Browsing by Author "LAI, CH"

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ANALYSIS OF THE AC ELECTRICAL RESPONSE FOR (BA,PB)TIO3 POSITIVE TEMPERATURE-COEFFICIENT CERAMICS
(1994-06-01)
LAI, CH
;
TSENG, TY
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
AC electrical properties of positive temperature coefficient of resistance (PTCR) ceramics of composition Ba0.347Pb0.65La0.003Ca0.005Ti0.995O3 with Curie point Tc almost-equal-to 380-degrees-C were studied. A combined imaginary impedance/modulus plot was adopted to identify the appropriate equivalent circuit. Analysis of such circuit modified with a frequency dependent capacitor C(n) gives satisfactory interpretation of the commonly observed inclined semicircle in the complex impedance plane. The exponent n, which is extracted from the depression angle and suggested to be associated with the ''loss'' degree of the material, shows a temperature dependence similar to that of a useful parameter extracted from the ac conductivity-frequency measurements. Both are ascribed to he conduction characteristics of the grain boundary, whose resistivity anomaly is the main contribution to PTCR jump. The validity of the present methodology is assessed not only by fitting well the observed non-ideality of real ac response, but also by confirming the Curie-Weiss behavior of the calculated grain boundary capacitance, which is the essence of well-accepted barrier theory (Heywang-Jonker model) proposed to explain the PTCR effect.
CALCULATIONS AND MODELING OF GRAIN-BOUNDARY ACCEPTOR STATES FOR (BA,PB)TIO3 POSITIVE TEMPERATURE-COEFFICIENT CERAMICS
(1993-09-01)
LAI, CH
;
LU, YY
;
TSENG, TY
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
The electrical properties of high-Curie-point positive temperature coefficient of resistance ceramics of composition Ba0.497Pb0.5La0.003Ca0.005Ti0.995O3 prepared from commercial BaTiO3 powder were studied. The acceptor-state energy and acceptor-state density were determined from the calculated rho(max) vs T(max) plot, where rho(max) and T(max) are the resistivity and temperature corresponding to the maximum in resistivity. These two parameters, combined with the effect of nonvanishing spontaneous polarization below the Curie point, were treated quantitatively to demonstrate the dependence of measured resistivity rho(dc) on the measured permittivity epsilon(m). The obtained values of potential barrier height and acceptor-state density based on the procedure above were found to be in excellent agreement with those extracted from the slope in the Arrhenius plots of resistivity versus 1/(Tepsilon(m)), where T is the absolute temperature. Also, the deviation of the measured dielectric constant epsilon(m) from the Curie-Weiss law's prediction was explained by the narrowing of the grain-boundary barrier layers.
EFFECT OF SOAKING TIME ON THE TEMPERATURE-COEFFICIENT OF RESISTIVITY OF SEMICONDUCTING BARIUM-TITANATE PTCR CERAMICS
(1994-09-01)
LU, YY
;
LAI, CH
;
TSENG, TY
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
The influence of soaking time at 1200 degrees C on the temperature coefficient of resistivity (TCR) of semiconducting barium titanate ceramics has been investigated. It is found that soaking duration has influence on the room-temperature resistivity, the maximum resistivity, the temperature (T-max) at which the maximum resistivity appears, and TCR of the positive temperature coefficient of resistivity (PTCR) ceramics. The increased room-temperature resistivity with the increase of soaking time is contributed mainly by the increase of grain boundary resistivity examined by the complex-plane impedance method. The obtained surface acceptor density (N-s) by capacitance-voltage measurement is found to increase with the soaking duration. The higher N-s contributes to higher built-in potential and results in lower T-max and higher maximum resistivity. Therefore, the increased surface acceptor density from increasing the soaking time at 1200 degrees C increases the temperature coefficient of resistivity of PTCR ceramics.
INFLUENCE OF SINTERING PROFILE ON THE RESISTIVITY OF LOW-CURIE-POINT PTCR CERAMICS
(1993-08-01)
LU, YY
;
LAI, CH
;
TSENG, TY
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
The influence of sintering profile, with emphasis on the effect of soaking treatment and cooling rate, on the resistivity of low-Curie-point positive temperature coefficient of resistivity (PTCR) ceramics has been investigated. Soaking treatment at 1200-degrees-C or slower cooling rate were found to result in an increase in the room-temperature and maximum values of resistivity in the resistivity-temperature characteristics. The surface acceptor density (N(s)), whose value was extracted from the capacitance-voltage measurement, was found to increase with soaking treatment at 1200-degrees-C or slower cooling rate. From derivations based on the Heywang-Jonker model, higher room-temperature and maximum resistivities are expected to occur as a result of increasing N(s), as confirmed by the experimental data.
INVESTIGATION OF RESISTIVITY AND PERMITTIVITY FOR (BA,PB)TIO3 PTCR CERAMICS
(1994-09-01)
LAI, CH
;
TSENG, TY
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
A scheme to examine quantitatively the interdependence between the measured resistance and permittivity data for semiconducting (Ba,Pb)TiO3 samples having Curie points above 300 degrees C was developed based on the Schottky-type potential barrier model. Specifically, the nonvanishing spontaneous polarization in the ferroelectric state was considered in terms of a useful parameter to explain the low resistivity below the Curie point. The resistivity derived from the measured permittivity shows a consistent temperature dependence with the de measured one, i.e., the positive temperature coefficient of resistance (PTCR) characteristics. It is also observed that more satisfactory agreement can be achieved when the acceptor states are assumed to be distributed over a certain energy interval, The values of acceptor-state densities thus obtained are found to be in excellent agreement with those extracted from the slope in the Arrhenius plots of resistivity vs 1/(T epsilon(m)), where T is the absolute temperature and epsilon(m) the dielectric constant. The validity of the present methodology is justified by obedience to the Curie-Weiss law for the calculated bulk dielectric constant.
PREPARATION AND AC ELECTRICAL RESPONSE ANALYSIS FOR (BA,PB)TIO3 PTCR CERAMICS
(1993-03-01)
LAI, CH
;
TSENG, TY
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
For those PTCR materials with higher Pb content, the thermodynamic derivation of increasing values for ''inversion temperature'' and cation vacancy diffusion rate accounts for the increases in the calculated acceptor-state densities N(s) (cm-2). The influence of frequency-dependent properties for the dielectric constant on the calculation of acceptor-state densities is also presented. Applying the concept of suitable value for N(s) to obtain a strong PTC effect, (Ba,Pb)TiO3 PTCR ceramics with Curie point T(c) over 340-degrees-C up to 420-degrees-C have been successfully prepared by adopting higher cooling rates.
THE EFFECTS OF ND2O3 ADDITIVES AND AL2O3-SIO2-TIO2 SINTERING AIDS ON THE ELECTRICAL-RESISTIVITY OF (BA,SR)TIO3 PTCR CERAMICS
(1995-04-01)
LAI, CH
;
WENG, CT
;
TSENG, TY
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
The influences of Nd donors and Al2O3-SiO2-TiO2 (AST) sintering aids on the electrical properties of (Ba,Sr)TiO3 materials have been investigated. The room-temperature resistivity and the temperature coefficient of the anomalous resistivity rise were used to characterize the performance of the samples. From derivations based on the Heywang-Jonker barrier model, both characterizing parameters are expected to increase as a result of higher acceptor-state density at the grain boundary. The surface acceptor density, whose value was extracted from the slope in the Arrhenius plot of resistivity versus 1/(T epsilon(m)), where epsilon(m) is the measured permittivity and T the absolute temperature, was found to decrease with the Nd content and increase with the AST dopant. A satisfactory interpretation of the observed variations in resistivity-temperature curves caused by additions of various dopants was thus obtained in the light of the resultant acceptor state density.