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1.
Yu-Dong Lu Xin Wang Zhi-Qiang Zhuang Bao-Ling Liu 《Journal of the American Ceramic Society》2007,90(11):3628-3631
The equilibrium defect chemistry of polycrystalline, undoped, and acceptor-doped BaPbO3 was studied by measurement of the equilibrium electrical conductivity as a function of temperature, 800°–900°C, and oxygen activity, 10−18 –1 atm. Both equilibrium electrical conductivity data of undoped and acceptor-doped samples were quantitatively fit to a defect model involving only doubly ionized oxygen vacancies, lead vacancies, holes, and acceptor impurities. The results in low and midrange of oxygen activity are dominated by acceptor impurities, whether deliberately added or not. Only in the highly oxidized condition is the conductivity independent of impurity content, confirming that this region represents the intrinsic behavior of BaPbO3 . 相似文献
2.
Equilibrium electrical conductivity of nonstoichiometric poly-crystalline BaTiO3 with varying Ba:Ti ratios was investigated at temperatures between 800° and 1200°C and P o2 from 10−22 to I atm. A transition from p -type to n -type conductivity was observed. Although the electrical conductivity of different specimens varied slightly, these differences did not appear to be a function of the Ba:Ti ratio in the region investigated. An intrinsic band-gap energy of ∼3.1 eV was calculated from the temperature dependence of the minimum conductivity. The O2 partial pressure dependence of the isothermal n -type conductivity cannot be described by simple defect models incorporating only singly or doubly ionized O vacancies. Likewise, simple defect models incorporating cation vacancies are not consistent with the observed pressure dependence of the p -type conductivity. More complex defect models which correspond to the observed behavior over the entire range of temperature and P o2 will be discussed in a subsequent paper. 相似文献
3.
The defect structure of monoclinic ZrO2 was studied by measuring the transfer numbers and electrical conductivity as functions of O2 pressure and temperature. The data suggest a defect structure of doubly ionized oxygen vacancies at low pressures, i.e. <10−19 atm, and singly ionized oxygen interstitials at pressures >10−9 atm. Zirconia is primarily an ionic conductor below #700°C and an electronic conductor at 700° to 1000°C for 10−22 ≤Po2 ≤1 atm. 相似文献
4.
The electrical conductivity and thermoelectric power of single-crystalline Ba0.03 Sr0.97 TiO3 were measured over a wide temperature (800° to 1100°C) and oxygen partial pressure (105 to 10-15 Pa) range. Our experimental data, like those of previous workers on nominally undoped BaTiO3 or SrTiO3 , support a defect model based on doubly ionized oxygen vacancies, electrons, holes, and accidental acceptor impurities. The simultaneous measurement of electrical conductivity and thermoelectric power, together with precise experimental data obtained with an advanced thermoectric power measurement technique, enabled us to determine for the first time reliable values for the preexponential factors and the activation energies which characterize the defect equilibrium constants. These calculated values, together with the defect model, were found to give an excellent fit to the experimental data, and were used to generate the boundaries, in P o2 -1/ T space, of the various defect regimes. 相似文献
5.
The defect structure of high-purity, polycrystalline HfO2 was investigated by measuring the oxygen partial pressure dependence of the electrical conductivity and the sample weight. From 1000° to 1500°C and above oxygen partial pressures of 10 −6 , the conductivity is electronic and proportional to p o2 1/5 . The predominant defect is completely ionized hafnium vacancies. At lower oxygen partial pressures a broad shallow minimum in the lower temperature conductivity isotherms indicates the presence of an oxygen pressure independent source of electronic charge carriers. By combining the weight change and conductivity data, mobility values were found to vary from 1.6 × 10−3 to 3 × 10−4 cm2 /V-sec. The activation energies for the hole mobilities were calculated to be 0.2 ev above 1300° C and 0.7 ev below this temperature. 相似文献
6.
The effect of added Al, as an acceptor impurity, on the equilibrium electrical conductivity of large-grained, polycrystalline BaTiO3 , is consistent with a previously proposed defect model which involves only doubly ionized oxygen vacancies, electrons, holes, and acceptor impurities. The behavior is an extension of that of undoped BaTiO3 , in which an accidental net acceptor excess already plays an important role. Comparison of the derived active acceptor content with the amount of added Al indicates that Al is <50% effective in creating acceptor levels. The magnitude of a small Po2 -independent conductivity component, necessary to fit the observed conductivity minima, increases with added Al content. This is consistent with a contribution from extrinsic oxygen vacancy conduction. 相似文献
7.
Complex impedance analysis at cryogenic temperatures has revealed that the bulk and grain boundary properties of BaTiO3 polycrystals are very sensitive to the oxygen partial pressure during sintering. Polycrystals sintered at P O2 as low as 10−15 atm were already electrically heterogeneous. The activation energy of the bulk conductivity in the rhombohedral phase was found to be close to that of the reduced undoped single crystal (i.e., 0.093 eV). The activation energy of the grain boundary conductivity increases with the temperature of the postsinter oxidation treatment from 0.064 to 0.113 eV. Analysis of polycrystalline BaTiO3 sintered in reducing atmosphere and then annealed at P O2 = 0.2 atm has shown that the onset of the PTCR effect occurs at much higher temperatures than expected in the framework of the oxygen chemisorption model. The EPR intensity of barium and titanium vacancies increases after oxidation at T > 1000°C. A substantial PTCR effect is achieved only after prolonged annealing of the ceramic in air at temperatures as high as 1200–1250°C. This result suggests that the PTCR effect in polycrystalline BaTiO3 is associated with interfacial segregation of cation vacancies during oxidation of the grain boundaries. 相似文献
8.
Studies of the oxidation of Gd and Dy at P O2 's from 10−0.3 to 10−14.5 atm and temperatures from 727° to 1327°C indicate both semiconducting and ionic-conducting domains in the sesquioxides formed. At higher temperatures, where dense coarsegrained oxide layers developed, the rate of oxidation in the high- P 02 semiconducting domain yielded oxygen diffusion coefficients in Dy2 O3 in excellent agreement with literature values derived from oxidation of partially reduced oxide single crystals. Under the same conditions, the oxidation of Gd yielded oxygen diffusion coefficients in cubic Gd2 O3 which are considerably below literature values for monoclinic single-crystal Gd2 O3 . At lower temperatures, porous scales were formed, and apparent diffusion coefficients derived from oxidation rates show a smaller temperature dependence than the high-temperature data. At low P O2 , the oxides behave as ionic conductors, and metal oxidation rates result in estimates of the electronic contribution to the electrical conductivity of the order of 10−6 to 10−7 Ω−1 cm−1 . 相似文献
9.
Bernhard Ostrick Maximilian Fleischer Hans Meixner 《Journal of the American Ceramic Society》1997,80(8):2153-2156
Simultaneous Hall and conductivity measurements were performed in situ between 650° and 1050°C on n-type semiconducting BaSnO3 ceramics. The variation of the Hall mobility and the Hall carrier density as a function of oxygen partial pressure between 102 and 105 Pa and of temperature was investigated. At temperatures below 900°C the conductivity exhibits a dependence on temperature and oxygen partial pressure which is mainly determined by variations of the Hall mobility. Above 900°C most of the significant dependence is due to a variation in carrier density. Furthermore, a simple defect model assuming doubly ionized oxygen vacancies and acceptor impurities is discussed for BaSnO3 . 相似文献
10.
The oxygen vacancy concentration of BaTiO3 doped with acceptors (Cr to Ni) is determined gravimetrically as a function of the O2 partial pressure during and after annealing at 700° to 1300°C. The oxygen vacancy concentration of these materials is larger than that of undoped and donor-doped BaTiO3 . The oxygen vacancies are doubly ionized and they compensate the acceptors of lower valence. Both the vacancy concentration and the valence of the acceptor dopants depend on the annealing conditions. The electronic energy levels of the acceptors within the BaTiO3 band gap are derived from the gravimetric measurements. The electrical properties of the acceptor-doped ceramics are favorable for base-metal-electrode multilayer capacitors, which require sintering in reducing atmospheres. 相似文献
11.
JUNICHIRO MIZUSAKI TADASHI SASAMOTO† W. R. CANNON‡ H. KENT BOWEN 《Journal of the American Ceramic Society》1982,65(8):363-368
Electronic conductivity and Seebeck coefficients of LaFeO3 were measured as a function of temperature (1000° to 1400°C) and P ( O 2 ) (105 to 10−13 Pa). Electronic conduction was found to be n-type in the lower P ( O2 ) range, and p -type in a higher P(O2 ) range. The calculated carrier mobilities suggest a hopping-type conduction mechanism. The carrier concentrations were calculated as a function of P ( O2 ) and the defect structure was described. It was found that the electrical properties of LaFeO3 are determined not only by the concentration of oxygen vacancies, but also by the La/Fe ratio. 相似文献
12.
Guarded measurements of the electrical conductivity of high-purity, polycrystalline Y2 O3 in thermodynamic equilibrium with the gas phase were made under controlled temperature and oxygen partial pressure conditions. Data are presented as isobars from 1200° to 1600°C, and as isotherms from oxygen partial pressures of 10−1 to 10−17 atm. The ionic contribution to the total conductivity, determined by the blocking electrode polarization technique, was less than 1% over the entire range of temperatures and oxygen partial pressures studied. Yttria is shown to be an amphoteric semiconductor with the region of predominant hole conduction shifting to higher pressures at higher temperatures. In the region of p -type conduction, the conductivity is represented by the expression σ= 1.3 × 103 p O2 3/16 exp (-1.94/kT). The observed pressure dependence is attributed to the predominance of fully ionized yttrium vacancies. Yttria is shown to be a mixed conductor below 900°C. 相似文献
13.
YOUNG HO HAN JOHN B. APPLEBY DONALD M. SMYTH 《Journal of the American Ceramic Society》1987,70(2):96-100
The equilibrium electrical conductivity of polycrystalline, calcium-doped BaTiO3 was studied over the oxygen partial pressure range 10-13 to 105 Pa and the temperature range 800° to 1000°C. There is little effect if CaO is substituted for a corresponding amount of BaO, i.e., Ba, 1-x Cax TiO3 . If CaO is substituted for a corresponding amount of the TiO2 content, i.e., BaTi1-x Cax O3-x , the equilibrium conductivity shows strong evidence of acceptor-doped behavior. If the corresponding amount of excess CaO is added to stoichiometric BaTiO3 , i.e., BaCax TiO3+x , the conductivity profiles are very close to those for samples with TiO2 replaced by CaO, and show highly acceptor-doped behavior. This is in agreement with the replacement of a small amount of Ti by Ca2+ on the octahedral B-sites of BaTiO3 , where it acts as an acceptor center, CaT 相似文献
14.
The effects of heat treatment in Ar-O2 and H2 -H2 O atmospheres on the flexural strength of hot isostatically pressed Si3 N4 were investigated. Increases in room-temperature strength, to values significantly above that of the aspolished material, were observed when the Si3 N4 was exposed at 1400°C to (1) H2 with water vapor pressure ( P H2O ) greater than 1 × 10−4 MPa or (2) Ar with oxygen partial pressure ( P O2 ) of between 7 × 10−6 and 1.5 × 10−5 MPa. However, the strength of the material was degraded when the P H2O in H2 was lower than 1 × 10−4 MPa, and essentially unaffected when the P O2 in Ar was higher than 1.5 × 10−5 MPa. We suggest that the observed strength increases are the result of strength-limiting surface flaws being healed by a Y2 Si2 O7 layer formed during exposure. 相似文献
15.
The electrical conductivity of polycrystalline Y2 O3 has been studied as a function of the partial pressure of oxygen (10–14 to 105 Pa) at 900° to 1500°C in atmospheres saturated with water vapor at 12°C or dried with P2 O5 . Yttria is a p -conductor at high oxygen activities. The p -conductivity increases with increasing P O2 and decreases with increasing PH2O . At low oxygen activities the oxide is a mixed ionic/electronic conductor. The ionic conductivity is approximately independent of P O2 and increases with increasing P H2O . In the Y2 O3 samples, excesses of lower-valent cation impurities (in the 10 to 100 mol-ppm range) are the dominating negatively charged defects, and in the presence of water vapor they are compensated by interstitial protons. At high P H2O levels additional protons are probably compensated by interstitial oxygen ions. At high temperatures (±1100°C) and for high P O2 and low P H2O , the protons are no longer dominant, and the lower-valent cations are mainly compensated by electron holes. The electrical conductivity exhibits hysteresis-like effects which are interpreted in terms of segregation/desegregation of impurities at grain boundaries. The mobility of electron holes in yttria at 1500°C is estimated to be of the order of magnitude of 0.05 cm2 . s–1 . V–1 相似文献
16.
Thermogravimetric measurements were made on NiO from 800° to 1100°C over the oxygen pressure range 10−1 to 10−4 atm. On the basis of complementary conductivity measurements showing a P (O2 )1/5 oxygen pressure dependence, it is proposed that the predominant defects are described by an electroneutrality condition involving doubly ionized metal vacancies, impurities, and electron holes, 2[ V"M ]=[ FM ]+ p. It is shown that for this defect model, the weight change relative to a low oxygen pressure reference weight is a measure of the effective vacancy concentration, defined as [ V"M ]eff ≡[ V"M ]-≡[ FM ], and therefore has the same oxygen pressure dependence as the electron hole concentration followed in the conductivity measurements. The expression [ V"M ]eff =0.168 P (O2 )1/5 exp (−0.86±0.15/ kT ) is derived to express the effective vacancy concentration in NiO. The probable effective impurity content of the specimens used is calculated. 相似文献
17.
The defect structure of acceptor-doped (Fe, Al, Cr) polycrystalline strontium titanate was investigated by measuring the equilibrium electrical conductivity as a function of oxygen activity (10−21 ≤ao2 ≤1) and temperature (85°C≤ T ≤ 1050°C). The electrical conductivity was n type with ∼−1/4 dependence on a 02 for a02 <10−10 . For a02 >10−8 , the observed data for Fe- or Al-doped samples were proportional to ∼1/4.5 power of a02 . In this region for Cr-doped samples, the value of m in.σp αa02 +1/m varies from ∼4.80 to ∼9.00 as the concentration of Cr is increased from 460 to 10 000 ppm. The onset of p -type conductivity depends on the amount. of acceptor impurity added to the sample. The absolute values of the conductivity in the acceptor-doped samples were lower in the n-type region than those for undoped SrTiO3 . The conductivity minima shift toward lower oxygen activity with increasing acceptor concentration. For the entire oxygen activity rangae used in this study, the defect structure of SrTiO3 is dominated by the added acceptor impurities. 相似文献
18.
A combination of ac and dc electrical conductivity measurements was used to characterize the charge transport in single crystals of CaW04 which were equilibrated with H2 -H2 O-Ar gas mixtures. Measurements were made from 900° to 1300°C and at PH2 O /PH2 ratios from 0.02 to 3.0. The ac conductivity at 1000°C varied from 51.4×10−6 to 5.89×10−6 mho/cm for PH2 O /PH2 =0.02 and 2.0, respectively; the dc conductivity changed from 51.0±10−6 to 5.42×10−6 mho/cm under the same experimental conditions. The partial logarithmic derivative of dc conductivity with respect to PH2 O /PH2 was −1/2 between 1000° and 1300°C. The results may be described by a paired-defect model of oxygen vacancies and oxygen interstitials (majority defects and minority electrons). 相似文献
19.
The electrical conductivity of polycrystalline TiTa2 O7 was measured at 1000° and 1050°C as a function of oxygen partial pressure from 10−1 to 10−21 MPa. In the apparent n-type region, the value of m in σn ∝PO2 −1/m was found to be ∼6 in the region >10−17 MPa and ∼4from 10−16 to 10−9 MPa. The conductivity appeared to be p-type for P02 <10−5 MPa. The measured data are explained on the basis of the presence of small amounts of acceptor impurities in the undoped sample. 相似文献
20.
The chemical diffusivity of 1.8 mol% aluminum-doped BaTiO3−δ was measured on single-crystal specimens, as a function of ambient oxygen partial pressure, in the range 10−18 atm ≤ P O2 ≤ 1 atm and at temperatures of 800°≤ T ≤ 1100°C, via a conductivity-relaxation technique. As in the polycrystalline, undoped BaTiO3−δ described in Part II of this work, the chemical diffusivity exhibited a maximum, of thermodynamic origin, approximately at the stoichiometric composition (δ= 0). The measured diffusivity was analyzed, based on the defect structure proposed and Wagner's classic theory of chemical diffusion, and the mobilities of the electrons and holes, as well as all of the relevant defect-equilibrium constants, then were evaluated with no prior assumptions. The evaluated parameters were compared with those for the undoped BaTiO3−δ . 相似文献