The temperature dependence of resistivity of Bi₂(Sr, Ca, Y)₃Cu₂O_y solid solutions has been measured from room temperature to 1073K in both air and N₂ atmosphere. The effects of the substitution of Y ions for Ca ions are discussed using the experimental data of electrical resistivity of both Bi₂Sr₂Ca_1-xY_xCu₂O_y and Bi₂Sr_1.5Ca_1.5-xY_xCu₂O_y. Therefore it is likely that the Sr/Ca ratio in these solid solutions is related to the effects of the substitution of Y ions for Ca ions.

View full abstract

Changes in the structural modulation and superconducting properties were examined in Bi_2-xPb_xSr₂CaCu₂O_8+δ and Bi_2-yPb_ySr₂YCu₂O_8+δ Systems. In both systems, b-axis length decreased and the periodicity of the structural modulation along b-axis increased with the increasing Pb content, and the structural modulation disappeared in the high Pb concentration region. The changes in the structural modulation were explained by the decrease in the additional oxygen in the Bi-O layer. Besides the relaxation of the structural modulation, the superconducting transition became sharp and a slight increase in T_c was observed in Bi_2-xPb_xSr₂CaCu₂O_8+δ system.

View full abstract

High-T_c Bi-based superconductive oxides are prepared by doping a small amount of fluorine. Starting materials including fluoride powders are heated in air. Under the optimum synthesis condition a superconductor with T_c=113K and T_cend=106K is obtained; the firing temperature is 860°C, which is lower by about 20°C as compared to the case without doping fluorines. During the heating process most fluorines are released from the crystal, while the grains with the (243) phase grow very large at the expense of the (232) grains. High-resolution transmission electron microscopy (HRTEM) reveals a frequent intergrowth of layers as well as a local deformation of the supercell from the orthorhombic to monoclinic system. Another series of synthsis in a closed reaction system resulted in a less favorable result.

View full abstract

The influences of additives such as Pb or Sb on the superconducting properties and crystallization process of Bi-Sr-Ca-Cu-O glass-ceramics were investigated. T_c (end) was influenced only by Pb, but not by K, Na, Sn, Sb or Li. The addition of Pb increased T_c (end) above 100K. The results of differential thermal analysis and X-ray diffraction analysis showed that the addition of Pb, Sn or Pb and Sb enhanced crystallization of the superconducting phases, but no clear exothermic crystallization peak was observed in the samples containing K or Na oxide.

View full abstract

The crystallization mechanism of an amorphous Bi₂Sr₂Ca₂Cu₃O_y phase was studied from the relations between crystallization and volume changes by dilatometry. Further, the effect of addition of CdO on the crystallization mechanism and superconductivity was discussed. The glass transition and crystallization temperatures for the amorphous phase with composition of Bi₂Sr₂Ca_2-xCd_xCu₃O_y were between 420° and 360°C, and between 440° and 420°C, respectively. Both values tend to decrease with increasing CdO content. The shrinkage of the amorphous Bi₂Sr₂Ca_2-xCd_xCu₃O_y phase occurred with the crystallization of a c₀=2.5nm phase. Furthermore, the amount of the c₀=2.5nm phase decreased with increasing CdO content with a minimum at x=0.4. Better superconductivity was obtained in the specimens containing less amount of the c₀=2.5nm phase.

View full abstract

Crystals of Bi₂Sr₂Ca_n-1Cu_nO_y superconductors were grown by the floating zone technique. The effects of oxygen pressure and growth rate were investigated. Oxygen pressures were varied from P_O2=0.005 to 1atm, and the growth rate from 60 to 0.8mm/h. It was found that higher oxygen pressures were important in growing single phase specimens of both the 232 (n=2) and the 221 (n=1) phases. It was also demonstrated that a lower growth rate was required to form single phase specimens of both the 232 and the 221 phases. Single crystals of the 232 phase up to 4.1×1.3×0.1mm³ and of the 221 phase up to 1.3×0.06×0.01mm³ were grown at growth rates of 0.8mm/h and 3mm/h, respectively. However, no single crystals of the 243 (n=3) phase have been obtained so far by the present method.

View full abstract

High-T_c superconducting Bi-Sr-Ca-Cu-O films were prepared on some metal substrates by pyrolysis of 2-ethylhexanoates, with or without ZrO₂ as a buffer layer. With respect to superconducting transition temperature (T_c), the films prepared on ZrO₂/Ag substrates have the highest of all the present films, those were T_c (onset)=90K and T_c (end)=80K. Without the buffer layer, only the films on Ag, stainless steel and hastelloy substrates showed the superconducting transition, and with the layer of ZrO₂, T_c (end) distinctly increased for them and, moreover, some films on other metals such as Cu have T_c (onset) above 10K. The ZrO₂ buffer layer seems to be appropriate to form homogeneous films on metal substrates.

View full abstract

The influences of microstructure on the critical current density of a high T_c superconductor YBa₂Cu₃O_x were discussed on the basis of J_C·R measured by the standard four-probe resistive method as well as of J_C·M calculated from the magnetization curve at 77K. The microstructure was changed by controlling the sintering temperature and cooling rates. For the specimens sintered at 920°-960°C, J_C·R increased and the resistivity decreased as the bulk density increased. A decrease in J_C·R for the specimen sintered at 990°C resulted from the incongruent melting of YBa₂Cu₃O_x. For the specimens sintered at 990° to 1030°C, J_C·R increased again by the recrystallization during slow cooling. The intrinsic J_C of YBa₂Cu₃O_x was examined by the Josephson weak links theory from the magnetization data. As a result, the grain size measured with an optical microscope is more suitable compared with the thickness of specimens for the calculation of J_C·M. Moreover, the area surrounded by weak links is larger than the grain size of the specimens sintered at 940° to 975°C. The intrinsic J_C calculated from the area surrounded by the weak links was almost constant, 2.5×10⁴A·cm^-2 at 0.7kOe at the sintering temperature from 960° to 1030°C. J_C·R increased by changing the cooling process from 1030°C. This is due to the increase in the area surrounded by the weak links corresponding to the grain growth.

View full abstract

The synthesis conditions of a partial melting process for a bulk YBa₂Cu₃O_x (123) superconductor are discussed in terms of the microstructure and superconducting properties. The overall process consists of the partial melting, in which Y₂BaCuO₅ (211) and a liquid phase form, and subsequent slow cooling for the formation of the 123 grains by a peritectic reaction of 211 with liquid. Melt-quenched and sintered starting materials were examined to understand the difference in microstructural development. During partial melting, the grain size of 211 was considerably dependent on the starting materials. Melt-quench-derived materials produced much finer 211 than sinter-derived materials. After the peritectic reaction, the residual 211 inclusions were trapped inside the 123 grains. The size of the 211 inclusions was almost equivalent to that during the partial melting. The 123 grains also included defects such as cracks and streaks which may have formed due to inhomogeneities in the peritectic reaction during the growth of the 123 grains. The melt-quench-derived materials included fewer defects than the sinter-derived materials. The finely and homogeneously dispersed 211 grains present in melt-quench-derived samples are considered to be effective in reducing defects by promoting a smooth and uniform peritectic reaction. As a result, melt-quench-derived materials showed better critical current densities than sinter-derived materials.

View full abstract

The high T_c superconductor (YBa₂Cu₃O_7-x) was heated to 920°C in various atmospheres by the same heating schedule, and the change of oxygen concentration in the exhaust gas was continuously monitored with a mass spectrometer. When the superconductors were heated in an oxygen partial pressure above 0.02atm, the tetragonal to orthorhombic transformation occurred completely. However, the superconductivity was lost when the oxygen partial pressure was lower than 0.01atm. These results suggested that the rate of oxygen absorption by the superconductor during cooling was too low at low P_O2 to recover sufficient lattice oxygen. The curve of O₂ evolution from the superconductor during heating consisted of two peaks. The desorption of O₂ at a lower temperature resulted in the transformation from orthorhombic to tetragonal phases.

View full abstract

BYCO was synthesized from a mixture using Ba(NO₃)₂ as a Ba source. The thermal analysis, phase changes and thermal shrinkage during heating of the mixture were investigated, and compared with those of a mixture using BaCO₃. For small specimen sizes, BaCO₃ was formed after the decomposition of Ba(NO₃)₂, whereas for large specimen sizes, BaCO₃ was not detected. An unknown phase of Ba-Cu compounds was also recognized as an intermediate product. In specimens using Ba(NO₃)₂, BYCO was synthesized at a lower temperature and the formation was almost completed after the calcination at 900°C for 2h. The synthesized powder was fine and weakly agglomerated. The fired density and the superconductivity were almost the same regardless the source materials, Ba(NO₃)₂ and BaCO₃.

View full abstract

YBa₂Cu₃O_x superconducting ceramic containing various oxides were prepared. The superconducting properties such as critical temperature and current density were measured, and degradation in moisture was discussed. A sample containing 1 to 5wt% of Ag₂O showed increases both in relative density up to 95% of the theoretical density and in the critical current density by 2-3 times than that without addition. The sample containing Ag₂O was improved the degradation in moisture compared the without addition sample. A superconducting wire was prepared using Ag pipe and a superconducting powder containing Ag₂O. The critical current density of the obtained superconducting wire was 92A/cm², about 2 times larger than that without addition.

View full abstract

C-axis oriented bulk samples in Y-Ba-Cu-O system have been prepared by the uniaxial pressing technique with various pre-sintering temperatures and pressures. The c-axis orientation by pressing was compared with that of highly c-axis oriented films made by the sol-gel method. Suitable starting compositions and sintering temperatures for preparing c-axis oriented samples were similar in both techniques. The origin of the c-axis orientation was attributed to the growth of plate crystals under the partial melting condition in the sintering process.

View full abstract

Ba₂YCu₃O_7-y⋅Ag metal composite thick film superconductors were prepared by the screen-printing method. The influence of Ag addition on superconducting properties and the structure of the thick films were investigated. A film paste, prepared from Ba₂YCu₃O_7-y, Ag₂O (the weight ratio=100: X, X=0-40) and an organic ve hicle, was fired on YSZ substrate at 985°C for 3min in an oxygen atomosphere. At the firing stage, Ag₂O was decomposed into Ag, which was segregated at grain boundaries and the interface between the film and the substrate. Addition of Ag₂O significantly improved the superconducting properties and the adhesive strength. The composite film has superconducting properties of T_c, on=93K, T_c, end=91K and J_c (at 77K)=1300A/cm². The increment in J_c can be attributed to the film densification by molten Ag and the prevention of the reaction between the superconducting phase and the substrate. The segregated Ag is analogous to a glass frit in a conventional thick film tecnology and improves the adhesion of thick films.

View full abstract

Fabrication of YBCO Josephson devices requires the micro-fabrication techniques for submicron pattern definition. In this paper we described a new procedure for the fabrication of Josephson devices which make use of metal patterns in the substrate to define the length of the bridge. Using the electrophoretic and lift-off techniques, we fabricated the array-type Josephson microbridges and estimated their superconducting properties.

View full abstract

Four non-superconducting oxides, i.e. LaBa₂Cu₂TaO₈, LaBa₂Cu₂NbO₈, PrBa₂Cu₂TaO₈ and PrBa₂Cu₂NbO₈, were synthesized. Their crystallographic structures were determined by means of the Rietveld analysis using X-ray powder diffraction data. In these compounds, Ta and Nb ions were found to preferentially occupy the Cu 1 site between the two Ba layers in the LaBa₂Cu₃O₇-type structure. The absence of superconductivity in these materials was discussed in terms of the bond valence sum of each cation in the crystal and a possibility to exhibit superconductivity by modifying these materials was suggested.

View full abstract

Two-phase composite ceramics consisting of 8mol% Y₂O₃ stabilized ZrO₂ and 10 to 70wt% of ZrSiO₄ were fabricated. The ceramics showed ionic conduction above 500°C. The conductivity of the ceramics decreased with increasing ZrSiO₄ content, but the ceramics with 70wt% of ZrSiO₄ still had a high conductivity of 3×10^-1S·m^-1 at 1000°C. The activation energy of conductivity of the ceramics was 104kJ·mol^-1, which agreed closely with that of a pure 8mol% Y₂O₃ stabilized ZrO₂. The thermal expansion coefficient of the ceramics decreased with increasing ZrSiO₄ content, and the ceramics with 70wt% of ZrSiO₄ showed a low average linear thermal expansion coefficient of 5.5×10^-6°C^-1 between room temperature and 1000°C. The 8mol% Y₂O₃ stabilized ZrO₂-ZrSiO₄ ceramics showed higher ionic conductivity than the conventional 8mol% Y₂O₃ stabilized ZrO₂-Al₂O₃ ceramics, when they have the same thermal expansion coefficients.

View full abstract

Thin films of ZrO₂, MgO and magnesia stabilized zirconia (MSZ) were prepared in a reduced pressure (5-20 Torr) on glass and alumina substrates by organometallic chemical vapor deposition (OMCVD) using zirconium alkoxide, Zr(O-tBu)₄, and magnesium acetylacetonate, Mg(C₅H₇O₂)₂, as the starting organometallic compounds. ZrO₂ films of various phases (amorphous, tetragonal and monoclinic phases) were formed at the deposition temperatures from 200° to 600°C by pyrolysis of Zr(O-tBu)₄. Amorphous MgO films were grown at the deposition temperatures from 450° to 700°C by pyrolysis of Mg(C₅H₇O₂)₂ which was transported with oxygen carrier gas. Heat treatment of the as-grown films was required in order to eliminate the residual free carbon. Fully or partially stabilized zirconia films were obtained at 10 Torr by a simultaneous pyrolysis of Zr(O-tBu)₄ and Mg(C₅H₇O₂)₂ in the deposition temperatures from 450° to 700°C, where the MgO concentration (0-18.5mol%) in MSZ was controlled by the respective carrier gas flow rates at a given saturation temperature of each complex. The electrical conductivity of fully stabilized films (cubic phase) which were prepared above 700°C on an alumina substrate, increased slightly with MgO concentration in MSZ and showed an activation energy for conduction of about 1.2eV.

View full abstract

In order to produce ion conducting beta-alumina fibers by sol-gel method, the sols of Na₂O-Al₂O₃ system were prepared by heating the slurries consisting of Al(NO₃)₃⋅9H₂O, Al powder, NaNO₃, H₂O and HNO₃ in reflux. Long gel fibers of 5-250μm in diameter were drawn from the spinnable viscous sols. When the gel fibers were heated in an open cruicible, the amount of Na₂O vaporization from fibers increased as the fiber diameter decreased. When the gel fibers derived from the sol corresponding to calculated oxide composition Na₂O⋅7.3Al₂O₃ were heated at 1100°C for 10min with powder of high Na₂O content in covered cruicible, ceramic fibers consisting solely of fine beta-alumina crystals were produced.

View full abstract

The influences of yttrium-substitution on the protonic and O^2- conduction in ceramic yttrium substituted oxyhydroxyapatites with the formula [Ca_10-xY_x](PO₄)₆[(OH)_2-x-2yO_x+y_??__y](YHAp(x)) were studied. YHAp (0.65) showed the highest value of conductivity (5×10^-4S/cm). This result suggests that the introduction of O^2- and vacancies into OH sites changed the conduction mechanism. Specimens without OH^-, e. g. YHAp (1.0) indicated O^2- conduction, while protonic conduction was observed in the YHAp (x) samples with x<0.8. Especially, YHAp (0.4) and YHAp (0.65), with the remaining OH^- of more than 0.5 on the OH sites, were purely protonic conductor. These results suggest that the variation of the amounts of OH^-, O^2- and _??_ will be attributable to the change in the conduction mechanism. Because of the rapid response of YHAp (0.65) to the change in the partial pressure of H₂, YHAp can be expected to be applicable to high temperature fuel cells.

View full abstract

In the system Na₂O-R₂O₃-P₂O₅-SiO₂ (R=Y, Sm, Gd), new fast Na⁺-conducting glass-ceramics were obtained. These materials were structurally analogous to the silicates of Na₃YSi₃O₉ (N₃YS), Na₅YSi₄O₁₂ (N₅YS) and Na₉YSi₆O₁₈ (N₉YS). The ionic conductivities of these materials were strongly dependent on the type of crystal phase. Among them N₅YPS-type glass-ceramic showed the highest ionic conductivity, σ₃₀₀=9.8×10^-2S/cm. The complex admittance analysis indicated that the N₅YPS-type glass-ceramics consisted of two phases, crystallized grains and residual glassy matrix. Both time and temperature for crystallization had considerable influences on the conduction properties, thus, annealing at a higher temperature for longer time gave higher ionic conductivity of crystallized grains and higher activation energy of residual glass. Microstructure was considered to understand the thermal effects. The optimum conditions for crystallization were discussed with reference to the conduction properties.

View full abstract

The electrical conductivity of Na₂O-ZrO₂-P₂O₅-SiO₂ system was high (σ=1-2×10^-3S·cm^-1 at 300°C), and it increased further with 1-2 order by crystallization. The conductivity of the glass ceramics was σ=1-2×10^-2S·cm^-1 at 300°C and the activation energy was 0.3eV. This conductivity was equivalent to that of NaSbO₃ polycrystals. The crystals present in the glass ceramics were the hexagonal and monoclinic phases of Nasicon family. It was found that the electrical conductivity related markedly to the microstructure of glass ceramics composed of the crystal phases and glass matrix. The role of ZrO₂ in these glasses was also discussed.

View full abstract

Cuprous ion-conducting glasses were prepared in the pseudoternary system CuI-Cu₂MoO₄-Cu₃PO₄. These glasses had high ionic conductivities of 10^-3 to 10⁰ Sm^-1 at room temperature. The infrared spectra showed that these glasses were composed of Cu⁺, I^-, monomer MoO₄^2-, and monomer PO₄^3- ions, and classified as “ionic glasses” containing one type of cations and three different types of anions. The conductivity of these glasses increased with an increase in the amount of Cu⁺ ions which were generated from the CuI component. Substitution of PO₄^3- anions for MoO₄^2- anions caused a decrease in the conductivity of the glasses containing a given CuI content. The variation of the conductivity mentioned above can be explained in terms of the valence of oxo-anions contained in these glasses.

View full abstract

Lithium-containing fluorozirconate glasses of the composition (100-x) (0.75ZrF₄⋅0.25BaF₂)⋅xLiF, where x ranged from 0 to 60, were prepared in a dry box filled with N₂. Electrical properties of these glasses were measured under dry N₂ or Ar atmosphere and the following two issues were discussed. Firstly the influence of annealing on the electrical conductivity of these glasses was examined, and found that the strain generated during glass preparation should be removed by annealing the glasses for a couple of hours at temperatures 10°-20°C below their glass transition temperatures in order to obtain their true conductivities. Secondly the mobile ion species in these glasses were determined by the DC electrolysis method and also by the DC electrical conductivity measurements using reversible and ion-blocking electrodes, and found that the mobile ions in these glasses changed from F^- to Li⁺ at around 10-20mol% LiF.

View full abstract

Semiconductive crystalline PZT powders, doped with bismuth and potassium, were synthesized by the hydrothermal reaction. The synthesis of crystalline powders became difficult with increasing amount of Bi, K doped and with increasing Zr/(Ti+Zr) ratio. A substantial amount of Pb remained in the solution after the hydrothermal reaction. Therefore, the addition of Pb in excess of 25% was necessary to obtain the stoichiometric composition of the PZT solid solution. The Bi contents in the obtaind powders were identical to those of the starting compositions, while the K contents were less than those expected for large amounts of Bi, K-doped. The electromechanical properties of the sintered bodies were measured by the “Admittance-Curve-Fitting” method, where the material constants were refined by fitting the observed data to a theoretical expression. The electromechanical coupling factors and piezoelectric coefficients had maximum values at a particular Bi content. This behavior was similar to that of the specimens prepared by a solid state reaction, but the change in the conductivity with Bi content was different between the specimens prepared by the hydrothermal and the solid state reactions.

View full abstract

La₂CuO₄ ceramics with a different atomic ratios of La to Cu (La/Cu) ranging from 1.86 to 2.01 were prepared by the solid state reaction, and the electrical conductivities and Seebeck coefficients were measured under the oxygen partial pressure (p_O2) from 1.00 to 10^-4atm. The La/Cu of grains in the samples prepared with different La/Cu was almost the same (=2.00). The excess cation does not seem to enter the grains but segregate in the grain boundaries. The electrical properties of La₂CuO₄ ceramics were affected by the secondary phases in the grain boundaries. The oxygen content deviation and the electrical conductivity varied proportional to 1/6 power of p_O2. The carrier concentration calculated from the Seebeck coefficient was in a linear relation with 1/6 power of p_O2, and p_O2 dependent term was about twice as much as the oxygen content deviation.

View full abstract

The perovskite powder of (La, Sr)NiO₃ system was prepared using acetate precursors and then reacted with Na₂CO₃ flux at 900°C for 24 hours to produce the perovskite single phase after washing away excess Na⁺ ion. The obtained powder was sintered at temperatures between 860° and 1180°C for 10 hours in the flow of O₂ gas by hot press. The maximum temperature of the stability of the perovskite LaNiO₃ phase is limited to the temperature at about 1040°C in this hot pressing condition. The bulk density of LaNiO₃ which was sintered at 1040°C was greatly increased to 97 percentage of the theoretical value with a small amount of Na addition. The maximum stability temperature of perovskite La_1-xSr_xNiO₃ phase seems to be lower than that of LaNiO₃.

View full abstract

VO₂ (A), a metastable phase of VO₂, was prepared by hydrothermal treatment of an aqueous VOSO₄ solution. (VO₂ (A) has tetragonal symmetry but the crystal structure remains unknown. DTA and magnetic susceptibility measurements revealed a transition at 162°C which was detectable on heating but not on cooling. Semiconducting behavior was observed and the high-tempertature phase was characterized by lower electric resistivity. The transition on heating expanded the a-axis and contracted the c-axis. It was also observed that the lattice spacing along the c-axis was reduced to the half at the transition. The transition was found to be similar to the metal-insulator transition in the rutile-type VO₂.

View full abstract

Vanadium bronze is a nonstoichiometric compound having with a general formula M_xV₂O₅, where M is Li, Na, K, Cu, Ag, Ca, Cd or Pb. The value x varies from 0 to 1. This work was done in order to prepare conductive ceramics containing vanadium bronze (M_xV₂O₅) and to investigate their electrical properties. (1) Films containing lithium and sodium vanadium bronze: A series of films were prepared from gels by quenching of mixed molten oxides Li₂O-V₂O₅ and Na₂O-V₂O₅ into water and heat-treated at various temperatures of 500° to 600°C. The conductivity of these films was about 2S/cm at 40°C. The charge carrier in these films was confirmed to be electron from Seebeck coefficient. (2) The ceramics containing copper and silver vanadium bronze: A series of amorphous materials were prepared by roller-quenching of mixed molten oxides Cu₂O-V₂O₅ and Ag₂O-V₂O₅. Powders of these amorphous were pressed into tablets and heat-treated at various temperatures from 400° to 600°C. The ceramic with the composition of 24Cu₂O-76V₂O₅ showed metallic conduction and its conductivity was about 1S/cm at 30°C. The ceramic with the composition of 14Ag₂O-86V₂O₅ showed a semi-conductive character and its conductivity was about 1S/cm at 30°C.

View full abstract

V₂O₅-TeO₂ glasses containing more than 80mol% V₂O₅ were obtained by moderate quenching of melts with addition of Fe₂O₃. These glasses are semiconductive by the small polaron hopping mechanism. The hopping process has been concluded to be adiabatic for [V₂O₅]≥50mol%. Fe ions, as well as V ions, seem to contribute to the hopping conduction, as suggested for Fe₂O₃-V₂O₅-P₂O₅ glasses. Dc conductivity of the tellurite glasses was more than 1 order of magnitude higher than that of phosphate glasses for [V₂O₅]≥70mol%. However, conductivities of both glasses were in the same order for [V₂O₅]≅80mol%. Relatively low conductivity of the tellurite glasses containing about 80mol% V₂O₅ was attributed to the very low value of the fraction of reduced V ion.

View full abstract

The structure of V₂O₅-TeO₂ glasses containing Fe₂O₃ was investigated. Addition of Fe₂O₃ extended the glass formation region up to 80mol% V₂O₅. The Mössbauer spectra indicated that Fe ions were 3-valent and tetrahedrally coordinated. An IR absorption peak or the shoulder attributed to V=O was observed. The O1s photoelectron spectra were measured using MgKα emission and analyzed. Two O1s spectra were separated, and the difference in the binding energies of these two peaks was about the same as that between O1s spectra of bridging oxygen, BO, and of non-bridging oxygen, NBO, or of double-bonded oxygen, DBO. The high energy peak corresponded to BO and the low energy peak to NBO and DBO. These results suggest that these glasses have a layerlike vanadate network structure.

View full abstract

A high-conductivity oxide thin film of the spinel type Li_1+xTi_2-xO₄ (0≤x≤1/3) was prepared by the sol-gel method. Lithium acetate and tetra-n-butyl orthotitanate were used as raw materials, and diethanolamine was used as a solvent. A thin film was prepared by dip-coating of silica glass substrates into a solution. The composition depended on the atmosphere of synthesis. The insulator composition with x equal to 1/3 was obtained by heat-treatment at 750°C in the air. The conductive compositions with x equal to 0.2 to 0.05 were obtained by heat-treatment at 750°C in the H₂ atmosphere. The average film thickness was an 1.2μm. The conductivity increased with decreasing x. When x is 0.05, logσ was 2.1 (S/cm). The charge carrier in the thin film, was confirmed to be electron by the Seebeck coefficient measurements. The electric conduction of the thin film showed a higher value than that of powder of the same composition.

View full abstract

Highly conductive transparent films of F-doped ZnO were prepared by an ion plating method in which metals were evaporated in vacuo in the presence of inductively coupled r. f. oxygen plasma. The reaction gases were pure oxygen and mixtures of oxygen and fluorine containing gases such as nitrogen trifluoride and silicon tetrafluoride. Crystalline films (2000Å) were prepared at a substrate temperature of 100°C, and they had transmittances of about 90% and conductivities around 1000S/cm.

View full abstract

The tunneling current-voltage characteristics of SrTiO₃ ceramics were determined in air and in vacuum. The experimental results obtained immediately after polishing and etching with a HF solution showed that inhomogeneous adsorption occured on the surface, and that the surface states were affected by the adsorption. The results obtained after the thermal treatment in vacuum showed that there was no inhomogeneous adsorption layer. In both cases, it was suggested that space charge layers were formed at the surface of SrTiO₃ ceramics.

View full abstract

The effects of sintering additives on the thermoelectric properties of SiC ceramics were studied. Porous SiC ceramics with -60% relative density were fabricated by sintering the pressed β-SiC powder compacts with additives (B, AlN) at 1900°-2100°C for 0.5-5h in nitrogen atmosphere. The sintered bodies were analyzed by means of XRD and SEM. In the case of B addition, XRD analysis showed that little phase transformation occurred during sintering and that BN was formed by the reaction with N₂. Lattice parameter measurements revealed incorporation of a certain amount of added B into the β-SiC lattice. On the other hand, in the case of 10wt% AlN addition, XRD and SEM analyses showed a phase transformation of β-SiC to mainly 4 H-SiC during sintering. The electrical conductivity and the Seebeck coefficient were measured at 350°-1050°C in argon atmosphere. The average grain size and the quantity of additives had significant effects on the thermoelectric properties of SiC.

View full abstract

Effects of sintering time on the electrical conductivity of sintered silicon carbide containing boron carbide and carbon were investigated. The silicon carbide was sintered in the temperature range of 1900° to 2000°C for under 10h in a vacuum furnace. SiC sintered at 1950°C for 3-10h and at 2000°C for 0.5-10h have high density. The electrical conductivity at 300K of the low density specimens was higher than that of the high density specimens. The electrical conductivity of the specimens sintered at 1900°C for under 10h was independent of the measuring temperature and the frequency. However, the conductivity of the specimens sintered at 1950°C for 3-10h and sintered at 2000°C for 0.5-10h depended on the temperature and the frequency. These characteristics may be explained as follows. The current through the sintered SiC is classified into two groups. One is a current along the grainboundary layer, and another is a current across the Schottky barrier formed around the grainboundary. The current across the grainboundary is dominant in the high density specimens, then only these specimen show the temperature and frequency dependence of electrical conductivity.

View full abstract

Layer structured crystals ZrNX (X=Br and I) were prepared by the reaction of ZrH₂ with NH₄X (X=Br and I) vapors at temperatures ranging from 550° to 750°C. The reaction proceeded in a manner very similar to that reported previously on the preparation of β-ZrNCl from ZrH₂ and NH₄Cl. ZrNBr was obtained as the α polymorph. It was chemically transported to the higher temperature zone in a glass tube sealed with NH₄Br. In a temperature gradient of 450° to 550°C, α-ZrNBr was transported as the α-form, whereas in a temperature gradient of 750° to 850°C, it was transported as the β-form with the 3R type stacking. Only the α type layered structure was known for ZrNI, and the β type layered structure was not obtained even in the chemical transport in the temperature gradient of 750° to 850°C. Electrical properties of β-ZrNBr and β-ZrNCl are very similar in the sense that both of them are n-type semiconductors with relatively high electrical conductivities for their large optical band gaps. The conductivities at room temperature and the activation energies for the conduction are -0.3Scm^-1 and 0.10eV for β-ZrNBr, and -1Scm^-1 and 0.060eV for β-ZrNCl. The band gap energies are estimated to be 3.2 and 3.4eV, respectively from the measurement of optical absorption spectra. In contrast to the high conductivities of the β-forms, α-ZrNBr and ZrNI showed conductivities smaller than 10^-9Scm^-1. A schematic band structural model was proposed for the valence electrons of ZrNX (X=Cl, Br and I).

View full abstract

The electrochemical behavior of ZrN in 1M H₂SO₄, 1M Na₂SO₄ and 1M NaOH solutions was investigated using cyclic voltammetry. A fine ZrN powder with the average particle size of 26nm was synthesized by the reduction-nitridation of ZrO₂ in the presence of Mg, and hot-pressed under 20MPa at 1400° to 1900°C for 1h. Ceramics with the relative density of 93% were obtained by hot-pressing at 1480°C, and the electrical resistivity was nearly equal to that of Ti. Since the ZrN electrodes showed a stable and reproducible voltammogram under cathodic polarization, the hydrogen evolution potential and Tafel's slope were examined. On the other hand, under anodic polarization the electrodes showed the dissolution of ZrN and a passive state.

View full abstract

Dense SiC-TiB₂ composites were prepared by reactive hot-pressing of spray-dried SiC-TiO₂-B₄C-C powder mixtures. The composite containing 20vol% TiB₂ exhibited an average four-point flexural strength of more than 700MPa at both 20° and 1400°C. The fracture toughness of the composites increased with increasing TiB₂ content and at 30vol% TiB₂, twice as high as that of a single phase SiC. The composites with more than 20vol% TiB₂ exhibited an electrical resistivity of less than 0.1Ω·cm, and could be machined easily by electrical discharge machining.

View full abstract

A new bonding method for metal-ceramic joints was developed for the flexible bonding interface of metal-ceramic composites, using the electrical discharge machining (EDM) technique. This method consists of two procedures, the surface machining of ceramic (electrical conductive ceramic, TiB₂) with EDM by the metal electrode (Cu, Ni), and the diffusion bonding at high temperatures. The bonding strength was evaluated from the tensile test. The following results were obtained,
(1) In the TiB₂-Ni composite, the bonded materials were obtained using the EDMed surface.
(2) The bonding strength depended on the bonding temperature, cooling rate, surface geometry and electrode materials.
(3) Different fracture modes were observed at different bonding temperature, probably because of the variation in the residual stress and interface strength.

View full abstract

Sintering conditions and mechanical properties of TiB₂-Al₂O₃ composites produced by pressureless sintering at 1600° to 1900°C by using two types of TiB₂ powder with different average particle size were investigated. The content of more than 40wt% Al₂O₃ yielded the composites having relative density of more than 95%. The bending strength of the composite containing coarse TiB₂ powder and 60wt% Al₂O₃ was 300MPa. And that of the composite containing fine TiB₂ powder and 50wt% Al₂O₃ was 450MPa. The Vickers hardness of the composites were about 15GPa. The electrical resistivity increased with increasing amount of Al₂O₃ and changed drastically from about 10^-3Ω·cm to 10⁵Ω·cm for 75wt% to 95wt% addition of Al₂O₃.

View full abstract

The Deep Level Transient Spectroscopy (DLTS) technique was applied to characterize the interface states at the grain boundaries of ZnO varistors. Two types of commercially available varistors, Bi-type and Pr-type, were tested. The ordinary DLTS method for both varistors gave similar spectra showing two peaks. These two peaks were assigned as bulk traps located at approximately 0.2eV and 0.3eV below the conduction band. In addition to these experiments, a new technique of DLTS method, called zero-biased DLTS method, was tried for observing the interface states at the grain boundaries. By this method a peak for the interface states of ZnO varistors was detected at higher temperatures for the first time. The Arrhenius plots of the emission rate gave the depths of the interface states of 1.03eV and 0.94eV for Pr-type and Bi-type varistors, respectively. Numerical simulation of the DLTS peak revealed that the interface states are distributed monoenergetically. Other samples which contained one of Ce, Pr, Nd, Sm, Gd, and Tb showed similar peaks for the interface states. Although these peaks gave similar values around 1eV, the peak height depended on the element of the rare earth group. Furthermore, since the nonlinear exponent (α) depended strongly on the peak height, the concentration of the interface states should give a large effect on the nonlinearity of the ZnO varistors. The origin of the interface states was suggested to be identified as chemisorbed oxygen ion at the ZnO grain boundaries.

View full abstract

The isothermal capacitance transient spectroscopy (ICTS) was applied to ZnO-Bi₂O₃-MnO₂ ternary varistor system. The ICTS signals caused by three trap levels were detected in three temperature ranges. The detected traps, which were calculated from the slope of Arrhenius plots of ln (τ·T²) vs 1000/T, located at 0.19, 0.29, and 0.60eV below the conduction band edge. The ICTS signals caused by the traps of 0.19 and 0.29eV had no dependence on the bias voltage, so that the detected levels were referred to the bulk traps. On the other hand, the signal caused by the trap of 0.60eV was referred to the interface state because the signal had the dependence on the bias voltage.

View full abstract

The degradation phenomenon in ZnO varistor caused by dc biasing was studied from the view point of voltage-current characteristics and isothermal capacitance transient spectroscopy (ICTS). After dc biasing, the ICTS signals caused by bulk traps did not change, whereas the signals caused by thee interface state changed. We found that the interface trap center shifted toward the conduction band edge, and that the distribution of state density became broad. The degradation phenomenon was related to the interface states of the grain boundaries.

View full abstract

Donor concentrations were obtained from the capacitance-vs-voltage characteristics of ZnO-varistor systems with and without bismuth ions. The α-values increased with decreasing donor concentration in both systems, and the Bi-containing system had higher values than that without bismuth ions at the same donor concentration. According to the scanning Auger electron spectro scopy, the bismuth ions segregated at the grain boundaries, and manganese, cobalt and chromium ions dissolved in zinc oxide lattice. The solute ions such as Mn or Co decreased the donor concentration, but bismuth ions did not affect it. It was suggested that bismuth ions increased the α-value by changing the grain boundary structure or forming an insulatung layer at the grain boundary.

View full abstract

Ceramic varistors in ZnO-SrO system containing CoO and MnO as a third component were prepared, and the effect of these additives on the electrical properties and microstructure of the specimens was investigated. The barrier height and the donor concentration were calculated from the 1/C²-V plot, and the nonlinearity of V-I characteristics was explained by the back-to-back Schottky barrier model. The second component SrO is considered to form trap levels at the interface of ZnO grains. The barrier height increases and the donor concentration decreases with increasing CoO content, and the nonlinear exponent α increases. On the other hand, the addition of MnO only decreases the donor concentration. In this case, however, a small amount of MnO (less than SrO content) is not effective, because of the formation of conductive SrMnO_3-x in the vicinity of grain boundaries. SrMnO_3-x only suppresses the grain growth.

View full abstract

Effects of simultaneous addition of BN and Bi₂O₃ on the microstructure and the PTCR characteristic in Ba_0.4Pb_0.6 TiO₃ ceramics were studied. Different effects of addition were observed depending on the element (Sb or La) used for making the materials semiconductive. Stable PTCR characteristics with room temperature resistivities of less than 10Ω·cm and a resistivity jump of more than 10³ were obtained for the materials containing 0.04 to 0.10mol% Bi₂O₃. These materials were characterized by twofold microstructures, in which large grains were surrounded by small ones.

View full abstract

The elementary distribution near the grain boundary and the oxygen diffusion in semiconducting barium titanate ceramics were investigated. Auger electron spectroscopy detected a Ba-O rich layer at grain boundaries. Measurement of the volume diffusion coefficient in barium titanate ceramics, suggested that the dissociation of oxygen at a high temperature is surpressed in a Ba-O rich layer. The relationship between a Ba-O rich layer at the grain boundary and the PTC effect was discussed.

View full abstract

Properties of an infrared (IR) detector using a positive temperature coefficient (PTC) thermistor, Ba_0.997La_0.003TiO₃+2mol%SiO₂+0.07mol%MnO, operated under the self-regulating heating conditions were investigated. The temperature coefficient of resistance was 18% K^-1, and the Curie point was 122°C. It was shown that a PTC thermistor can detect IR radiation from a 600K blackbody furnace under the self-regulating conditions. However, the response and the thermal time constant were greatly affected by the variation in the ambient temperatures (15°-124°C) and the applied voltage (1-100V). Theoretical expressions for the response and the thermal time constant were discussed, and the calculated results were consistent with the experimental ones.

View full abstract

A chemical sensor using both fluoride ion conductor, PbSnF₄, and silver ion conductor, Ag₆I₄WO₄, is proposed for monitering the concentration of oxygen gas at room temperature. The sensor of the Ag/Ag₆I₄WO₄/PbSnF₄/Pt type was assembled by pressing powders of silver and the electrolytes into a pellet and by sputtering platinum thereon. The potentiometric and amperometric response characteristics were examined by measuring the electromotive force and the short-circuit current. The sensor could detect oxygen gas ranging from 0.01 to 1atm at room temperature. The 90% response time of the amperometric response, 3min on average, was 20 times shorter than that of the potentiometric one. The short-circuit current increased linearly with increasing partial pressure of oxygen, whereas the electromotive force varied directly with the logarithm of partial pressure of oxygen following a Nernst-type equation. The Nernstian slope suggested one-electron reduction of adsorped oxygen molecules on the Pt sensing electrode. The influence of humidity, the use of fast ion conductors, the preparation of PbSnF₄ and the response mechanism were also reported and discussed. The sensor showed the best amperometric response characteristics among this kind of sensor reported so far. The results obtained here confirmed the utility of PbSnF₄ and Ag₆I₄WO₄ as a solid electrolyte for ambient temperature oxygen sensors.

View full abstract