Finite size effects in submicron barium titanate particles

An experimental and theoretical analysis of finite size effects in submicron barium titanate particles is presented. The dielectric data show that the dielectric constant (ε = 734) of small particles (d = 20 nm) remains very similar to that of single crystals provided that the particles are grown from an amorphous phase and not mechanically crushed into powder. This shows that what has in the past been mistaken for true size effects (i.e., depolarization fields) is due to lattice strain. The X-ray data show that the Curie temperature (cubic-tetragonal transition temperature) decreases monotonically to zero degree Kelvin as particle size is reduced. However this is shown to be a simple function of lattice constant: The lattice constant expands as the particle diameter d contracts. This is similar to surface relaxation in thin films.     

Calculations of finite size effects in barium titanate

The polarisation and permittivity of barium titanate are calculated as a function of temperature from Landau-Ginzburg-Devonshire theory, taking into account effects due to the finite volume of coherently polarising regions. For regions of less than 10 nm in diameter, in which spatially uniform thermal fluctuations of the polarisation become significant, there are marked departures from the single crystal behaviour, particularly in the vicinity of the ferroelectric phase transitions. The behaviour is similar to aspects of that reported for fine-grained ceramics and thin films, suggesting that limited coherence of the magnitude of polarisation is responsible for a reduction in the permittivity and the appearance of diffuseness of the ferroelectric phase transitions.     

Effect of vibro-milling time on phase formation and particle size of barium titanate nanopowders

Barium titanate (BT) nanopowder was synthesized by a solid state reaction via a rapid vibro-milling technique. The effect of milling time on phase formation and particle size of BT powder was investigated. Powder samples were characterized using XRD (X-ray diffraction) and SEM techniques. It was found that the resulting BT powders have a range of particle size depending on milling times. Production of a single-phase BT nanopowder can be successfully achieved by employing a combination of 30 h milling time and calcination conditions of 1200 °C for 2 h.     

粉体的燃烧合成技术及其在钛酸钡粉体制备中的应用

综述了燃烧法合成陶瓷粉体的原理、方法及其在钛酸钡粉体制备中的应用 ,比较了各种燃烧法合成钛酸钡粉体工艺的优缺点 ,并指出目前经济、有效地制备钛酸钡粉体的方法是以有机燃料和金属硝酸盐为原料的低温燃烧合成法。     

In-situ observation of needle domain evolution in barium titanate single crystals

We report in-situ optical observations of the ferroelastic switching in a barium titanate single crystal under compressive stress. Optical micrographs were captured in two regions showing distinct arrangements of domains. Coarsely spaced needle domains in a matrix consisting of a differently oriented large single domain were found to retreat under the application of compressive stress of around 1–2 MPa. However, a comb of closely spaced needle domains was found to be more stable, retreating only slightly under a similar magnitude of applied load. The observations show that the pattern of needle domains influences the ferroelastic switching process and the observed coercive stress can depend strongly on domain arrangement.     

Non-destructive three-dimensional Raman analysis of domain orientation in barium titanate single-crystal

Ferroelectric domain orientation in barium titanate (BaTiO₃; BT) single-crystal has been investigated with respect to its geometrical distribution in the solid angle by using polarised Raman spectroscopy. After retrieving the angular dependence of the intensity of selected Raman modes of BT and their defocusing properties at selected laser wavelength (in-depth probe response function), both in-plane and out-of-plane domain fractions could be visualised and mapped with microscopic resolution by Raman spectroscopic assessments. It is demonstrated that polarised Raman spectroscopy is a valuable and efficient tool for fully 3D, non-destructive assessments of domain orientation in ferroelectrics.     

Engineering grain size and electrical properties of donor-doped barium titanate ceramics

A semiconducting lanthanum-doped barium titanate ceramic has been fabricated for battery safety applications by simple means from nanoparticles prepared at room temperature by kinetically controlled vapor diffusion catalysis. The material, characterized by electron microscopy, X-ray diffraction and electrical measurements, exhibits a difficult to achieve combination of submicron grain size (∼500 nm) and attractive electrical properties of room temperature resistivity below 100 Ω cm and a 12-fold increase in resistivity through the Curie temperature (positive thermal coefficient of resistivity, PTCR). Systematic investigation of sintering conditions revealed that a short period of heating at 1350 °C under air is necessary to suppress abnormal grain growth, while precise control of the cooling rate is needed to achieve the targeted electrical properties. Cooling must be sufficiently fast to avoid complete back-oxidation, yet slow enough to facilitate oxygen adsorption at the grain boundaries to produce the thin oxide layer apparently responsible for the observed PTCR.     

Effect of mechanical stress on phase stability and polarization states in ferroelectric barium titanate and lead titanate

The effect of normal and shear stress on phase transitions in BaTiO₃ and PbTiO₃ has been investigated using a modified Landau–Ginzburg–Devonshire phenomenological model based on assumption of constant stress boundary conditions. Stress–temperature phase diagrams have been developed, and the influence of stress on polarization stability has been analyzed. The results show monoclinic phases with various polarization states absent in stress-free BaTiO₃ may exist under uniaxial, biaxial, anisotropic three-dimensional, and shear stress conditions. For PbTiO₃, our calculations show that, under normal stress new phases cannot be generated and the only stable ferroelectric phase has tetragonal symmetry, but under shear stress orthorhombic, rhombohedral, and monoclinic phases can be stabilized.     

Inversion domain network stabilization and spinel phase suppression in ZnO

The development of inversion domain networks consisting of basal‐plane and pyramidal‐plane inversion domain boundary (b‐IDB and p‐IDB) interfaces within grains in Sn‐Al dual‐doped ZnO (Zn_0.98Sn_0.01Al_0.01O) polycrystalline ceramics has been confirmed using transmission electron microscopy. The atomic structure of the b‐IDB and p‐IDB interfaces has been analyzed using atomic‐resolution scanning transmission electron microscopy. The localization of Sn and Al at the respective sites of the b‐IDBs and p‐IDBs was confirmed by energy‐dispersive X‐ray spectroscopy. In contrast to Sn or Al single‐dopant addition to ZnO, which results in the formation of spinel phase precipitates without the development of inversion domain networks, Sn‐Al dual‐doping caused the suppression of spinel phase formation and the formation of monophasic inversion domain networks composed of RMO₃(ZnO)_n homologous phase compound members, where R and M represent dopants substituting at the b‐IDB and p‐IDB sites, with a general formula of SnAlO₃(ZnO)_n. The results of this study demonstrate that the formation of inversion domain networks in ZnO‐based ceramics can be stabilized via multiple‐dopant addition. This finding has potential implications for the modification of the bulk or nanoscale properties based on the choice of the specific dopants, R and M, the control of the ratio R:M and the value of n in the RMO₃(ZnO)_n homologous phase compound members constituting the inversion domain networks.     

Combined shock-wave synthesis of noble spinel and laves cubic phase

The paper described the shock-wave synthesis of high-pressure high-temperature mineral MgAl₂O₄ with the parameter a = 8.085(3) Å and the Laves phase MgCu₂ with a = 7.076(2) Å from a mixture of MgO and aluminum powder placed into a copper insert inside a steel conservation ampoule. Spinel MgAl₂O₄ with a smaller lattice parameter a = 8.0798 Å has been formed in an aluminum insert cup. __________ Translated from Steklo i Keramika, No. 6, pp. 21–22, June, 2006.     

Hydrothermal synthesis of carbonate-free submicron-sized barium titanate from an amorphous precursor: Synthesis and characterization

In this paper, the amorphous barium titanate precursor was prepared by the peroxo-hydroxide method and post-treated by various drying procedures, such as: room temperature drying, room temperature vacuum drying and vacuum drying at 50 °C. The objective in the latter two treatments was to increase the Ti-O-Ba bonds of the precursor. The post-treated precursors were compared with the untreated (i.e., ‘wet’) precursor. Also, a barium titanate precursor was prepared by an alkoxide route. Afterwards, the precursors were hydrothermally treated at 200 °C in a 10 M NaOH solution. Vacuum drying of the precursor seemingly promoted the formation of Ti-O-Ti bonds in the hydrothermal end-product. The low Ba:Ti ratio (0.66) of the alkoxide-route prepared precursor lead to a multi-phase hydrothermal product with BaTiO₃ as the main phase. In contrast, phase pure BaTiO₃, i.e. without BaCO₃ contamination, was obtained for the precursor which was dried at room temperature. Cube-shaped and highly crystalline BaTiO₃ particles were observed by electron microscopy for the hydrothermally treated peroxo-hydroxide-route prepared precursor.     

Self-propagating high-temperature synthesis of barium titanate and subsequent densification by spark plasma sintering (SPS)

This paper describes the self-propagating high-temperature synthesis (SHS) of perovskitic oxides, specifically BaTiO₃, and their subsequent densification by spark plasma sintering. With the final goal of obtaining dense nanostructured materials, SHS products were mechanically treated at different milling time conditions, before densification. It was found that the grain size of ball milled powders decreases with increasing milling time, this effect being more evident at early stages of milling. Depending upon the ball milling (BM) conditions adopted, crystallite size in the range 15–70 nm was obtained. After milling for 5 h, the resulting powders (20–30 nm) were sintered by SPS, at 700 A, for different periods of time. By properly varying sintering time in the interval 70–140 s, it is possible to obtain products with relative density in the range 66–99%, respectively. In particular, grain growth during sintering was found to be limited (below 50 nm) if the electric current is applied for time intervals equal to or less than 100 s. The observed dielectric properties are typical of a nanocrystalline BaTiO₃ ceramic.     

Influence of barium titanate content and particle size on electromechanical coupling coefficient of lead-free piezoelectric ceramic-Portland cement composites

The 0–3 lead-free piezoelectric ceramic-Portland cement composites were prepared by mixing and pressing the Portland cement (PC) and barium titanate (BT) ceramic powder. The influences of BT particle size and BT content on the electromechanical coupling coefficient (K_t) of the composites were investigated. The results indicate that the particle size of BT used to produce the composite under the conditions of the same BT content (at 50% BT) and fabrication technique has an influence on the K_t values. The electromechanical coupling coefficient was found to increase with the particle size of BT used where the values of K_t are found to be at 10.8% and 14.1% for composites with median particle size of 75 μm and 425 μm, respectively. Furthermore, K_t of composites increase with increasing content of BT (at the same particle size of 425 μm) when the content of BT reaches 70%, K_t is 16.6%. In addition, the acoustic impedance of the composite also increase with an increase of BT content.     

Role of defect structures in stabilization of ferroelectric phase in tin‐substituted lead zirconate titanate

This study reports the influence of B‐site acceptor dopants, manganese and copper, on the sequence of phase transformations in antiferroelectric (AFE) Pb(Zr_0.60Sn_0.30Ti_0.10)O₃. The sequence of phase transformations below the Curie point have been examined by dielectric, polarization‐electric field and strain‐electric field studies. The parent compound and B‐site Cu²⁺‐doped composition exhibit the same sequence, FE ← AFE ← MCC with incommensuration in the multicell cubic (MCC) state, whereas in the case of B‐site Mn³⁺‐doped system, incommensuration is found to be suppressed and only ferroelectric (FE) phase is observed below the Curie point. The underlying mechanism is related to the nature of defect complexes present in the system through detailed Electron Paramagnetic Resonance studies.     

Amphibious water‐soluble copolymer. I. Its synthesis and dispersing ability on barium titanate

An amphibious water‐soluble copolymer, polyacrylamide/(α‐N,N‐dimethyl‐N‐acryloyloxyethyl) ammonium ethanate (PAAM/DAAE), was synthesized and used as a dispersion agent for BaTiO₃ particles. PAAM/DAAE was prepared from acrylamide and (α‐N,N‐dimethyl‐N‐acryloyloxyethyl) ammonium ethanate under basic conditions through a free‐radical polymerization. The structure of this copolymer was verified with IR and ¹H‐NMR spectra. The dispersing effects of PAAM/DAAE were examined through the measurement of the viscosity and sedimentation of BaTiO₃ suspensions and the green density. The results indicated that this copolymer could uniformly disperse the particles, reduce the viscosity, stabilize the suspensions, and produce high‐density green compacts. In comparison with a commercial dispersant, the ammonium salt of poly(methylacrylic acid), PAAM/DAAE was clearly more effective. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 2232–2239, 2004     

PTCR effect in donor doped barium titanate: review of compositions,microstructures, processing and properties

Abstract

Abstract

Barium titanate is widely used in the fabrication of thermistors with a positive temperature coefficient of resistivity (PTCR). The resistivity can increase by several orders of magnitude near the phase transition temperature T_c for the ferroelectric tetragonal to the paraelectric cubic phase transformation. There is general agreement that the anomaly in the change of electrical resistivity of donor doped BaTiO₃ around T_c is due to the grain boundary effect. The Heywang–Jonker model and other mechanisms involving the nature of the electrical barrier formed across the grain boundaries of polycrystalline BaTiO₃ are reviewed. The compositional effect on BaTiO₃ based PTCR properties is listed and discussed. The influences of manufacturing methods under different stages including the initial doping methods, sample forming methods and final heat treatments on PTCR properties are compared. The complex interrelationships between compositions, microstructures, processing and PTCR characteristics are well discussed.     

A semiempirical derivation of phase domain size in interpenetrating polymer networks

In previous papers, interpenetrating polymer networks were shown to display a cellular structure. The phase domain size of polymer II was shown to depend inversely on the crosslink density of polymer I. The present paper presents a semiempirical derivation of equations which show quantitatively the dependence of the phase domain size of polymer II on the crosslinking density of polymer I, and also on the interfacial energy and the overall composition. If polymer II is linear, the dependence on the molecular weight of polymer II is also included. The values of the phase domain sizes so estimated are compared with experimental results. While theory and experiment yield good agreement, the semiempirical nature of the equations must be borne in mind.     

X射线荧光光谱法测定钛酸钡中主次元素含量

用粉末压片X射线荧光光谱法测定BaTiO3中Ba、Ti、Mg、Si、Fe、Sr含量。根据钛酸钡样品中主次成分含量范围,采用在高纯钛酸钡样品(纯度≥99.99%)中加入梯度量各杂质元素标准溶液的方法制作校准样品系列,解决了无钛酸钡标准样品的问题。采用经验影响系数法纠正基体效应,并对较准样品的配制方法、压片机压力、粒度效应等进行了探讨。方法用于钛酸钡中主次元素含量测定时,测定结果与化学分析法相符,相对标准偏差均小于6.73%。     

Diffraction,structure and phase stability studies on aluminium titanate

The crystal structure studies of aluminium titanate (Al₂TiO₅) in the temperature range of 25–1200 °C have been investigated by X-ray and neutron powder diffraction. The use of simultaneous refinement of the X-ray and neutron data has enhanced the refinement precision of the structure model of Al₂TiO₅ with the concomitant improvement in values such as atomic coordinates, site occupancies and unit-cell parameters. Results from Rietveld refinements showed that the disordering of Ti and Al atoms in the metal 1 and metal 2 sites. High-temperature neutron diffraction studies of the Al₂TiO₅ crystal structure revealed an increasing substitution of Al for the Ti atoms in the crystal structure. It is proposed that this increased substitution is responsible for the decomposition of Al₂TiO₅ that is observed between 900 and 1280 °C.     

立方体纳米氧化镁的可控合成及其粒度影响因素的探究

采用溶胶-凝胶法,以Mg(CH_3COO)_2·4H_2O为原料,草酸为络合剂和酸性试剂,从晶体形成机理入手,详细研究了不同粒径立方纳米氧化镁的制备工艺。采用XRD和SEM分别表征了纳米氧化镁的结构和形貌。结果表明,通过改变工艺条件可以制备出粒度可控的纳米氧化镁,反应温度是影响纳米氧化镁粒度与形貌的关键因素。