Size and Temperature Dependence of Dielectric Constant of Ultrafine PbTiO_(3) Particles

Using sol-gel method,we prepared ferroelectric PbTiO_(3) fine particles with particle size covering the range 20-200nm;the dielectric properties of powder compact were studied in the present work.The room temperature dielectric constant increases with the decrease in particle size and reaches a maximum value at the size of about 40nm.The temperature dependence of dielectric constant involving the evolution of particle size during the measurement is discussed.

Specific Heat Anomaly of Lithium Sodium Niobate at Low Temperature

A differential scanning calorimetric(DSC)study of the crystal Li_(0.02)Na_(0.98)NbO_(3) was carried out in the temperature range from 120 to 300K.The specific heat changes anomalously at about 180K on cooling and about 260K on heating,implying a first order phase transition.The heat of transition ΔQ=(212±50)J/mol and the corresponding change in entropy AS=0.82J/mol.K.The thermal hysteresis of this transition is about 80K,in agreement with that previously obtained from the dielectric and pyroelectric measurements.

Size Effect in Ferroelectric Long Cylinders

The Curie temperature and polarization in a ferroelectric cylinder with infinite length have been examined using Landau free energy expansion.The Curie temperature and polarization decrease with decreasing cylinder diameter for the positive extrapolation length,and reach zero at the critical size.For negative extrapolation length,both Curie temperature and polarization increase with decreasing cylinder diameter.

Surface Modification of the Spontaneous Polarization in Ferroelectric Thin Films

The surface effects on the spontaneous polarization in ferroelectric thin films were discussed based on the Ising model in a transverse field.Both the modifications of the transverse field and the strength of coupling in the surface layer were taken into consideration.At the temperature much lower than the Curie point,the behaviour of the spontaneous polarization deviates from the phenomenological description.The surface effects might lead to the spontaneous polarization and the Curie temperature changing in different direction.The theory gives a reasonable description of the experimental temperature dependence of the spontaneous polarization of triglycine sulfate thin films in the literature.

Flux Growth and Properties of KNPBN Crystal

Potasium sodium lead barium niobate KNPBN crystals have been grown successfully by flux method.The dielectric constant along the polar axis was measured at 50 kHz as a function of temperature in the range of 0 to 400℃.The flux system,diffuse phase transition and optical absorption were also discussed.

Barium Titanate Prepared by Stearic Acid-Gel Method

Stearic acid-gel method was used to prepare barium titanate.All the samples show a cubic perovskite structure and thus no ferroelectricity is detected at room temperature.X-ray diffraction reveals that the grains can hardly grow even firing at high temperature so that the grain size in most of the samples is below the ferroelectric critical size.A tentative explanation is proposed of the peculiarities based on the nonstoichiometry at the surface layer.

Surface Effects on Dielectric Susceptibility of a Semi-infinite Ferroelectrie

The profilc of susceptibility uear the surfuce of a senni-ifinite frrclectric is calculatel from Luidau free energy.In surfunce ferroclectric plase,the susceptibility is reduced at surface for|δ|<<βand much enhanced for|δ|-β,whereδandβare the.extrapolation le1ngth and tle coluerent lengtl respectiv:ly.In ferroelectric phase,the surface susceptibility is gcllcrally reduced for8<0.But the crurve is not molnotonic,there is a mininum in the surface layer.Forδ>0,the surface susceptibility is.redced forδ<<βand enhanced fo rδ-β.The curve has a maxiln11 in the srface layer.

Low Temperature Phase Transition of an Orthorhombic Tungsten Bronze Ferroelectric Sr_(2-x)Ca_(x)NaNb_(5)O_(15)

The dielectric and pyroelectric properties of a new ferroelectric crystal with orthorhombic tungsten bronze structure Sr_(2-x)Ca_(x)NaNb_(5)O_(15) have been investigated in the temperature range of 300 to 10 K.Based on anomalies in the properties and by use of Curie's principle of symmetry,a ferroelectric-ferroelectric phase transition between mm2 and m is found to occur at about 75K.On cooling through the transition,the spontaneous polarization deviates by about two degrees from the two-fold axis[001]to[h0l]direction in the mirror plane.

Sduties on Modification of Pb_(1-x)BaxNb_(2)O_(6) Crystals

This paper reports the growth of pqre and Na,K modified lead barium niobate crystals and the measurement of their dielectric and structure properties.Exprimental results show that both pure and Na,K modified lead barium niobate crystals have growth striations.All the crystals belong to 4mm point group of tetragonal system at room temperature.Their Curie temperature are 379℃,389℃ and 473℃,respectively.

Growth and Properties of Tungsten Bronze-Type Second Harmonic Generation Crystal LiBa_(2)Nb_(5)O_(15)

Striation-free LiBa_(2)Nb_(5)O_(15)(LBN)single crystal of good optical quality has been grown by Czochralski method.The crystal growth and dielectric,structural,spectral properties of LBN were measured.Experimental results show that LBN single crystal belongs to tetragonal tungsten bronze structure at room temperature.Experiments carried out with LBN powder samples have shown that this crystal has good second harmonic generation effect.At room temperature,the dielectric constant ε_(33) is about 87 and Curie temperature T_(c),is about 586℃.A very ideal uniaxial conoscope image can be observed in LBN single crystal under polarizing microscope.In the range of 600 to 2600nm,the transmission of single crystal is about 75%.

STUDIES ON PHOTOREFRACTIVE CRYSTAL LEAD BARIUM NIOBATE

The optical quality Pb^(1-x)Ba_(x)Nb_(2)O_(6) single crystals have been grown by Czochraski method from the melt.Their structure,dielectric,ferroelectric properties were studied.Transmission spectra,two-wave maxing properties were measured.This crystal will be an available material for photorefractive applications.

LOW TEMPERATURE PROPERTIES OF Tb-DOPED Sr_(1-x)Ba_(x)Nb_(2)O_(6)(SBN)CRYSTALS

Tb-doped SBN crystals were prepared by the Czochralski technique.Their dielectric,ferroelectric,and pyroelectric properties were studied.A low temperature phase transition between two polar phases was revealed by the anomalies of the dielectric and pyroelectric properties;the polar axis changes its direction during this transition.The point group of the low temperature phase may be mm2,m or 1.

CRYSTAL GROWTH AND FUNDAMENTAL PROPERTIES OF Cd_(x)(Sr_(0.61)Ba_(0.39))_(1-x)Nb_(2)O_(6)

Cd_(x)(Sr_(0.61)Ba_(0.39))_(1-x)Nb_(2)O_(6)(CdSBN)crystals have been grown successfully by Czochralski technique.The difficulty in crystal growth increased with the increase of the crystal diameter.Macroscopic defects,such as striations and crackings were observed in some crystals.Temperature dependence of dielectric constant ε′γ3 and pyroelectric coefficient P were studied.The relationship between grown behavior and melt compositions was also discussed.

DIELECTRIC RESPONSE OF A SEMICONDUCTING CERAMIC BASED ON BARIUM TITANATE

Complex dielectric constant ε=ε′-iε″ er-iert of a semiconducting BaTiO_(3) ceramics has been measured at room temperature over the frequency range of 10kHz-500MHz・This dielec-trzc response can be approximately described by the Debye type relaxation equations^whereas the conductzvzty dominates the imaginary part at lower frequencies・From the dielectr'ic measurement3 the resisti-vity and the dz~electric constant have been evaluated for the grains and grain boundaries separately.