Barium titanate is an inorganic substance with the chemical formula BaTiOthree, a strong dielectric compound material with high dielectric constant and low dielectric loss, is one of the most widely used materials in electronic ceramics, known as the "pillar of electronic ceramics industry".[1]
Chinese name
barium titanate
chemical formula
BaTiOthree
molecular weight
two hundred and thirty-three point one nine two two
Solubility: soluble in concentrated sulfuric acid, hydrochloric acid and hydrofluoric acid, insoluble in hot dilute nitric acid, water and alkali.
structure
Figure 1 BaTiO3 Crystal Structure
Barium titanate is a uniform melting compound with a melting point of 1618 ℃.Below this temperature, barium titanate crystallized above 1460 ℃ is nonferroelectricHexagonal system6/mmmPoint group。At this time, the hexagonal system is stable.Barium titanate transforms into cubic perovskite structure at 1460~130 ℃.In this structure Ti4+(TitaniumIons）In O2-Oxygen composed of (oxygen ions)OctahedronCentral, Ba2+(Barium ion) is located in the space enclosed by eight oxygen octahedrons (see Figure 1).At this time, the symmetry of barium titanate crystal structure is very high, so there is no dipole momentFerroelectricity, norPiezoelectricity。
The symmetry of the crystal decreases with the decrease of temperature.When the temperature drops to 130 ℃, barium titanate undergoes a paraelectric ferroelectric phase transition.In the temperature range of 130~5 ℃, barium titanate is a tetragonal system of 4mmPoint group, with significant subway electricitySpontaneous polarizationIn the direction of axis c, namely [001].Barium titanateCubic systemThe structure changes little when it transforms into tetragonal system.fromUnit cellLook, justUnit cellAlong the originalCubic systemThe first axis (axis c) of is lengthened and shortened along the other two axes.
When the temperature drops below 5 ℃,Barium titanate crystalIntoOrthorhombic systemMm2 point group, the crystal still hasFerroelectricity, its spontaneous polarization intensity follows the original cubicUnit cellDirection of the face diagonal [011] of the.For convenience, we usually useMonoclinic systemTo describeOrthorhombic systemOfMonad。The advantage of this treatment is that we can easilyMonadSee inSpontaneous polarizationThe situation.Barium titanateTetragonal systemThe structure changes little when it is transformed into orthorhombic crystal system.fromUnit cellIt is equivalent to the originalCubic systemThe diagonal line of one plane of is lengthened, the diagonal line of the other plane is shortened, and the c axis is unchanged.
Crystalline transformation
When the temperature continues to drop below - 90 ℃, the crystal changes fromOrthorhombic systemTransformed into trigonal system 3mPoint groupAt this time, the crystal still has ferroelectricity, and its spontaneous polarization intensity direction is the same as that of the original cubic cellBody diagonal[111] The direction is parallel.Barium titanateOrthorhombic systemThe structure changes little when it is transformed into a tripartite system.From the perspective of unit cell, it is equivalent to a piece of original cubic unit cellBody diagonalThe diagonal of the other root body is shortened.
Spontaneous polarization
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Barium titanate is a typical ferroelectrics, so its spontaneous polarization must be mentioned when referring to barium titanate[2]。Generally speaking, the electricity of dielectricPolarization processThere are three ways, namelyElectron displacement polarization、Ion displacement polarizationAnd intrinsicElectric momentSteering polarization.For barium titanate, afterphysical scientistIt is strictly estimated that the contribution of spontaneous polarization of barium titanate mainly comes from Ti4+Ion shift polarization and one O in oxygen octahedron2-Electronic displacement polarization.The specific calculation process is relatively simple, but the content is lengthy, which will not be described here. Please refer to relevant books.
Ferroelectric domain
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Barium titanate crystal is composed of countless barium titanate cells.When the cubic barium titanate crystal is cooled toCurie pointTc will start to generateSpontaneous polarizationAt the same time, the cubic phase is transformed into tetragonal phase.Occurs onSpontaneous polarizationSome of them are close to each otherUnit cellAlong one of the original cubic cellsCrystal axisproduceSpontaneous polarizationAnd another part of the adjacent cells may spontaneously polarize along another crystal axis of the original cubic cell.In this way, when barium titanate transforms into tetragonal phase, the crystal appears along different directionsSpontaneous polarizationWe call it domain.That is to say, by lowering the temperature, when the crystal changes from paraelectric phase to ferroelectric phaseSpontaneous polarization, resulting in the change of surface electrostatic interaction and the generation of domain structure.[3]
In addition to the structural symmetry of the crystal, the type of domain and the orientation of domain wall should also meet the following two conditions: ①Continuity: As a result of the formation of the electric domain, the lattice on the two surfaces generated by cutting the crystal along the domain wall is continuous and matched. ②Spontaneous polarization componentContinuity: The components of the spontaneous polarization intensity of two adjacent domains in the direction perpendicular to the domain wall are equal.Therefore, in tetragonal barium titanate single crystalSpontaneous polarizationThe directions can only intersect 180 ° or 90 °, that is, there are only 180 ° domains and 90 ° domains.In monoclinic barium titanateSpontaneous polarizationAlong the plane diagonal of the original cubic cell, there are 60 ° and 120 ° domains besides 180 ° and 90 ° domains.In the triclinic barium titanate system, there are not only 180 ° domains, but also 60 ° and 109 ° domains.
Dielectric property
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Fig. 2 Variation curve of single crystal dielectric constant with temperature
The dielectric properties of barium titanate here mainly refer toBarium titanate ceramicsDielectric properties of[4]。Barium titanateDielectric propertiesBasically similar to barium titanate single crystal.But because ceramics arePolycrystallineStructure, presencegrainandgrain boundary。The size and disordered orientation of the grain, the presence of glass phase and impurities in the grain boundary directly affect its dielectric properties, making it different from the single crystal.
Fig. 3 Variation of ε and tan δ of barium titanate ceramics with temperature T
pureBarium titanate ceramicsThe relationship between ε and temperature T in the a-axis and c-axis is shown in Figure 3.It is related tosingle crystalCompared with the change relationship of ε and tan δ with temperature T (Fig. 2), the characteristics of ε near Tc are similar, but at 5 ℃ and - 90 ℃Phase transition temperatureThere is no single crystal nearby that changes significantly.
Effect of particle size on ε - T
In addition, the grain size of barium titanate ceramics will also affect the relationship between ε and temperature T (Fig. 4).Fig. 4 shows that ε increases below Tc and ε peak decreases near Tc with the decrease of grain size.
research status
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Barium titanate is a strongDielectric materialIt is one of the most widely used materials in electronic ceramics and is known as the "pillar of electronic ceramics industry". There are too many researches on barium titanate. Many scholars at home and abroad have done a lot of research work on barium titanate. Through doping modification, a large number of new materials have been obtained, especially in the application of MLCC.Its application prospect is extremely broad, and we look forward to our joining.[1][5]
purpose
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Mainly used in electronic ceramicsPTC thermistor, capacitor, etcElectronic componentsAnd the reinforcement of some composite materials.[5-6]
