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.     

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 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 secondary-phase segregation on the positive temperature coefficient in resistance characteristics of n-BaTiO3 ceramics

Modifications in the positive temperature coefficient in resistance (PTCR) of n-BaTiO₃ ceramics are brought about by specific additives such as Al₂O₃, B₂O₃ or SiO₂, leading to the segregation of secondary phases such as BaAl₆TiO₁₂, BaB₆TiO₁₂ or BaTiSi₃O₉ at the grain boundaries. Segregation of barium aluminotitanates resulted in broad PTCR curves, whereas B₂O₃ addition gave rise to steeper jumps and SiO₂ addition did not result in much broadening compared with donor-only doped samples. Microstructural studies clearly show the formation of a structurally coherent expitaxial second phase layer of barium aluminotitanate surrounding the BaTiO₃ grains. Electron paramagnetic resonance investigations indicated barium vacancies, V_Ba, as the major electron trap centres which are activated across the tetragonal-to-cubic phase transition according to the process V^X _Ba + e V_Ba. The grain size dependence of the intensity of the V_Ba signal indicated the concentration of these trap centers in the grain-boundary layer (GBL) regions. Further, the charge occupancy of these centres is modified by the secondary phases formed through grain-boundary segregation layers. BaAl₆TiO₁₂ gave rise to Al-O^– hole centres whereas no paramagnetic centres corresponding to boron could be detected on B₂O₃ addition. Such secondary phases, forming epitaxial layers over the BaTiO₃ grains, modify the GBL region, rich in electron traps, surrounding the grain core. The complex impedance analyses support this three-layer structure, showing the corresponding contributions to the total resistance which can be assigned as R _g, R _gb and R _{secondary phase}. The epitaxial second phase layers bring about inhomogeneity in the spatial distribution of acceptor states between the grain boundary and the grain bulk resulting in extended diffuse phase transition characteristics for the GBL regions in n-BaTiO₃ ceramics. This can cause the GBL regions to have different transition temperatures from the grain bulk and a spread in energy levels of the associated GBL trap states, thus modifying the PTCR curves. An attempt has been made to explain the results based on the vibronic interactions applied to the mid-band-gap states in n-BaTiO₃.     

Effect of a small amount of liquid-forming additives on the microstructure of Al2O3 ceramics

Sintering behaviour and microstructure of Al₂O₃ ceramics without additives and with 0.02–0.25 mol% CaO + SiO₂ (CaO/SiO₂ = 1) were investigated. When Al₂O₃ bodies were sintered at 1400 °C, the sinterability and the grain size decreased as the content of CaO + Si₂ increased. When Al₂O₃ ceramics with 0.05 – 0.25 mol% CaO + SiO₂ were sintered at higher sintering temperature, both CaO and SiO₂ reacted with Al₂O₃ to produce the liquid phase along grain boundaries, and exaggerated platelet Al₂O₃ grains, with an aspect ratio of about 4.5, were formed. Because the size of platelet grains decreased as the content of CaO + SiO₂ increased, the distribution of either SiO₂ particles or this intergranular phase of CaO – Al₂O₃ – SiO₂ might control the microstructure.     

Low-temperature sintering of SiC reticulated porous ceramics with MgO–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript> additives as sintering aids

SiC reticulated porous ceramics (SiC RPCs) was fabricated with polymer replicas method by using MgO–Al₂O₃–SiO₂ additives as sintering aids at 1,000∼1,450 °C. The MgO–Al₂O₃–SiO₂ additives were from alumina, kaolin and Talc powders. By employing various experimental techniques, zeta potential, viscosity and rheological measurements, the dispersion of mixed powders (SiC, Al₂O₃, talc and kaolin) in aqueous media using silica sol as a binder was studied. The pH value of the optimum dispersion was found to be around pH 10 for the mixtures. The optimum condition of the slurry suitable for impregnating the polymeric sponge was obtained. At the same time, the influence of the sintering temperature and holding time on the properties of SiC RPCs was investigated. According to the properties of SiC RPCs, the optimal sintering temperature was chosen at 1,300 °C, which was lower than that with Al₂O₃–SiO₂ additives as sintering aids.     

Microstructure and PTCR effect of La-doped BaPbO3 ceramics

La-doped BaPbO₃ ceramics were prepared by using BaCO₃ and PbO. Electrical properties and microstructure of the ceramics were studied. The results show a thin surface layer with a very low resistivity, and the interior of the ceramics with the characteristics of positive temperature coefficient of resistivity (PTCR). The PTCR behavior was related to the La content. The investigation by scanning electron microscopy and transmission electron microscopy revealed that the low resistivity of the surface layer was due to formation of a nano-size BaPbO₃ phase with metallic properties.     

Preparation and electrical properties of semiconducting strontium—lead titanate PTCR ceramics

A typical positive temperature coefficient of resistance (PTCR) effect in yttrium-doped (Sr, Pb)TiO₃ ceramics made by chemical processing was obtained for the first time. The results show that the room temperature resistivity is lower than 10² Ω cm and the resistivity jump is above 10⁶. The breakdown voltage is above 340 V mm⁻¹ (a.c.). The sintering temperature is about 1100°C, lower than for conventional BaTiO₃ ceramics. This revised version was published online in July 2006 with corrections to the Cover Date.     

Influence of the sintering process on the PTCR effect of chip-type Ba1.022–x Sm x TiO3 ceramics prepared by the reduction sintering-reoxidation method

We investigated the effects of sintering temperature and reoxidation annealing on the positive temperature coefficient of resistance (PTCR) effect of Ba_1.022–x Sm _x TiO₃ ceramics that were sintered at 1,180–1,260 °C for 30 min in a reducing atmosphere and reoxidized at 800 °C for 1 h. Results indicated that the room-temperature (RT) resistivity and resistance jump of the ceramics decreased with increasing sintering temperature; moreover, the samples exhibited a remarkable PTCR effect with a resistance jump of 3.3 orders of magnitude and achieved a low RT resistivity of 374.4 Ω cm at a lower sintering temperature. Furthermore, the higher grain-boundary resistivity of the ceramics obtained at a high reoxidation temperature after sintering at low temperature was estimated using an impedance analyzer. In addition, the voltage versus current behavior was investigated in present study.     

PTCR properties of Sb2O3-doped (Ba,Sr)TiO3

Perovskite barium–strontium titanate, (Ba,Sr)TiO₃ was prepared and effects of Sb₂O₃ additives on its PTCR properties were investigated. The (Ba,Sr)TiO₃ with 0.05∼0.25 mol% Sb₂O₃ showed semiconducting PTCR behavior and anomalous grain growth was also observed when sintered at 1360 °C. It was considered that charge compensation by doping Sb₂O₃ as well as anomalous grain growth by sintering leads to resistivity reduction from insulating to semiconducting transition.     

Effect of donor concentration on the PTCR behavior of Y-doped BaTiO3–(Bi1/2Na1/2)TiO3 ceramics

Lead-free positive temperature coefficient of resistivity (PTCR) ceramics of 99 mol%BaTiO₃–1 mol%(Bi_1/2Na_1/2)TiO₃ (BBNT1) and 90 mol%BaTiO₃–10 mol%(Bi_1/2Na_1/2)TiO₃ (BBNT10) were prepared by the conventional solid state reaction method. The effect of donor concentration on the microstructure and PTCR behavior of the BBNT ceramics were investigated. The results show that all BBNT1 and BBNT10 ceramics have formed a single perovskite structure with tetragonal phase when sintered in air or N₂. 0.2 mol% Y-doped BBNT1, sintered at 1,330 °C for 30 min in air, has room-temperature resistivity (ρ₂₅) of ~60 Ω cm and resistivity jump [maximum resistivity (ρ_max)/minimum resistivity (ρ_min)] of ~4.2 orders of magnitude with T_c about 150 °C. 0.4 mol% Y-doped BBNT10, sintered at 1,250 °C for 3 h in N₂, has ρ₂₅ of ~100 Ω cm and resistivity jump of ~4.7 orders of magnitude with T_c about 180 °C. The BBNT10 has higher breakdown voltage of 200 V/mm (a.c.) than that of BBNT1, making these BBNT10 ceramics promising candidates for high temperature lead-free PTCR materials.     

Positive temperature coefficient of resistance effects in BaPbO3/polyethylene composites

The positive temperature coefficient of resistance (PTCR) properties of BaPbO₃/polyethylene composites have been investigated as a function of the volume ratio of BaPbO₃ to polyethylene. The PTCR effect increased up to nine orders of magnitude greater than that of semiconducting BaTiO₃ ceramics. However, the PTCR effect decreased with increasing BaPbO₃ content. The PTCR effect was dependent on the tunnelling current in the BaPbO₃ intergrain gap. When the PTCR effect was measured repeatedly, the resistivity at room temperature increased, the PTCR effect decreased, and the transition temperature shifted to a lower value. These results could be explained by the thermal properties of polyethylene and the large structural change of composites in the repeated measurements.     

Effects of Kaolinite additions on sintering behavior and dielectric properties of CaCu3Ti4O12 ceramics

Commercial Kaolinite was employed as sintering aid to reduce the sintering temperature of CaCu₃Ti₄O₁₂ (CCTO) ceramics. The effects of Kaolinite content and sintering temperature on the densification, microstructure and dielectric properties of CCTO ceramics have been investigated. The density characterization results show that the addition of Kaolinite significantly enhanced the relative density of CCTO ceramics to about 92 %. X-ray diffraction results show CCTO ceramics with a low amount of Kaolinite exhibited perovskite-like structure, but 1.0 wt% Kaolinite additions resulted in the formation of a secondary phase, CaO–TiO₂–Al₂O₃–SiO₂ glass phase was formed and improved the dielectric constant of ceramics, which was supported by scanning electron microscopy–energy dispersive X-ray results. CCTO ceramic with 1.0 wt% Kaolinite addition possessed well temperature and frequency stability of dielectric constant. It was found that Kaolinite lowered the dielectric loss of the samples.     

A study of the stabilization of aluminium titanate

A tialite ceramics (Al₂TiO₅) was synthesized at a temperature of 1500 °C, incorporating CaF₂, La₂O₃, SiO₂ or kaolin and MgO additives. Its thermal stability was investigated by thermocycling in a reducing medium. The batches containing SiO₂ or kaolin additives underwent a decomposition to rutile and corundum. A stabilized ceramic with added MgO was produced. X-ray and electron-probe microanalysis established the presence of Mg/Al, Ti_z/O₄, Al_2–x-y Ti_1+x Mg _y O₅ and Ca_1–x La _x /Al_12–y-z Mg _y Ti _z /O₁₉ solid solutions, which retained their chemical composition after thermocycling in a reducing medium.     

Preparation and properties of mullite-bonded porous SiC ceramics using porous alumina as oxide

Mullite-bonded porous silicon carbide ceramics were prepared by an in situ reaction bonding technique and sintering in air with SiC, porous Al₂O₃, and graphite as starting materials. The pores in the ceramics were formed by burning graphite and by stacking particles of SiC and Al₂O₃. The surface of SiC was oxidized to SiO₂ at high temperature. With a further increase in temperature, SiO₂ reacted with Al₂O₃ to form mullite. The reaction-bonding characteristics, phase composition, open porosity, mechanical strength as well as the microstructure of porous SiC ceramics were investigated.     

Effects of TiO2 doping on microstructure and properties of directed laser deposition alumina/aluminum titanate composites

ABSTRACT

Al₂O₃-based composite ceramics have excellent high temperature performance and are ideal materials for preparing hot end components. However, poor fracture toughness and thermal shock resistance limit its applications. Based on the excellent low thermal expansion characteristics and thermal shock resistance of Al₂TiO₅ ceramic, different composition ratios of Al₂O₃/Al₂TiO₅ composite ceramics were prepared by directed laser deposition (DLD) technology. Effects of TiO₂ doping amount on microstructure and properties of the composite ceramics were investigated. Results show that α-Al₂O₃ phase is discretely distributed in the continuous aluminum titanate matrix when TiO₂ doping amount between 2 and 30?mol%. With the increase of TiO₂ doping amount, content of Al₂O₃ gradually decreases and its morphology changes from cellular to dendritic. When TiO₂ doping amount reaches 43.9?mol%, the microstructure transforms into fine Al₂TiO₅/Al₆Ti₂O₁₃ eutectic structure. Property test results show that Al₂O₃/Al₂TiO₅ composite ceramics have good comprehensive mechanical properties when TiO₂ doping amount between 2 and 6?mol%.     

Effects of additives on sintering and some properties of calcium phosphates with various Ca/P ratios

The effects were studied of additives on the sintering and properties such as the compressive strength and chemical durability of ceramics of calcium pyrophosphate, tricalcium phosphate and hydroxyapatite. In improving the compressive strength of these calcium phosphates, the effect of additives was found to be in the order Na₂OCaF₂>MgOgAl₂O₃, SiO₂ for the case of single-doping, and was most marked in the combination of Na₂O-MgO-Al₂O₃ for the complex addition. It was also found that the chemical durability of the calcium phosphates was improved greatly by using a single or complex additives.     

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.     

Glasses and glass-ceramics in the SrO–TiO2–Al2O3–SiO2–B2O3 system and the effect of P2O5 additions

The glass formation abilities of various compositions in SrO–TiO₂–Al₂O₃–SiO₂, SrO–TiO₂–B₂O₃–SiO₂, SrO–TiO₂–Al₂O₃–B₂O₃, and SrO–TiO₂–Al₂O₃–SiO₂–B₂O₃ systems were studied. Many new compositions were found to be suitable for the casting of crack-free, optically clear glasses of different color and with glass transition temperatures ranging from 595 to 775 °C. The crystallization behavior, structure, and thermal expansion behavior of selected glasses were analyzed by DTA, XRD, dilatometry, and heat treatment. The effect of P₂O₅ on the glass structure and crystallization behavior was also studied. P₂O₅ played a dual role depending on composition. In some glasses it acted as a nucleating agent while in others it suppressed crystallization. Heat treatment of borate and borosilicate glasses transformed them into glass-ceramics while comparable SrO–TiO₂–Al₂O₃–SiO₂ glasses showed a lower tendency to crystallize and form glass-ceramics under the same conditions.     

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.