PTCR ceramics produced from oxalate-derived barium titanate

BaTiO₃ powders with an average crystallite size from 68 to 2000 nm have been prepared by calcining barium titanyl oxalate at temperatures from 700 to 1150°C, and their morphology and recrystallization kinetics have been studied. The powders have been used to produce positive temperature coefficient of resistance (Ba,Ca,Pb)TiO₃ ceramics, and the microstructure and electrical properties of the ceramics have been investigated. The results indicate that improving the crystallinity of the barium titanate powder suppresses recrystallization in the ceramics, increases their resistivity, and has a significant effect on their resistance jump and electric strength. We have identified the optimal temperature range of barium titanyl oxalate calcination, which insures the highest electric strength of PTC thermistors with a resistance of 31 Ω. The corresponding crystallite size of the barium titanate powder is ?200 nm.     

Water-based sol–gel nanocrystalline barium titanate: controlling the crystal structure and phase transformation by Ba:Ti atomic ratio

Highly stable, water-based barium titanate (BaTiO₃) sols were developed by a low cost and straightforward sol–gel process. Nanocrystalline barium titanate thin films and powders with various Ba:Ti atomic ratios were produced from the aqueous sols. The prepared sols had a narrow particle size distribution in the range 21–23 nm and they were stable over 5 months. X-ray diffraction pattern revealed that powders contained mixture of hexagonal- or perovskite-BaTiO₃ as well as a trace of Ba₂Ti₁₃O₂₂ and Ba₄Ti₂O₂₇ phases, depending on annealing temperature and Ba:Ti atomic ratio. Highly pure barium titanate with cubic perovskite structure achieved with Ba:Ti = 50:50 atomic ratio at the high temperature of 800 °C, whereas pure barium titanate with hexagonal structure obtained for the same atomic ratio at the low temperature of 500 °C. Transmission electron microscope revealed that the crystallite size of both hexagonal- and perovskite-BaTiO₃ phases reduced with increasing the Ba:Ti atomic ratio, being in the range 2–3 nm. Scanning electron microscope analysis revealed that the average grain size of barium titanate thin films decreased with an increase in the Ba:Ti atomic ratio, being in the range 28–35 nm. Moreover, based on atomic force microscope images, BaTiO₃ thin films had a columnar-like morphology with high roughness. One of the highest specific surface area reported in the literature was obtained for annealed powders at 550 °C in the range 257–353 m²g⁻¹.     

Synthesis of nm-sized barium titanate crystallites using a new LTDS method and their characterization

A new LTDS method was established to synthesize nm-sized barium titanate crystallites with particle sizes around 10 nm. This LTDS method was the following two features, i.e., (1) a heat of neutralization in a neutralization reaction between strong acid and base can be used as a driving force for a formation of barium titanate, and (2) barium titanate crystallites can be directly synthesized from Ti and Ba ions, not via intermediates. Therefore, using the LTDS method, it can be expected that there is no minimum limit in particle sizes, and also no contamination in barium titanate crystallites. At first, optimum conditions for a formation of barium titanate were screened at various reaction temperatures and Ba/Ti atomic ratios in the starting materials. As a result, the formation of barium titanate was found at higher reaction temperatures than 50°C and higher Ba/Ti atomic ratios than 5. Under a constant temperature, particle sizes decreased with increasing Ba/Ti atomic ratios while under a constant Ba/Ti atomic ratio, particle sizes were independent of reaction temperatures. Finally, barium titanate crystallites with particle sizes below 10 nm were first prepared. These particles were also characterized using various methods.     

Effects of surfactant additives during ball milling on the microstructural,electrical, and piezoelectric properties of 0.4Ba(Zr0.2Ti0.8)O3-0.6(Ba0.7Ca0.3)TiO3 ceramics

Journal of Materials Science: Materials in Electronics - In this work, calcium barium zirconate titanate piezoelectric ceramics 0.4Ba(Zr0.2Ti0.8)O3- 0.6(Ba0.7Ca0.3)TiO3((1?x)BZT-xBCT,...     

溶胶-凝胶法制备Ba_(3.99)Sm_(9.34)Ti_(18)O_(54)微波介质陶瓷粉体

用柠檬酸(C6H8O7)为络合剂,以硝酸钐(Sm(NO3)3)、钛酸四丁酯(C16H36O4Ti)、硝酸钡(Ba(NO3)2)为原料,采用溶胶-凝胶法制备单相Ba3.99Sm9.34Ti18O54微波介质陶瓷粉体。采用差热分析、X射线衍射表征Ba3.99Sm9.34Ti18O54前驱体的热行为和晶型转化过程,用场发射扫描电镜对粉体的形貌进行表征。结果表明:在1000℃下保温3h可以获得纯Ba3.99Sm9.34Ti18O54陶瓷粉体,形状为不规则片状,平均粒径为100~200nm。     

纳米钛酸钡粉体的水热合成研究

采用氢氧化钡和四氯化钛作为钡源和钛源,氨水作矿化剂,在200～280℃保温4h条件下合成粒径为60～90nm钛酸钡粉体。TEM、XRD、钡/钛摩尔比(Ba/Ti)和比表面积的测试结果表明,在单一水介质中水热合成钛酸钡晶体的结晶过程遵循“溶解-结晶”机制,存在第II类聚集长大过程。前驱体浓度、合成温度和合成反应时间是影响粉体粒径的重要因素。粉体的Ba/Ti比过低会导致粉体中出现BaCO3杂质相,且Ba/Ti越低,钛酸钡晶体晶胞膨胀程度越大。     

Effect of the crystallinity of BaTiO<Subscript>3</Subscript> powders prepared using the merker method on the properties of PTCR ceramics

Barium titanate powders differing in particle size (110–740 nm) were prepared by calcining barium titanyl oxalate precipitated by the Merker method. The powders were sintered to produce PTCR ceramics with the composition 100(Ba_0.89Ca_0.08Pb_0.03)TiO₃ + ^0.8TiO₂ + ^0.7Y + ^0.1Mn + ^2.5SiO₂ and electrical properties of the ceramics were studied. The results demonstrate that improving the crystallinity of the barium titanate powder suppresses recrystallization of the ceramics and has a significant effect on their resistance ratio and electric strength. We found the optimal range of calcination temperatures (950–1000°C) for barium titanyl oxalate which ensures the highest electric strength of thermistors with a resistance of 31 Ω. The average crystallite size of the parent barium titanate powder is ∼250–320 nm.     

Microstructure and dielectric characteristics of Ni ion doped BaTiO3 nanoparticles

In this article, we report the effect of nickel oxide (NiO) addition on the microstructure and the dielectric properties of nano-barium titanate (BaTiO₃). Nickel ion doped barium titanate nanoparticles have been synthesized through a chemical route using polyvinyl alcohol (PVA). The concentration of Ni²⁺ varies from 0 to 1.6 mol% in the specimens. The particle diameters in the specimen lie in the range of 24 nm to 35 nm. It is seen that the dielectric permittivity in doped specimens is enhanced compared to that of undoped BT ceramics. The dielectric permittivity shows maxima in the specimen containing 0.6 mol% of dopant concentration. The results are explained in terms of the change in lattice parameter c and a of the specimens.     

Synthesis and characterization of microwave-hydrothermally derived Ba1−xSrxTiO3 powders

Submicron-sized, spherical, stoichiometric strontium substituted barium titanate: Ba_0.75Sr_0.25TiO₃ (BST) powders are prepared by microwave-hydrothermal (MH) route under the MH conditions of 200 °C, 200 psi, and 30 min in strongly alkaline conditions (pH>12) using potassium titanyl oxalate (KTO) and nitrates of Ba and Sr as the starting precursors and KOH as the mineralizer. The characterization studies by XRD, XRF and SEM indicated that stoichiometric cubic BST powders (a₀=3.992 Å) were obtained by adjusting MH conditions mentioned earlier and keeping the (Ba+Sr)/Ti ratio in starting solution slightly in excess (∼1.06).     

Preparation of barium titanate by homogeneous precipitation

The barium titanyl oxalate, which was the precursor of barium titanate, was prepared by homogeneous precipitation using the diethyl oxalate as a precipitating agent. The oxalate dried at 80 °C was the crystalline phase, which was converted to amorphous-like phase after drying at 120 °C. The weight loss of oxalate was 47.3 wt%. The chemical formula of the oxalate could correspond to BaTiO(C₂O₄)₂ · 4H₂O. The barium titanate obtained from the calcination of oxalate at 850 °C was a tetragonal phase with a particle size of 0.2 m.     

Influence of microstructure on dielectric properties of Nd-doped barium titanate synthesized by hydrothermal method

Nd-doped barium titanates were successfully synthesized via a hydrothermal route. The as-prepared barium titanate was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), fourier transformation infrared spectroscopy (FTIR), and Vis–NIR spectroscopy respectively. The results show that pure and Nd-doped barium titanate powders have cubic perovskite structure. After sintering at a temperature of 1,250 °C for 2 h, the phase compositions of all barium titanate are tetragonal phase structure. Vis–NIR spectra well confirmed that Nd³⁺ have been doped into barium titanate. The particle diameters of Nd-doped barium titanate powders and ceramics become samller with the increase of Nd³⁺ content. When Nd/Ba molar ratio is 0.02, the dielectric loss (0.0008) of the powder measured at 1 MHz and room temperature dramatically decreases by 99 % comparing with pure barium titanate (0.083) and shows frequency independence with the frequency increasing from 40 Hz to 1 MHz. The dielectric constant and dielectric loss are 436 and 0.09 after sintering. The Nd-doped BaTiO₃ show an improvement in the dielectric quality which possess a decreased sensitivity to frequency for both the dielectric constant and dielectric loss. Such improvements are of potential importance for high energy density and low loss.     

X-ray diffraction and microstructural studies on hydrothermally synthesized cubic barium titanate from TiO2-Ba(OH)2-H2O system

In our earlier studies synthesis of pure barium titanate (BT) from TiO₂-Ba(OH)₂-NH₃ system was reported. This work describes a novel route for preparing cubic barium titanate from TiO₂-Ba(OH)₂-H₂O system without addition of any mineraliser. The experimental parameters varied were: reaction time (half-an-hour to 3 h), reaction temperature (80 to 150 °C) and [Ba/Ti] ratio (0.92 to 1.64). The progress of reaction for formation of BT was monitored by analyzing the X-ray diffraction data obtained under different processing conditions. Mono-phasic barium titanate powder having a mean crystallite diameter (MCD) of 26 nm along (101) plane was formed when the reaction was carried out for 3 h at 150 °C. The estimated strains on the planes of the BT nano-crystals were found to be negligible. The microstructure of pure BT showed the particles to be of single crystallite nature with average size matching with the MCD value calculated from the XRD data.     

水基金属有机物分解法制备Ba0.5Sr0.5TiO3薄膜的研究

制备了水基Ｂａ＾２＋、Ｓｒ＾２＋、Ｔｉ＾４＋三元有机物溶液。根据红外光 谱测定及对比实验分析了溶液配制过程中化学反应机理。采用金属有机物分解法（ＭＯＤ）制备Ｂａ０．５Ｓｒ０．５ＴｉＯ３（ＢＳＴ）薄膜。通过ＸＲＤ、ＳＥ趱 阻抗分析仪等分析测试手段,薄膜的相结构、微观形态及电性能。结果表明,所制备ＢＳＴ薄 数矿晶相结构,结晶完整晶粒小（１０－５０ｎｍ）,显微结构均匀致密,并具有良好的电性能（电容密度为     

Synthesis of single-crystalline perovskite barium titanate nanorods by a combined route based on sol-gel and surfactant-templated methods

The single-crystalline perovskite barium titanate nanorods were successfully synthesized by a combined route based on sol-gel and surfactant-templated methods at low temperature. The synthesis was accomplished by using barium acetate (Ba(CH₃COO)₂) and tetrabutyl titanate (Ti[O(CH₂)₃CH₃]₄) as the starting materials and laurylamine as the surfactant, respectively. Well-isolated single-crystalline cubic perovskite BaTiO₃ nanorods with diameters ranging from 20 to 80 nm and lengths reaching up to > 10 μm can be easily fabricated by this route. The crystal form and morphology of the nanorods were characterized by X-ray diffraction, TEM and HRTEM. The mechanism of the formation of the single-crystalline cubic perovskite BaTiO₃ nanorods was discussed based on the theory of oriented attachment.     

低电阻率Ba0.92Ca0.08Ti1.02O3PTCR陶瓷界面缺陷态研究

用TEM和EDS相结合的测试手段,研究了低电阻率Ba0.92Ca0.08Ti1.02O3PTCR陶瓷材料的界面元素分布。根据界面元素分布的情况,对低电阻率Ba0.92Ca0.08Ti1.02O3PTCR陶瓷界面可能存在的缺陷态进行分析,认为在低电阻率Ba0.92Ca0.08Ti1.02O3PTCR陶瓷界面上主要存在以下缺陷结构：Mn“Ti,Mn‘Ti或Al‘Ti,V”Ba。     

PTCR barium titanate ceramics obtained from oxalate-derived powders with varying crystallinity

Barium titanate powders with average crystallite sizes of 68–2000 nm have been prepared by the calcination of barium titanyl oxalate (BTO) at temperatures of 700–1150 °C. The morphology and recrystallization kinetics of the powders have been studied using the SEM and X-ray methods. Samples of PTCR (BaCaPb)TiO₃ ceramics have been made from these powders and their microstructure and electrical properties have been investigated. It has been found that the increase of the crystallinity of the starting powders suppresses recrystallization of the ceramics, leading to growth in resistivity and significantly influencing on the resistance jump and breakdown strength of the ceramics. An optimal temperature range for the calcination of BTO has been found to ensure maximum breakdown strength of the PTC thermistors with the resistance of 31 Ω. At this temperature range the barium titanate powders had crystallite sizes of ~200 nm.     

Electronic structure of nanograin barium titanate ceramics

The density of states and band structure of 20 nm barium titanate (BaTiO₃, BT) ceramics are investigated by first-principles calculation. The full potential linearized augmented plane wave (FLAPW) method is used and the exchange correlation effects are treated by the generalized gradient approximation (GGA). The results show that there is substantial hybridization between the Ti 3d and O 2p states in 20 nm BT ceramics and the interaction between barium and oxygen is typically ionic.     

Microwave synthesis of nano-sized barium titanate

Nano-sized barium titanate powders have been synthesized by microwave processing at 2.45 GHz. Using barium titanyl oxalate (BTO) as a precursor, microwave processing was carried out by heating the precursor to a temperature between 600 °C and 750 °C with different heating rates from 10 °C/min to 20 °C/min without isothermal holding. X-ray diffraction analysis indicates that the decomposed product at 680 °C was pure cubic BaTiO₃. The BET specific surface area of barium titanate powder, after microwave heating to 680 °C, was 14.2 ± 0.5 m²/g, corresponding to an average particle size of 70 nm. This particle size was confirmed by the scanning electron microscopy (SEM). Parallel study shows that the conventional heating in a regular resistance furnace using a similar heating schedule did not result in complete conversion of BTO. This study shows that the microwave processing significantly accelerated the decomposition of barium titanyl oxalate and reduced the temperature of barium titanate nano-powder formation, resulting in nano-sized pure cubic barium titanate powder.     

Effect of Ultrasonic Processing on the Synthesis of Barium Titanyl Oxalate and the Characteristics of the BaTiO<Subscript>3</Subscript> Powder Prepared from It

Barium titanyl oxalate (BTO) with small deviations from stoichiometry has been synthesized by a chemical and a sonochemical method (under ultrasonication). Ultrasonic processing has been shown to reduce the particle size of the resultant BTO powder by about ten times and ensure a nearly spherical shape of the particles. The morphology of barium titanate powders prepared by decomposing the BTO at a temperature of 900°C is similar to that of the parent BTO and independent of stoichiometry. The powders have a barium to titanium ratio Ba/Ti = 1.002 and 0.987. The barium titanate powders synthesized using the sonochemical method contain a smaller amount of residual phases and have a larger specific surface area, smaller crystallite size (~100 nm), and smaller unit-cell parameters than do the powders prepared without ultrasonication.     

Characterization of fine grain Ba0.995Y0.005TiO3 ceramics obtained from gel-precursor nanopowder

Using an acetate-alkoxide sol-gel route in which the precursors are barium acetate, yttrium isopropoxide and titanium diisopropoxide bis-acetylacetonate, we prepared a ferroelectric material with the formula: Ba1-xYxTiO3, x = 0.005. SEM analysis showed a polymeric microstructure of the gel due to the chelated titanium alkoxide precursor used as starting materials. The evolution of the structure and microstructure of the precursor gel heated at temperatures up to 1000 degrees C was studied by various techniques. The powder obtained by heating the gel at 1100 degrees C presented a homogeneous structure consisting of submicronic particles (approximately 200 nm). XRD and SAED analyses revealed that Ba0.995Y0.005TiO3 nanocrystals of about 5-10 nm appeared at 600 degrees C, together with BaCO3. The presence of barium carbonate was identified also by IR spectroscopy and thermal analyses. The ceramics obtained from the as-prepared powder presented good dielectric properties (capacitance = 840 pF/dielectric constant = 3860 and dielectric loss (tandelta) = 0.078 at Curie temperatures of 120-121 degrees C).