Effect of potato-starch on the microstructure and PTCR characteristics of (Ba,Sr)TiO3

Porous semiconducting (Ba, Sr)TiO₃ ceramics were fabricated by the addition of potato-starch to the (Ba, Sr)TiO₃ powders. The effects of potato-starch on the microstructure and PTCR characteristics of the porous (Ba, Sr)TiO₃ ceramics have been investigated. The porosity largely increased and the grain size slightly decreased with increasing potato-starch content. The crystalline structure of (Ba, Sr)TiO₃ ceramics was independent of the potato-starch content. The (Ba, Sr)TiO₃ ceramics containing potato-starch showed higher PTCR (10⁶) characteristics than that of the (Ba, Sr)TiO₃ ceramics without potato-starch. It was found that the development of PTCR characteristic in the porous (Ba, Sr)TiO₃ ceramics containing various amounts of potato-starch, in the same way as normal BaTiO₃ ceramics without potato-starch, is associated with grain boundaries, from the impedance analysis.     

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.     

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 Isovalent Ba-Site Substitutions on the Properties of (Ba1 – x – y M y Y x )TiO3(M = Ca, Sr, Pb) PTCR Ceramics

An approach is proposed for fabricating fine-grained, low-resistivity BaTiO₃-based PTCR ceramics via partial isovalent substitutions on the Ba site. The grain size of the ceramics thus prepared is shown to decrease as the ratio of ionic radii r(Ba²⁺)/r(M²⁺) (M = Ca, Sr, Pb) increases. Isovalent substitutions on the Ba site narrow down the range of donor dopant (yttrium) concentrations in which PTCR materials can be prepared. The experimental results agree well with thermodynamic calculations under the assumption that the materials contain the Y³⁺Ti³⁺O₃ phase, as suggested by ESR data, which point to the presence of Y³⁺–Ti³⁺ associates. Partial calcium, strontium, and lead substitutions on the Ba site reduce the average grain size of PTCR ceramics, which is probably due to the lattice strain arising from the isovalent substitution. Partial replacement of Ba²⁺ with mixtures of different isovalent elements (e.g., Sr²⁺ and Pb²⁺) offers the possibility of obtaining fine-grained, low-resistivity PTCR ceramics, without changing the phase transition temperature.     

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.     

Dielectric properties of Sb2O3-doped Ba0.672Sr0.32Y0.008TiO3

Sb₂O₃-doped Ba_0.672Sr_0.32Y_0.008TiO₃ (BSYT) dielectric ceramics were prepared by conventional solid state method, and their dielectric properties were investigated with variation of Sb₂O₃ doping content and sintering temperature. The X-ray diffraction patterns indicated that all the BSYT specimens possessed the perovskite polycrystalline structure. The experimental results reveal that the introduction of Sb₂O₃ into Ba_0.672Sr_0.32Y_0.008TiO₃ can control the grain growth, reduce the relative dielectric constant and dielectric loss, shift the Curie temperature to lower temperature and significantly improve the thermal stability of the BSYT ceramics. The samples doped with 1.6 wt.% Sb₂O₃ sintered at 1320 °C for 2 h exhibited attractive properties, including high relative dielectric constant (> 1500), low dielectric loss (< 40 × 10^− 4), low temperature coefficient of capacitor(< ± 35%) over a wide temperature range from − 25 °C to + 85 °C.     

Effect of Bi2O3 and Y2O3 doping methods on electrical properties and PTCR behavior of Ba0.95Ca0.05TiO3 ceramics

Lead-free PTCR ceramics based on Bi₂O₃ and Y₂O₃ doped Ba_0.95Ca_0.05TiO₃ were fabricated by the conventional mixed oxide method, while Bi₂O₃ and Y₂O₃ were doped directly or after pre-calcining, in the molar ratio of Bi₂O₃:Y₂O₃ = 1:1. There were two synthesizing route, i.e. the materials were pre-calcined at 900 °C to obtain BiYO₃ firstly and then doped into the basic materials, and the materials were directly doped into the starting materials, both of which could obtain samples with different electrical properties and PTCR behavior. The samples were characterized by using X-ray diffraction, scanning electron microscope, dielectric constant-temperature and resistivity-temperature measurement instrument. It was revealed that the perovskite lattice, the microstructure and the PTCR behavior of Ba_0.95Ca_0.05TiO₃ varied with different doping contents and methods. A further research was conducted so as to study the electrical properties of ceramics by impedance spectroscopy.     

Effect of pore-forming agent on PTCR characteristics of <Emphasis Type="Italic">n</Emphasis>-BaTiO<Subscript>3</Subscript>

Porous n-BaTiO₃ ceramics are synthesized by the addition of pore-forming agent into the (Ba,Sr)TiO₃ powder. From the DTA-TGA analysis for samples containing the PEG and corn-starch, it was found that an exotherm occurred at 262 and 315°C, respectively, weight loss commenced at 165 and 252°C, and was virtually complete by 265 and 472°C, respectively. The porosity of n-BaTiO₃ ceramics increased and the grain size decreased with increasing the pore-forming agent. From the XRD analysis at high angles, all the samples with and without the pore-forming agent at room-temperature exhibit the tetragonal structure. PTCR jump of the samples containing pore-forming agent is 1-2 orders higher than that of sample without the pore-forming agent. It is also found that the development of PTCR characteristic in the porous n-BaTiO₃ ceramics containing pore-forming agent is related to grain boundaries, which basically equals that in ordinary BaTiO₃ without pore-forming agent, from the complex impedance results.     

Pyroelectric properties of arrayed BaTiO3 system thick film for uncooled IR detector

The Sr and Ca added to BaTiO₃ in order to shift transition temperature near room temperature. The donor (Yb₂O₃) and acceptor (MnCO₃) impurities were added to the (Ba,Sr,Ca)TiO₃ powder for the improvement of structural and electrical properties. The (Ba,Sr,Ca)TiO₃ powder was made by sol-gel method and the thick films were fabricated by screen-printing. We fabricated array type thick films. The 1 mm × 3 mm array thick films were arranged 2 × 8. Relative dielectric constant and dielectric loss of Yb₂O₃ 0.1 mol% doped (Ba,Sr,Ca)TiO₃ array thick film were 1068 and 2.8%, respectively at Curie temperature, 44 °C. Pyroelectric coefficient and F.M.D* showed 21.7 × 10⁻⁹ C/cm² K and 3.2 × 10⁻⁹ C cm/J, respectively.     

Compositional analysis and electrical properties of Sb,Mn-doped barium strontium titanate PTCR ceramics with TiO<Subscript>2</Subscript> and SiO<Subscript>2</Subscript> sintering additives

The secondary phase compositions in Sb-doped (Ba, Sr)TiO₃ ceramics containing SiO₂ and excess TiO₂ sintering additives have been examined by XRD, BEI and EPMA techniques. It is shown that alongside the primary (Ba_0.797Sr_0.2Sb_0.003)TiO₃ phase, (Ba_1.95Sr_0.05)₂(Ti_1.2Si_1.8)O₈ and Sb-doped (Ba_0.99Sr_0.01)₆Ti₁₇O₄₀ phases form at the intergranular regions. Substitution of up to 0.04 mol% MnO₂ enhances the PTCR effect, giving a ratio ρ_max/ρ_min ∼ 7 × 10⁵ for the optimum 0.04% MnO₂ composition. At this level Mn cannot be detected in the microstructure by EPMA, however in insulating samples containing 0.08 mol% of MnO₂, it was detected in the (Ba_0.99Sr_0.01)₆Ti₁₇O₄₀ intergranular phase.     

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.     

Effect of segregative additives on the positive temperature coefficient in resistance characteristics of n-BaTiO3 ceramics

The influence of B₂O₃, and Al₂O₃ as segregative additives in modifying the ρ–T characteristics has been studied in BaTiO₃ ceramics with positive temperature coefficient of resistance (PTCR). Reaction of Al₂O₃ at the grain boundary regions of BaTiO₃ ceramics leads to the segregation of the secondary phase, BaAl₆TiO₁₂ resulting in broad PTCR jump, whereas B₂O₃ addition gives rise to steeper resistivity jump. Microstructure studies by SEM reveal the formation of coherent second phase layer of barium aluminotitanate surrounding the BaTiO₃ grains. The EDX results shows varying Al to Ti ratio in the early stage of phase formation in BaAl₆TiO₁₂ resulting in electrically active layer around the BaTiO₃ grains. The TiO₂-excess melt formation results in lower resistivity for 2–4% Al₂O₃ containing n-BaTiO₃ ceramics whereas at higher alumina contents, BaAl₆TiO₁₂ phase becomes dominant leading to higher resistivity in the sample. Complex impedance analyses support the three-layer regions, corresponding to the contributions from grain interior resistance (R _g), grain boundary resistance (R _gb), and that from secondary phase (R _sec). Electron paramagnetic resonance spectroscopy (EPR) indicated barium vacancies, V _Ba ^/ as the major electron trap centers which are activated across the tetragonal-to-cubic phase transition. A charge trapping mechanism is proposed wherein the segregation of secondary phases bring carrier redistribution among the various acceptor states thereby affecting the electrical conductivity of n-BaTiO₃ ceramics. The presence of Al³⁺–O⁻–Al³⁺ or Ti⁴⁺–O⁻–Al³⁺ type hole centers at the grain boundary layer (GBL) regions results in charge redistribution across the modified phase transition temperature due to symmetry-related vibronic interactions resulting in broad PTCR characteristics extending to higher temperatures.     

蒸汽掺杂-一种新的钛酸钡基PTCR陶瓷的掺杂方法

晶界铲应是陶瓷材料所固的的特性,利用某些氧化物在高温下具有较高的蒸汽压,在烧成过程对陶瓷材料进行掺杂改性,可以有效地控制晶界行为,改善材料性能,钛酸钡基半导体陶瓷中存在的ＰＴＣＲ效应,是一种典型的晶界效应。     

Grain growth kinetic in xTiO2–6 wt.% Bi2O3–(94 − x) ZnO (x=0, 2, 4) ceramic system

The grain growth kinetics in the 0, 2 and 4 wt.% TiO₂ added ZnO–6 wt.% Bi₂O₃ system was studied using the simplified phenomenological grain growth kinetics equation together with the physical properties of the sintered samples. The grain growth exponent value (n) and the apparent activation energy for the ZnO–6 wt.% Bi₂O₃ system was 5 and 218 kJ/ mol, respectively. The addition of the TiO₂ to the ZnO–6 wt.% Bi₂O₃ system inhibited the ZnO grain growth. At 2 and 4 wt.% TiO₂ additions, the apparent activation energies were calculated as 467 and 346 kJ/ mol, respectively. The addition of TiO₂ to the system inhibited the grain growth of ZnO ceramics.     

Structure, infrared and Raman spectroscopic studies of new Sr0.50SbFe(PO4)3 and SrSb0.50Fe1.50(PO4)3 Nasicon phases

Two newly synthesised Sr_0.50SbFe(PO₄)₃ [Sr_0.5.] and SrSb_0.50Fe_1.50(PO₄)₃ [Sr.] phases were obtained by conventional solid-state reaction techniques at 1000 °C in air atmosphere. Their crystallographic structures were determined at room temperature from X-ray powder diffraction (XRPD) data using the Rietveld analysis. Both compounds belong to the Nasicon structural family. [Sr_0.5.] and [Sr.] crystallise in rhombohedral system with \textR[`3] {\text{R}}\overline{3} and \textR[`3] \textc {\text{R}}\overline{3} {\text{c}} space group, respectively. Hexagonal cell parameters for [Sr_0.5.] and [Sr.] are: a = 8.227(1) ?, c = 22.767(2) ? and a = 8.339(1) ?, c = 22.704(2) ?, respectively. Sr²⁺ and vacancies in {[Sr_0.50]_3a[□_0.50]_3b}_M1SbFe(PO₄)₃ are practically ordered within the two positions, 3a and 3b, of M1 sites. Structure refinements show also an ordered distribution of Sb⁵⁺ and Fe³⁺ ions within the Nasicon framework. Within the structure, each Sr_(3a)O₆ octahedron shares two faces with two Fe³⁺O₆ octahedra and each vacancy (□_(3b)O₆) site is located between two Sb⁵⁺O₆ octahedra. In [Sr]_M1Sb_0.50Fe_1.50(PO₄)₃ compound, all M1 sites are occupied by Sr²⁺ and the Sb⁵⁺ and Fe³⁺ ions are randomly distributed within the Nasicon framework. A Raman and infrared spectroscopic study was used to obtain further structural information about the nature of bonding in both selected compositions.     

Effect of the Distribution of Manganese Ions on the Properties of Mn-Doped (Ba,Y)TiO3 PTCR Ceramics

The electrical properties and microstructure of (Ba,Y)TiO₃ PTCR ceramics were studied. The results indicate that the Mn ions increase the intergranular barrier height and produce a high-resistance layer on the grain surface. The temperature-dependent resistances of the grain bulk, surface layer, and grain boundaries, the temperature coefficient of resistance, and the magnitude of the varistor effect were assessed as a function of Mn content.     

Preparation of Sb<Subscript>2</Subscript>S<Subscript>3</Subscript> film on functional organic self-assembled monolayers by chemical bath deposition

In this work, self-assembled monolayers (SAMs) of octadecyltrichlorosilane (OTS) were applied to induce the nucleation and growth of the antimony sulfide (Sb₂S₃) films on the functional ITO glass substrate at low temperature. The structure, morphology, and optical properties of the Sb₂S₃ films were investigated by X-ray diffraction, scanning electron microscopy, X-ray energy dispersive spectroscopy, and UV–vis spectroscopy. After thermal treatment at 200 °C for 1 h in air, the orthorhombic Sb₂S₃ was formed as a predominant phase in the deposited thin films. When the deposited films were thermally treated at 400 °C for 1 h in air, the orthorhombic Sb₂S₃ was decomposed and a cubic Sb₂O₃ was formed. The optical band energies of the as-deposited and thermally treated Sb₂S₃ films at 200 °C for 1 h in air and nitrogen were found to be 2.05 eV, 1.77, and 1.76 eV, respectively. As chemical templates, the OTS-functionalized SAMs played an important role in controlling the nucleation and growth of Sb₂S₃ films at low temperature. The results obtained from different preparation parameters applied in the present work will allow controlling the growth of the Sb₂S₃ films with uniform surface.     

Thickness and temperature effects on electrical resistivity of (Bi0.5Sb0.5)2Te3 thin films

Thin films of (Bi_0.5Sb_0.5)₂Te₃ of different thickness were deposited on glass substrate by the flash evaporation method in a vacuum of 1 × 10^− 5 Torr. X-ray diffraction and transmission electron microscope analysis indicates that these films are polycrystalline even in the as-deposited state and the post-deposition annealing leads to grain growth. Electrical resistivity studies were carried out on these films as a function of temperature (300– 450 K) and film thickness (450–2000 Å). Temperature dependence of electrical resistivity shows that (Bi_0.5Sb_0.5)₂Te₃ films are semiconducting. It is found that electrical conduction activation energy decreases with increase of film thickness and this observation is explained based on the Slater model. Thickness dependence of electrical resistivity is analyzed using the effective mean free path model of size effect with perfect diffuse scattering. This analysis leads to the evaluation of the important physical parameters i.e., mean free path and bulk resistivity of hypothetical bulk.     

The influence of firing profile and additives on the PTCR effect and microstructure of BaTiO3 ceramics

Nonstoichiometric BaTiO₃ PTCR type materials are investigated with various amounts of MnO₂, Sb₂O₃ and MgO dopants. Specimens fired with a nonisothermal rate-controlled sintering profile exhibit a rather fine and uniform microstructure as compared to those processed by conventional sintering techniques. The temperature at which the resistivity anomaly begins is observed to decrease with Sb₂O₃ and MgO contents. The Curie point of BaTiO₃-based ceramics can be altered by addition of Sb₂O₃, and the dielectric peak is maintained by the presence of MgO additive. Magnesium ions act as acceptors in the BaTiO₂ lattice, while antimony ions as donors. The presence of magnesium compensates some of the antimony, hence the doped-BaTiO₃ seminonductive region is pushed to higher contents.     

Effects of SiO2 and Cr2O3 on the formation process of ZnO varistors

The effects of SiO₂ and Cr₂O₃ on the formation process of ZnO varistors were investigated. Prior to formation of the Zn_2.33Sb_0.67O₄ spinel phase (Sp-phase), a spinel-like phase forms. However, this phase does not control the varistor microstructure. The Sp-phase and the Bi₂O₃-phase which were formed by the decomposition of the Bi₂(Zn_4/3Sb_2/3)O₆ pyrochlore phase played important parts in the control of the varistor microstructure. That is, the Bi₂O₃ phase produced in the reaction promotes the initial sintering of the varistor and the Sp-phase inhibits the ZnO grain growth. In this reaction, SiO₂ and Cr₂O₃ play a role in decreasing the decomposition temperature of the pyrochlore phase. Decreasing the decomposition temperature below 900° C (where ZnO grain growth begins) leads to the inhibition of ZnO grain growth.