Complete series of solid solutions exist in the mixed oxide pyrochlore system, Er₂³⁺ (V_1−x³⁺ Fe_x³⁺)_4/3 W₂^3/6+O₇, with the crystal structure changing from cubic to rhombohedral atx=0·25. The compounds are semiconductors at 300 K exhibitingp- orn-type behaviour depending on the value ofx. The observed resistivity and Seebeck coefficient data as a function of temperature have been interpreted in terms of a hopping mechanism.

High silica fabric with a silica content >98% can be prepared by removing the non-siliceous ions from the E-glass fabric using HCl as the leachant. The presence of Si-OH groups makes the material hydrophilic. The extent of moisture absorption depends on the extent of leaching. The moisture present in the leached fabric decreases the volume and surface resistivity values.

An overview of the semiconductor based photoelectrochemical (pec) cells for solar energy conversion is presented.pec cells are of two types: photoelectrolysis cells and photovoltaic cells. The principles involved, electrode and electrode/electrolyte interface characteristics, experimental methods of investigation and energy conversion efficiency are discussed in detail. Up-to-date data on variouspec cells are also presented and discussed.

A survey of the known highT_c ternary systems of compounds is presented. Special emphasis has been placed on the part played by the crystal structure and bonding in the exhibition of highT_c superconductivity. The superconducting and other physical properties of the ternary borides have been discussed in relatively more detail. Finally, the available guidelines from crystal and solid state chemistry points of view, have been reiterated for a search for new highT_c materials.

A series of ternary equiatiomic transition metal silicidesMM′Si (M=Zr, Nb, Ta;M′=Ru, Re) and Zr_1/2M_1/2 RuSi (M=Nb, Ta) have been synthesized and characterized. Guinier x-ray data clearly indicate that the compoundsMM′Si (M=Nb, Ta;M′=Ru, Re) crystallize with the orthorhombic TiFeSi-structure. This is contrary to one of the literature reports ascribing the hexagonal ZrRuSi structure toMReSi,M=Nb, Ta. ZrRuSi and Zr_1/2M_1/2RuSi (M=Nb, Ta), however, crystallize with the ZrRuSi structure. All the compounds are metallic and exhibit small and negative Seebeck coefficients. Superconductivity has been discovered inMM′Si,M=Nb, Ta;M′=Ru, Re and the transition temperatures, as measured by the low frequency a.c. susceptibility technique, range from 2·7 to 5·1 K. Zr_1/2Nb_1/2RuSi is not superconducting above 2·2 K. The observed properties are explained in terms of the electronic charge transfer effects and crystal structures adopted by these metal excess ternary silicides.

Thick films of the highT_csuperconducting oxides, LnBa₂Cu₃O₇, Ln = Eu, Y, have been fabricated by screen printing on alumina and SrTiO₃ substrates. Conditions for optimum superconductivity behaviour of the films have been established.T_c^onset varies from 90–94 K for all the films but zero resistance was observed only in a few cases.

A brief survey is presented of the recent developments and emerging trends in research in photoelectrochemical cell for solar energy conversion utilizing semiconductor electrodes.

PEC studies on the single crystals of the metal-cluster oxide compounds. A₂Mo₃O₈ (A = Zn, Mg, Fe), and polycrystalline LiYMo₃O₈ are reported. The photoresponse behaviour is attributed to the Mod-d transition. The photopotential, the photocurrent vs applied voltage and the wavelength data indicate thatn-Zn₂Mo₃O₈ is stable and possesses a small and indirect band gap of 1·55 eV and a direct band gap of 1·9 eV. With change in A ions in A₂Mo₃O₈, there is no significant change in the PEC properties. LiYMo₃O₈ is found to be ofp-type. PEC studies show that excepting for poor electronic conductivity, A₂Mo₃O₈ possesses all the requisitie characteristics of an ideal photoanode for PAE of water for trapping solar energy.

The oxygen-deficient phase of the highT_c superconductor, YBa₂Cu₃O₇, was oxygen-enriched using the fluidization technique to give good superconducting properties. The normal method of oxygen treatment at 900°C for 24 h and at 600°C for 24 h has been reduced to just one treatment at 600°C for 12 h by the fluidization technique to achieve almost the same strength of superconducting signal for the YBa₂Cu₃O₇ powder, which establishes the attractiveness of the latter route for the large-scale preparation of superconducting material. The particle sizes were in the range 0–90, 90–180 and 180–420 µm. The fluidized particles were crystalline with orthorhombic distortion.T_c^onset, estimated using the a.c. magnetic susceptibility method, was 91·3 K. The volume fraction of superconducting material in the product was 83·7–85·3%, one of the highest values reported so far for YBa₂Cu₃O₇.

The effect of oxide additives-CuO, SiO₂, Y₂O₃, Bi₂O₃ and ZnO in 1–10 mol% on the sintering and superconducting properties of YBa₂Cu₃O₇ was studied. SEM studies indicated improvement of grain size and interconnectivity due to the additives, the best results being obtained with Bi₂O₃, SiO₂ and Y₂O₃. The superconducting transition temperature is unaffected (92±2 K) even with 10 mol % of the additives. ZnO, however, decreases theT_c as expected.

The layer-type compound, Ca_0·85Sr_0·15CuO₂, which is the prototype of the cuprate family of highT_c oxide superconductors is an electronic insulator, and has a low ordered moment at 300 K indicating that Cu²⁺ ions are highly correlated. Doping with lithium (up tox=0·15) decreases the room temperature resistivity drastically (from 10³ ohm·cm to 0·6 ohm·cm) but metallic behaviour is not induced below 300 K. The magnetic moment on Cu²⁺ remains low but our studies did not show any antiferromagnetic ordering below 300 K in any of the Li-doped phases.

Superconductivity with a maximumT_{c, zero} of 58 K forx > 0·75 has been observed in a nominal starting composition, Th_xTl₂Sr₂Ca₂Cu₃O_y. X-ray data show that the 1212 phase gets stabilized due to part substitution of Th at the Tl-site forx⩾0·25; however,T_c is low forx<0·75.

The compounds CaREBaCu₃O_7−y (RE=La and Sm) are tetragonal at room temperature withT_c between 60 and 70 K. The single-phase compounds were prepared by solid-state reaction. The resistivity was measured by a four-probe technique in a continuous flow cryostat with the temperature being controlled to an accuracy of 10 mK. The resistivity vs temperature showed a break in slope around 180 K in CaLaBaCu₃O_7−y and around 220 K in CaSmBaCu₃O_7−y. The results were analysed for fluctuation conductivity from 180 K downwards. A plot of dρ/dT vsT showed a sharp peak atT_m=69·69 K for La compound and 66·00 K for the Sm compound. Detailed analysis of the resistivity in the regionT_on to 180 K was carried out using the procedure due to Veira and Vidal. The results are discussed in this paper.

Silver addition to the highT_c superconductor, YBa₂Cu₃O₇ (YBCO) is known to improve its mechanical properties and critical current (J_c) characteristics. We have synthesized the superconductor-metal composite YBCO-Ag by an electroless process at room temperature (using sucrose or hydrazine hydrate in an alkaline medium). The latter method produces a non-random coating whereby each grain of YBCO is coated uniformly by silver. The intrinsic highT_c of 90 K of YBCO can be recovered by compaction of the composite and subsequent heat treatment (900°C and 600°C) in oxygen.

Magnetic shields of various high-temperature superconductors, YBa₂Cu₃O_7−x (YBCO), YBa₂Cu₃O_7−x-Ag composites (random inclusions as well as non-random coatings) and Bi₂Sr₂Ca₂Cu₃O_x (BSCCO) were prepared by uniaxial as well as isostatic compression with various dimensions. The shielding properties were measured at 77 K for dc and ac magnetic fields in the range of frequencies from 100 Hz to 10 kHz. The critical penetration field (CPF), defined as the value of the applied magnetic field at which a detectable field was observed inside the cylinder, varied from cylinder to cylinder and also with the ageing of the cylinders in the case of YBCO shields. The highest value of CPF was 16 G at 77 K for YBCO shield prepared by isostatic compression. Even though the stability of BSCCO shields with respect to ageing is good, the CPF values are very low compared to those for YBCO. Detailed studies were performed in the case of YBCO shields. The CPF decreased as a function of time over a period of 90 days. The CPF decreased as the frequency of the applied field was increased. The wave form of the field inside the pot for a sinusoidal applied field was highly distorted and showed the presence of higher harmonics with appreciable amplitude. The wave form was Fourier-analysed to yield the field inside the shield along with the harmonics. The shields with Ag addition seem to give better performance at high fields.

The photoelectrochemical behaviour of oxide bronzes of the type A₆Nb₈Ti₂O₃₀ (A = Ba. Sr) has been studied. They show good photoactivity with photopotentials of the order of 700–800 mV. The flat band potential has been estimated to be − 0·9 V vs SCE. The band gap estimated from the spectral response is 3·1 eV.