties [9–11]. Ahmed et al. [4] studied the optical energy gap of praseodymium phosphate glass. They analysed their data on the basis of modern theoretical models of amorphous solids. There are no other reports on rare earth oxide glasses. Thus it was of some interest to study the nature of the optical energy gap of rare earth doped glasses and to see the effect of composition on the position of the absorption edge and the value of the so-called optical energy gap. The present communication reports such studies on Pr-doped B2O3– P2O5–BaO glasses. Borophosphate glasses of final composition B2O3–P2O5–BaO in the ratio of 40, 45 and 15 mol % were prepared from analytical reagent grades of H3BO3, NH4H2PO4 and BaCo3 in 10 g batches. The praseodymium oxide added to the glass was Pr6O11 (99.99%). The chemicals were obtained from E. Merck (India) Ltd, Glaxo India Ltd and Aldrich Chemical Company, USA, respectively. Batches containing 0.1, 0.3, and 0.5 mol % of Pr in the base glass materials were mixed in an agate pestle mortar for 30 min, and were thermally treated in an alumina crucible up to 1000 8C in steps of 100 8C starting from 300 8C. A hold time of 6 h was given at each temperature step. Homogeneity of the melt was ensured by stirring the melt with an alumina rod from time to time. The melt was quenched by pouring into a rectangular-shaped depression in a copper plate. Glasses contained no crystalline phases as revealed by an X-ray diffractogram of the specimen. The glass samples were polished using cerium oxide power. The absorption spectra were recorded using Perkin Elmer Lambda-4B UV/VIS Spectrophotometer in the wavelength range 200–900 nm. Four sharp absorption peaks at 445.5, 470.5, 482.8 and 591 nm were observed in the visible region. Fig. 1 shows the optical absorption spectra for several compositions of Pr-doped borophosphate glasses. On the basis of a comparison of the present spectra with the work already reported [1–3], the absorption peaks observed at 442.0, 469.6, 481.8 and 587.8 nm may be assigned as arising due to the transitions H4? P2, H4? P1, H4? P0 and H4? D2, respectively. This assignment of peaks similar to the ones existing in the literature for different materials containing Pr ions, is justified if we remember that f–f transitions are only weakly affected by the environment of the praseodymium ion [2, 12, 13]. This also explains the observed sharpness of the peaks. In crystalline materials, the Eopt value is directly obtained from the absorption. However, for amorphous materials in general and oxide glasses in particular, the optical absorption at the fundamental edge may be used to estimate Eopt values using a theory due to Davis and Mott [14]. The absorption in amorphous semiconductors obeys a quadratic relation for the inter-band non-direct transitions

Investigations of the possible processes of charge transport common to ferroelectric ceramics and high-T/sub c/ superconductors are reported. Some potential experimental techniques for studying ferroelectrics and superconductors are discussed. Some common intrinsic features of these materials are highlighted. Results on dielectric behavior, microwave absorption, and surface impedance of some of these materials are discussed. >

This paper presents a rapid and novel technique of growing nanowires of Si0.75Ge0.25 alloy at 900 °C in less than 10 min by processing the constituents in a TE011 single mode cylindrical resonant cavity, operated at 2.45 GHz and ∼300 W. The microstructural, crystal structural, and x-ray photoelectron spectroscopic studies of the nanowires have been carried out to establish their dimensions, crystallographic structure, and composition, respectively. It is proposed that the growth of nanowires is due to electromagnetic field assisted morphological transformation.

A unique cavity perturbation method has been used to study the dielectric behavior of chromium oxide (Cr2O3) in the microwave region at ∼3.5GHz and temperatures up to ∼1100°C. The specimen in the form of pellet was heated by microwave power at 2.45GHz in e-field of a single mode cavity. In the temperature range of measurement, the dielectric constant is found to increase within 10% and loss factor within one and a half times the room temperature values. Results are discussed in the light of agitated ionic motion in the material.

Changes in magnetic parameters of nickel zinc ferrite (NZF) ceramics have been monitored in situ during microwave H heating using an indigenous technique developed in the laboratory. The technique utilizes the concept based on combining a small measurement signal with a high power-processing signal, which permits a study on materials heated either in an electric or in a magnetic field of a TM103 single mode cavity. Coupling and decoupling of these two signals are suggested by the use of a simple arrangement. Experimental results obtained on various compositions of NZF ceramics up to 1200°C are presented. The results indicate that the higher zinc end compositions of the ferrites demonstrate the phenomenon of decrystallization under the influence of microwave H.

Transparent strontium titanate thin films were prepared on fused silica substrate by spin-on sol–gel process using strontium 2-ethyl hexanoate and titanium isopropoxide as precursors in ethanol medium. The lattice constants of crystalline films were a=b=c=3.91 Å. Optical constants of the films in amorphous and polycrystalline forms were measured in the wavelength range of 300 nm to 900 nm with the aid of transmission spectra. The observed band gaps for amorphous and crystalline films were 4.07 eV and 3.62 eV respectively. The refractive index (n) and extinction coefficient (k) at 610 nm for the amorphous films are 1.72 and 0.0022 and for the crystalline films the corresponding values are 1.88 and 0.0062. Packing density of the amorphous and crystalline films were 0.65 and 0.75 respectively. The polycrystalline films have a lower n and k in a 350–850 nm wavelength range and higher band gap as compared to single crystal data. The dispersion data of the refractive index is interpreted with the single oscillator model. Using this model, the values obtained for oscillator strength (S 0) and oscillator energy (ε0) were 0.64 ×1014 m-2 and 7.31 eV respectively for the amorphous film and 0.43 ×1014 m-2 and 5.69 eV for crystalline films.

Polycrystalline PbTiO3 thin films having layered perovskite structure were fabricated by a sol–gel (spin coating) processing method. The substrates used were fused silica and stainless-steel. Surface morphology of the films was studied with scanning electron microscopy (SEM). The effect of post-deposition annealing on the electrical and structural properties were analyzed. Crystalline phase of PbTiO3 can only be obtained when the annealing temperature is higher than 500°C. The electrical measurements were conducted on metal–film–metal capacitors. Films annealed at 600°C exhibited crystallinity with perovskite phase, a dielectric constant of 132 and a dissipation factor of 0.039 at 1 kHz. I-V characteristics were found to be ohmic at low fields and space charge limited, controlled by traps at high fields. Transmission spectra of the films deposited on fused silica were recorded and from this, refractive index, extinction coefficient and thickness of the films were calculated. The standard deviation of the thickness was found to be 18.8 nm. The dispersion curve for the refractive index `n' is fairly flat beyond 500 nm and rises rapidly towards shorter wavelength region, showing the typical shape of dispersion curve near an interband transition. The refractive index was found to be in the range 2.00–2.20 for different wavelengths (300 to 900 nm).

ac magnetic susceptibility measurements is one of the processes by which the superconducting properties of mixed oxide superconductors can be characterized. The study of the superconductors at an externally applied magnetic field predicts the existence of two superconducting transitions due to: (i) the one reflecting the intragrain properties and (ii) the other describing intergrain interactions which might be assumed as due to Josephson tunneling. In this abstract, we report our work on the ac susceptibility measurements in PbxBi2−xSr2Ca2Cu3O10 superconductors to study the real part of χ’ as a function of temperature, Pb concentration, and sintering time. The results have been compared with the x‐ray diffraction (XRD) results. The material preparation and the characterization techniques used would be discussed. The XRD and the susceptibility measurements indicate that too high Pb concentrations and too low sintering time does not seem to improve the high Tc phase. The weak link coupling phase determined from the χ’‐T dependence is strongly dependent on both the Pb concentration and the sintering time. The transition temperature for the low Tc phase decreases with sintering time and increases with the increasing Pb concentration. The results would be discussed in the light of the best superconducting properties achieved by variation of the material preparation.

Measurements are reported of the complex dielectric permittivity, e(ω), of single crystal tri-glycine sulphate (TGS) at temperatures on either side of the ferroelectric Curie temperature, Tc, covering the frequency range ω/2π = 10-105 Hz. By contrast with the reported strong dispersion in the GHz region, the dielectric loss and the real part of the susceptibility, x(ω) - e(ω) — e∞, follow the empirical relation χ'(ω) ∞χ'(ω) ∞ ωx-1 which gives a frequency-independent ratio χ'(ω)/χ'(ω) = cot(ηπ/2) in conformity with the response of most other solid dielectrics. The physical significance of this result in relation to the interpretation of the polarization mechanisms in ferroelectrics is discussed.

The photovoltaic and rectification properties of CdS- and AgBr-sensitized Eriochrome Black T dye solar cells have been studied. The dependence of the short-circuit current and the open-circuit voltage on light intensity and electrode material are examined and the variations with electrode material are explained on the basis of the built-in potential developed at the metal-semiconductor interface. Conversion efficiency, fill factor, diode factor and reverse saturation current are also calculated for each cell.

A technique suitable for the measurement of electrical properties of films/sheets at microwave frequencies has been described. Groups of materials are identified for which the absence of any kind of electrodes in these microwave measurements can be utilised with advantage. Results are presented for a variety of materials such as metal films, superionic conductors, semiconductors and insulating polymers.

A microwave measurement technique has been developed for the characterization of high dielectric constant (100 to 10,000), and high dielectric loss (0.2 to 0.7) materials. The method is based on the frequency dependence of electromagnetic wave transmission through a dielectric slab. It is a useful technique for the dielectric measurement of relaxor ferroelectric materials, and an example is given for the Pb(Mg1/3Nb2/3)O3-PbTiO3 system. The rapid developments in network analysis have made this measurment method possible.

The theory of dark conduction in dyes assumes thermal excitation from the ground state to an excited state, followed by trapping of the hole near the electrode. Electrical conductivity studies on crystal violet (CV), an organic dye of industrial importance, have been carried out in both dark and light conditions. The present paper deals with the effect of field, temperature and electrode materials on the electrical conduction in CV as well as in CV sensitised with AgI. These studies suggest that conduction is mainly due to the charge carriers injected from the electrode into the dye (Schottky-Richardson mechanism) and the electron transfer mechanism. Photo and dark activation energies in the sensitised dye have been calculated.