Effect of atmosphere on the PTCR characteristics of porous Ba(Ti,Sb)O3 ceramics produced by adding corn-starch

Porous Ba(Ti,Sb)O₃ ceramics were fabricated by adding corn-starch at 20 wt %. The effect of atmosphere on the PTCR characteristics of the porous Ba(Ti,Sb)O₃ ceramics and the role of oxygen on the grain boundaries in the PTCR characteristics of the Ba(Ti,Sb)O₃ ceramics were investigated. In air, O₂, N₂, and H₂ atmospheres, the electrical resistivity of Ba(Ti,Sb)O₃ ceramics below 150 °C was independent of atmosphere, while it was strongly dependent on atmosphere above 200 °C. The low electrical resistivity in reducing atmospheres was due to a decrease in potential barrier height, which originated from an increase in the number of electrons owing to the desorption of chemisorbed oxygen atoms at the grain boundaries. In a N₂ atmosphere, the electrical resistivity of Ba(Ti,Sb)O₃ ceramics during the cooling cycle was lower than that during the heating cycle, and then the electrical resistivity of the porous Ba(Ti,Sb)O₃ ceramics during subsequent heating and cooling cycles was increased again by exposure to an O₂ atmosphere.     

Effect of partially oxidized Ti powders on the microstructure and PTCR characteristics of (Ba,Sr)TiO3

Rutile TiO₂ (a=4.594 å and c=2.958 å) phase was formed on the outer region of Ti powders after oxidation at 600 °C for 1–300 h. Porous (Ba,Sr)TiO₃ ceramics were fabricated by adding partially oxidized Ti powders (4–8 vol %) into (Ba,Sr)TiO₃ powders, and showed excellent positive temperature coefficient of resistivity (PTCR) characteristics after paste-baking treatment at 580 °C in air. The PTCR characteristics of the porous ceramics were mainly attributed to the adsorption of oxygen at the grain boundaries. The microstructure and electrical properties of the porous (Ba,Sr)TiO₃ ceramics containing the partially oxidized Ti powders oxidized at 600 °C for different oxidation times (1–300 h) were investigated.     

Influence of the additives and processing conditions on the characteristics of dense SnO2-based ceramics

Three different SnO₂-based powder mixtures, containing 2 wt% CuO as sintering aid and Sb₂O₃ in amounts from 0 to 4 wt% as activator of the electrical conductivity, were sintered to high density at temperatures in the range 1000–1400°C and soaking times from 1 to 6 h. Densification behaviour and microstructure development are strongly dependent on the presence of CuO, that gives rise to a liquid phase, and on Sb₂O₃ that retards the liquid phase formation and hinders grain growth. Cu and Sb cations can enter s.s. in the SnO₂ network with different oxidation states and in different positions, depending on the sintering conditions. The characteristics of the grain boundary phase, of the SnO₂ solid solutions and their modification depending on thermal treatments were analyzed. The electrical resistivity values varied in a wide range from 10^–1 to 10⁴ cm, depending on starting composition and processing conditions: in terms of the final density and of the electrical conductivity, the optimal sintering conditions were found to be 1200°C, for 1–3 h. The electrical resistivity was related to the microstructural features, particularly to the characteristics of the resulting SnO₂-based solid solutions.     

A structural model of Cr/(Ba,Pb)TiO3 positive temperature coefficient composite

The over-current protector is one of the main applications of the positive temperature coefficient (PTC) thermistor. Low room-temperature resistance and a PTC effect are required for the use of the over-current protection. As a result, lowering the room-temperature resistivity of PTC materials becomes very important. From a Japanese patent, the method of adding metal to BaTiO₃-based PTC ceramics to form composites has shown good results. But in recent publications, few papers were related to this area. Furthermore, in the limited literature, the resistance–temperature curve of the material expressed a negative temperature coefficient (NTC) effect when metal was present. In the present work, chromium (Cr), was added to (Ba,Pb)TiO₃ ceramics to form PTC composite with higher Curie temperature (T _C = 180°C). Under a given composition and method, the prepared composite had low room-temperature resistivity (p = 1.33cm) and PTC effect (P_max/P_max = 10). From the experimental results obtained, a structural model of the composite is proposed. The co-function of metal and ceramics, and sintering atmosphere factor on the PTC effect are discussed in this model. By employing this model, the resistance–temperature properties of the composites can be explained satisfactorily.     

Characterization and electrical properties of sol-gel synthesized (Sr, Pb)TiO3 powders

Y-doped (Sr, Pb)TiO₃ powders were prepared by a sol-gel route as well as the calcination of gel precursors. The results of DTA/TG showed that the thermal decomposition of dry precursors mainly occurred below 600°C. Meanwhile, infrared ray (IR) spectrum meter, X-ray diffraction (XRD) meter and transmission electron microscope (TEM) were used to characterize the synthesized powders, respectively. Using the synthesized powders as starting materials, Sr_0.5Pb_0.5TiO₃ semiconducting ceramics were fabricated at 1050°C. Sample's room temperature resistivity is 1.51 × 10² · cm, its resistivity jumps more than 5 orders of magnitude above the Curie temperature (T _c). With increasing the soaking time, the room temperature resistivity and the negative temperature coefficient of resistance (NTCR) effect below T _c increased, showing the electrical properties of (Sr, Pb)TiO₃ thermistors are obviously affected by PbO loss.     

Processing characteristics of PTCR ceramics with low sintering temperature

The processing behavior of PTCR ceramics of (Ba,Sr,Ca,Pb)TiO₃ solid solution composition with additives of lanthanum oxide (La₂O₃) and boron nitride (BN) was studied. The ceramics can be sintered at temperatures as low as 1100 °C and possess rather low room-temperature resistivity with good PTCR effect. The sample ball milled with de-ionized water exhibits a more uniform microstructure compared to the sample ball-milled with alcohol. Particle size of less than 1 m was found to be adequate for preparing the ceramics and the finer particles (0.45 m) do not significantly improve the PTCR behavior. The performance of the PTCR sample is not sensitive to the sintering parameters such as the sintering time and cooling rate. This may be ascribed to the presence of excess BaO in the sample and the low sintering temperature, thereby eliminating the effect of Ba ion vacancies on the properties of the PTCR sample.     

Electrocal,thermal, thermoelectric and related properties of magnesium silicide semiconductor prepared from rice husk

Polycrystalline, 10m size magnesium silicide was prepared by alloying 99.9% purity polycrystalline silicon obtained from rice husk ash and high-purity magnesium powder. The material in sintered pellet form was characterized for its structural, electrical, thermal, thermoelectric and other properties. A typical sintered pellet exhibited a room-temperature (30°C) thermoelectric power of 565 V K^–1 and an electrical resistivity of 35 cm. On the other hand, the material was found to be thermally quite stable up to 650°C with a room-temperature thermal conductivity of 6.3×10^–3cals^–1cm^–1K^–1 (2.6 J s^–1 m^–1 K^–1). These properties of the material indicate that the material can find potential applications as a thermoelectric generator and in other semiconductor devices. Furthermore, an indigenous technology for large-scale production of silanes (SiH₄) can be developed using this Mg₂Si which could be prepared in large quantities by a simple and low-cost process.     

Electrical Resistivity of La0.7Sr0.3MnO3 Ceramics Prepared via Chelate Homogenization

La_0.7Sr_0.3MnO₃ ceramics are prepared from powders produced via gelation and/or microwave processing of solutions of polynuclear chelates (La, Sr, and Mn diethylenetriaminepentaacetates), and their electrical resistivity is measured as a function of temperature. As the sintering temperature is raised from 800 to 1100°C, the average grain size of the ceramics, evaluated by the Debye–Scherrer method, increases by about a factor of 2.5 and their resistivity drops by about two orders of magnitude. The effect of the sintering temperature on the average grain size depends very little on the preparation procedure. For some of the samples, the room-temperature weak-field magnetoresistance is determined.     

Liquid-Phase Sintering and Properties of Si3N4–TiN Composites

Densification during liquid-phase sintering of Si₃N₄–TiN was studied in the presence of Y₂O₃. The content of TiN was varied from 0–50 mass%. During the densification Y-silicate was formed. The amount of silicate increased with both decreasing fraction of TiN and increasing isothermal heating time. Density, fracture toughness, and electrical resistivity were measured as a function of TiN content. It was found that the density and fracture toughness increased with increasing TiN content. The electrical resistivity drops drastically, from 10¹⁰ m for sintered Si₃N₄ to 10^–3 m for sintered Si₃N₄–TiN composite containing 30 vol% TiN.     

Preparation and properties of PTCR ceramics with low resistivity sintered at low temperature

Two types of positive temperature coefficient of resistance (PTCR) ceramics with relatively low room-temperature resistivity were prepared using the four-component system (Ba, Sr, Ca, Pb)TiO₃ with lanthanum oxide (La₂O₃) as the donor dopant and boron nitride (BN) as the sintering aid. The first type of materials was sintered at 1080 °C for 2 h. It has a Curie point , a room-temperature resistivity of 58 cm and a resistivity jump of around . For the second type of materials that were sintered at 1100 °C for 20 min, , and . These PTCR ceramics are considered to be suitable materials for fabricating multilayer PTCR devices by the co-firing process. Factors associated with the composition that influence the PTCR property of the materials are discussed.     

Soft chemical synthesis and characterization of lithium nickel oxide electrode materials

Soft chemistry was used to prepare ordered nanophase Li_0.6NiO₂ electrode materials by completing the oxidation of Ni²⁺ to Ni³⁺ and/or Ni⁴⁺ species with H₂O₂ oxidant during solution reactions at 50–60 °C and evaporation at 105–150 °C rather than during sintering. Both elemental analysis and electron spectroscopy for chemical analysis (ESCA) results indicate that oxidation was completed. The deconvoluted ESCA spectra of nickel ions exhibited a semi-quantitative ratio of Ni⁴⁺Ni³⁺=6040 which presented no significant change with increase of sintering time. After sintering for up to 11 h at 700°C, ordered Li_0.61Ni_0.96O_2.0 ceramics were formed (R3m, a ₀=0.2837 nm, c ₀=1.417 nm). Distribution of the crystallite size was in the range of 80–200 nm. As sintering times were increased, the crystallite shapes exhibited a more distinct morphology, and the ordering degree of the cations was enhanced, while the conductivity was sharply enhanced up to 2.0×10^{–1 –1} cm^–1 at 30 °C. Compared to conventional ceramic and solution methods, the ordered nanophase Li_0.61 Ni_0.96O_2.0 ceramics was obtained at 700 °C with shorter sintering times ( 11 h).     

Oxidation resistance and electrical properties of silicon carbide added with Al2O3, AlN, Y2O3 and NiO

Because of its excellent thermal, mechanical and electrical properties silicon carbide is widely used for heating elements. Nevertheless these elements are affected by electrical ageing (increase of electrical resistivity during use). This phenomenon is generally attributed to oxidation but no satisfactory answer has been presently found to reduce its effects. The aim of this study is to obtain a better understanding of the degradation of the electrical properties through the oxidation behavior of hot pressed samples containing various amount of additives. Eight dense SiC ceramic samples with Al₂O₃, AlN, Y₂O₃and NiO additives were prepared by hot pressing. The influence of these additives on sintering, oxidation behavior and electrical properties was evaluated. Formation of an yttrium garnet phase leads to liquid phase sintering but decreases the oxidation resistance. The dependence of electrical resistivity with temperature can be explained by the presence or not of a metallic phase formed between Ni and Si. This secondary phase permits a low (< 5 · cm) and almost constant value of the electrical resistivity from ambient temperature up to 950°C to be obtained.     

Effects of junction formation conditions on the photovoltaic properties of sintered CdS/CdTe solar cells

Microstructures and properties of sintered CdS films on glass substrates and sintered CdTe films on polycrystal CdS substrates have been investigated. The CdS films, which contained 9 wt % CdCl₂ as a sintering aid and were sintered at 650° C for 1 h in nitrogen, are transparent and have an average grain size of 15m and an electrical resistivity of 0.5cm. The CdTe films, which were coated on the sintered CdS substrate and were sintered above 610° C for 1 h in nitrogen, have a dense structure with an average grain size larger than 5m. All polycrystal CdS/CdTe solar cells were fabricated by this successive coating and sintering method. The sintering temperature of CdTe films on the sintered CdS films was varied from 585 to 700° C. Compositional interfaces and p-n juctions are formed during sintering. The highest solar efficiency (7.18%) was found in a solar cell made by sintering the composite layer of glass-CdS-CdTe at 625° C for 1 h. A fabrication temperature below 610° C resulted in poor solar cell efficiencies due to the porous structure of the CdTe films and above 650° C also resulted in poor efficiencies due to the formation of a CdS_1-x Te_x layer at the interface and a large p-n junction depth.     

Preparation of Barium Strontium Titanate Ceramic by Sol-Gel Method and Microwave Sintering

The (Ba,Sr)TiO₃ amorphous gel was prepared by sol-gel process and calcined in the 2.45-GHz multimode microwave furnace to synthesize (Ba,Sr)TiO₃ nanopowder. The calcination temperature of the (Ba,Sr)TiO₃ ceramic powders that convert the material into prevoskite phase can be reduced from 1100°C to 900°C, the nanopowder displays the highest sinterability. Using a new kind of insulator materials made of MgAl₂O₄–LaCrO₃, the crack-free and dense (Ba_0.80Sr_0.20)TiO₃ ceramics with fine grain size (<1 µm) were prepared by microwave sintering at 1310°C for 15 min. The fine (Ba,Sr)TiO₃ ceramics sintered by microwave sintering technique display lower dielectric loss than that of conventional samples, indicating a reduction of the influence of defects with the microwave process.     

Microstructure and mechanical properties of hot-pressed SiC-TiC composites

Hot-pressed SiC-TiC composite ceramics with 0–100 wt% TiC have been investigated to determine the effect of composition (amount of TiC) on the elastic modulus, hardness, flexural strength and fracture toughness,K _IC. The composites exhibited superior mechanical properties compared to monolithic SiC and TiC, especially in fracture toughness,K _IC, value for 30–50 wt% TiC composite. The maximum values ofK _IC and room-temperature flexural strength were 6 MPa m^1/2 for a 50 wt % TiC and 750 MPa for a 30 wt% TiC composite, respectively. The observed toughening could be attributed to the deflection of cracks due to dispersion of the different particles. Although no third phases were detected by both TEM and XRD studies, an EDAX study and resistivity measurements indicated some possibility of solid solutions being present. The composites containing more than 30 wt% TiC, exhibited resistivity lower than 10^–3 cm which is favourable for electro-discharge machining of ceramics.     

Nonlinear electrical characteristics and dielectric properties of (Ca, Ta)-doped TiO2 ceramics

TiO₂ ceramics doped with 1.0 mol% Ca and different concentrations of Ta were obtained by sintering processing at 1450°C. The microstructures, nonlinear electrical behavior and dielectric properties of the ceramics were investigated. The samples have nonlinear coefficients of = 2.0–5.0 and ultrahigh relative dielectric constants which is up to 10⁵. Especially, the effects of Ta dopant on the nonlinear electrical characteristics and dielectric properties of the (Ca, Ta)-doped TiO₂ ceramics were studied in detail. When the concentration of Ta is 2.0 mol%, the sample exhibits the highest nonlinear coefficient and a comparatively lower dielectric constant. By analogy to a grain-boundary atomic defect model, the effects of Ta and the nonlinear electrical behavior of the TiO₂ system were explained.     

Electrical and magnetic properties of Ni0.8Zn0.2Fe2O4/silica composite prepared by sol-gel method

Ultrafine Ni_0.8Zn_0.2Fe₂O₄ particles dispersed in silica (SiO₂) matrix are produced by sol-gel method. The powders were subjected to X-ray diffraction to confirm the formation of crystalline phases. The physical properties such as bulk density, true density, % of porosity and % of linear shrinkage were studied. The magnetic permeability as the function of frequency from 1 kHz to 13 MHz and temperatures from room temperature to 300°C were studied for samples sintered at temperature 1250°C. The AC electrical resistivity as the function of frequency and DC electrical resistivity as the function of temperature were studied. The AC-resistivity of the order of 10⁵ cm and DC-resistivity 10⁸ cm were obtained at room temperature. Microstructural features of sintered samples show the presence of ferrite grains of 1–2 m size.     

Preparation and Physical Properties of Ag<Subscript>5</Subscript>Pb<Subscript>2</Subscript>O<Subscript>6</Subscript>-Ag Ceramics

A new process for Ag₅Pb₂O₆ synthesis is described. Ag₅Pb₂O₆-Ag ceramics are prepared and characterized using x-ray diffraction, thermal analysis, and scanning electron microscopy. The room-temperature resistivity of Ag₅Pb₂O₆ ceramics is 0.35–0.37 m cm, and that of Ag₅Pb₂O₆-Ag ceramics is 0.03–0.04 µ cm.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 5, 2005, pp. 628–634.Original Russian Text Copyright © 2005 by Akimov, Savchuk.     

Nanosized hydroxyapatite powders derived from coprecipitation process

Nanosized hydoxyapatite (Ca₁₀(PO₄)₆(OH)₂ or HA) powders were prepared by a coprecipitation process using calcium nitrate and phosphoric acid as starting materials. The synthesized powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Brunauer-Emmett-Teller (BET) specific area measurment techniques. Single phase HA, with an average grain size of about 60 nm and a BET surface area of 62 m²/g, was obtained. No grain coarsening was observed when the HA powders were heated at 600°C for 4 hours. HA ceramics were obtained by sintering the powders at temperatures from 1000°C to 1200°C. Dense HA ceramics with a theoretical density of 98% and grain size of 6.5 m were achieved after sintering the HA powders at 1200°C for 2 hours. HA phase was observed to decompose into tricalcium phosphate when sintered at 1300°C. The microstructure development of the sintered HA ceramics with sintering temperature was also characterized and discussed.     

X-ray photoelectron spectroscopy and electrical properties studies of La2O3-doped strontium titanate ceramics prepared by sol-precipitation method

X-ray photoelectron spectroscopy has been used to investigate the existence of Ti³⁺ on the surface of La₂O₃-doped strontium titanate and to determine its surface characteristics. The surfaces, having Sr/Ti ratios significantly varying from the stoichiometric ratio, revealed the presence of carbon and suggested the presence of hydroxyl groups on the surface, whose concentration largely decreased in the bulk. Ti³⁺ species existed as a function of the sintering conditions and were detected on the surface of (La, Sr)TiO₃ sintered in air or in N₂ by natural cooling. These samples had a lower electrical resistivity, especially when sintered in a N₂ atmosphere. The surfaces of air oxidized SrTiO₃ and quenched from high temperature contained no detectable amount of Ti³⁺, resulting in higher resistivity. However, the N₂-sintered samples were dark blue in color and exhibited lower resistivity, semiconductivity, and lower valence oxidation state Ti existed when sintered above 1350°C.