Resistive random access memory (RRAM) based on reversible bistable resistance switching effect is attracting much attention for its superior properties such as nonvolatility, long retention time, simple device structure, small size, and low operating voltage. The unique resistance switching properties have been observed in some binary transition metal oxides, perovskite oxides, and organic molecular materials. This paper briefly reviews the status and new progress on binary transition metal oxide thin film materials such as NiO, TiO₂, ZrO₂, ZnO, and their multilayered thin films and metal nanocomposite thin films, for resistive random access memory applications, and also presents some of our own research work in this field. There is no doubt that study on transition metal oxide thin films for RRAM application will have been a topic of great interest in the forthcoming years, and it is expected that better understanding of physical mechanisms for the bistable resistance switching of the transition metal oxide thin films sandwiched between two metal electrodes can be achieved.

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SrO-containing calcium phosphate invert glasses, (60-x)CaO·xSrO·30P₂O₅·7Na₂O·3TiO₂ (mol%, x = 0-60), which are expected to inhibit bone resorption by osteoclast and enhance bone formation, were prepared and their ion release behaviors in Tris buffer solution were estimated. The glass containing 20 mol% of SrO showed the smallest amount of the dissolved ions in the present work. Laser Raman spectra showed that the peaks of phosphate groups and TiO_y polyhedral groups red-shifted with increasing the SrO content in the glasses. The shift was suggested to be due to decrease in bonding strength between cations and phosphate groups or TiO_y polyhedral groups in the glasses. In the case of the glasses containing SrO over 20 mol%, no shifts of Raman peaks due to the TiO_y polyhedra were observed. The TiO_y (y = 4 or 6) polyhedra in the glasses can coordinate with cations up to 18 mol% since they contain 3 mol% of TiO₂. Sr²⁺ ions are supposed from the shift behavior of the Raman peaks to preferentially coordinate with the TiO_y polyhedra; the formation of this structure would induce decrease in the ion amounts released from the glasses containing 0-20 mol% of SrO.

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Perovskite BiFeO₃-SrTiO₃ ceramics have been prepared using solid-state reactions. Also, effects of SrTiO₃ content on the crystal structure and magnetic properties of a BiFeO₃-SrTiO₃ system have been studied. Single-phase perovskite BiFeO₃-SrTiO₃ ceramics have been fabricated/ Pure BiFeO₃ ceramics contain a small amount of a second phase, such as Bi₃₆Fe₂O₅₇ in perovskite BiFeO₃. The crystallographic symmetry of BiFeO₃-SrTiO₃ changes from rhombohedral to cubic when SrTiO₃ exceeds 20 mol%. Moreover, Mn doping to BiFeO₃-SrTiO₃ is very effective in controlling the grain growth and improving the sintered density. In particular, dielectric loss for the resultant ceramics is greatly reduced by Mn doping. The rhombohedrally distorted 0.9BiFeO₃-0.1SrTiO₃ shows a high Curie temperature (> 750°C) and weak ferromagnetism at 10-300 K.

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The ceramic composites containing 1-25 vol% BaTiO₃ in a ferrite matrix phase of Ni_0.3Zn_0.7Fe₂O₄ were fabricated by sintering at 1250°C in air. The sintered composites indicated a steep increase in dielectric permittivity in a small BaTiO₃ ratio up to 5 vol%. In a limited BaTiO₃ ratio of 3-15 vol%, a reaction between Ni_0.3Zn_0.7Fe₂O₄ and BaTiO₃ resulted in the production of a liquid phase during sintering, which caused correspondingly the formation of secondary phases of BaFe₂O₄ ss and hexagonal BaTiO₃ ss. These solid phase and microstructure changes, however, brought about little contribution to the increasing permittivity of the composites. Electrical property evaluations combined with compositional analysis revealed that the remarkably increasing permittivity could be predominantly attributed to the electron hopping between Fe²⁺ and Fe³⁺ in the ferrite matrix, which was caused by the incorporation of a small amount of Ti⁴⁺ into the spinel sublattice.

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Low temperature crystallization of CSD-derived ferroelectric thin films was aimed by laser assisted annealing in this paper. Pb(Zr_xTi_1-x)O₃(PZT) thin films with a MPB composition (Zr/Ti = 53/47) and LaNiO₃ (LNO) thin film electrodes were deposited by the chemical solution deposition (CSD) method on a Si wafer. The thickness of PZT films was about 70 nm, and the thickness of LNO films as seeding layers and electrodes was about 250 nm. The PZT/LNO films were annealed at the substrate temperature ranging from 300°C to 500°C for 15 min with or without KrF excimier laser irradiation. Both the diffraction peaks of (100)- and (200)- planes for PZT and LNO were identified at 500°C. However, (100)- and (200)- peaks for PZT were not observed for the films without laser-irradiation below 400°C. On the other hand, these peaks were observed for the PZT films with laser-irradiation even below 400°C. The dielectric constant of the PZT film with laser assisted annealing at 350°C was measured to show about 700. These results demonstrated that the crystallization of ferroelectric PZT thin films was enhanced by KrF excimier laser irradiation and by using the oriented LNO thin film as a seeding layer.

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Phase transitions in Pb(Zn_1/3Nb_2/3)O₃-PbTiO₃ were investigated by means of the dielectric constant measurement under the zero-field heating after field cooling. A sharp phase transition between ferroelectric phases in Pb(Zn_1/3Nb_2/3)O₃ was confirmed to appear at 114°C, and to connect to the morphotropic phase boundary in the Pb(Zn_1/3Nb_2/3)O₃-PbTiO₃ solid solution system. It is therefore concluded that the phase transition between ferroelectric phases of Pb(Zn_1/3Nb_2/3)O₃ is the one between the tetragonal and rhombohedral phases.

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The crystallographic and magnetic properties of porous Ni_xZn_1-xFe₂O₄ (x = 0.1, 0.5 and 0.9) from wood templates were investigated in this study. The nitrates aqueous precursor solution was infiltrated into the wood specimen at 60°C to retain the original shape and the microstructure of wood. The sintered bodies contained elongated pores enclosed by a thin layer of wall with the thickness about 1-2 μm. The grain size of polycrystalline ferrite was observed to be in the range of 1-1.5 μm. Each grain was horizontally connected to form polycrystalline walls of single-layered grains. The VSM measurement showed that there is an obvious difference of magnetic properties in the parallel and perpendicular direction due to the anisotropic structure of wood template. The coercivity perpendicular to the pores orientation was larger compared to the coercivity parallel to the pores orientation. It is also discovered that the optimum saturation magnetization was achieved at x = 0.5.

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ZnO nanoparticle was successfully prepared by microwave irradiation method in various oxygen/nitrogen ratio atmospheres. In the present method, steel wool was used as a heat generator by microwave irradiation. The highest heating rate of ∼200°C/sec for the reaction was achieved under the highest oxygen concentration, 80%, whereas the lowest hating rate was ∼40°C/sec. The product prepared in a low oxygen ratio atmosphere showed tetra pod shape with high aspect ratio, c/a. On the other hand, the product prepared under high oxygen ratio atmosphere was relatively isotropic. PL spectra of the products with the higher aspect ratios showed higher UV emission intensity than the products with low aspect ratios i.e. isotropic shape.

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Dye-sensitized solar cells with organic dyes such as xylenol orange, rose bengal and erythrosine B were fabricated and characterized. Gel electrolyte based on polyacrylonitrile was used to improve its durability. A solar cell mixed with xylenol orange and rose bengal showed a wide optical absorption in the visible range, and yielded a high conversion efficiency compared to the other solar cells in the present work. Microstructures of TiO₂ electrode was also investigated by transmission electron microscopy and X-ray diffraction. A mechanism of photovoltaic properties of the dye-sensitized solar cells was discussed on experimental results and the energy level diagram.

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Chalcopyrite compounds/fullerene solar cells based on copper-indium-disulphide and C₆₀ were produced. Two-types of devices with heterojunction and bulk heterojunction structures were examined and characterized. The bulk heterojunction structure provided better efficiency compared to that of the heterojunction structure. Nanostructures of the solar cells were confirmed as mixed nanocrystals by transmission electron microscopy. A photovoltaic mechanism of the present solar cells was discussed on the basis of the experimental results.

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Tungsten doped VO₂ coated SiO₂ nano-particles with mono-dispersion were fabricated by the chemical solution deposition method. The resultant W-doped VO₂ coated SiO₂ particle size was about 40 nm. The transition temperature of non-doped VO₂ coated SiO₂ particles was 68°C, and the transition temperature of 1.1 atomic% tungsten doped VO₂ coated SiO₂ particles was 52°C.

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Ga₂O₃ thin films were fabricated using the Ga nitrate and GaOOH aqueous solutions. The films were crystallized as β-Ga₂O₃ phase by heating at 700°C and the band gap of resultant films was about 4.9 eV. The films were uniform and the film from Ga nitrate solution was dense with porosity of about 3%. The resistivity of the blank film was about 10⁷ Ω cm. The doped films with Ti and Zr exhibited rectifying properties on ITO sputtered glass substrate.

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This paper describes a single step sol-gel method, which is very useful for the manufacturing of various films. In this paper, ZnO-ZrO₂ films with excellent ultraviolet ray shielding property were fabricated by a single step deposition of sol-gel spin coating. Zinc acetate dihydrate [Zn(OCOCH₃)₂·2H₂O] and zirconium n-propoxide [Zr(OC₃H₇)₄] were used as starting materials. Monoethanolamine was added to dissolve zinc acetate in alcoholic solvent. Precursor solution with the molar ratio of [Zn]/[Zr] from 100/0 to 60/40 were prepared for the deposition. The effects of zirconium addition on the stability of the precursor solution, crystallization behavior of ZnO-ZrO₂ film, optical property and microstructure were examined. As a result, the stability of the precursor solution increased with increasing ZrO₂ content. Dried films with zirconia component were more stable to humidity in air than the films without zirconia. The addition of zirconia component to the ZnO film also improved the microstructure of the film surface. Namely, the surface of the film with the molar ratio [Zn]/[Zr] of 70/30 became flat without crack. Relatively thick ZnO-ZrO₂ film with thickness about 0.7 μm was obtained by the single step deposition. The resultant film with a [Zn]/[Zr] of 70/30 exhibited high transmittance in the visible region (T_VIS∼90%) and low transmittance in the ultraviolet region (T_UV < 10%). This suggested that the ZnO-ZrO₂ film with good UV-shielding property was successfully prepared by a single step sol-gel spin coating.

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A joining technique for oxide ion conducting ceramics, such as yttria-stabilized zirconia, was developed, utilizing oxidation of transiently formed aluminum melt. Two yttria-stabilized zirconia (YSZ) blocks were joined via Al/Ni alloy/Al interlayers at 1273 to 1473 K for 3.6 to 28.8 ks in a vacuum (< 0.2 Pa) with an applied load of 80 MPa. The average fracture strength increased with increasing the joining time and temperature. A strong four-point bend strength of 170 MPa was obtained for the specimen joined at 1473 K for 14.4 ks, which was fractured at YSZ block. Alumina interlayer, which is expected to act as a protective layer for further oxidation of Ni alloy, was formed between YSZ and Ni alloy in the specimen after joining.

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In order to find out conditions for fabricating green body with no defect in gelcasting method, effect of mixing conditions of additives on solidification process of ceramics slurry has been examined in this study. Flow pattern of alumina slurry is deduced by considering actual power consumption of an agitation mixer as well as Reynolds number of agitation calculated, and then compared with observation of solidified green body. The effects of mixing condition is described as a function of the rotation speed and related to flow regimes such as laminar, transition and turbulent flow. It has been found that the boundary between the transition and turbulent flow region is the most suitable for gelcasting method.

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A new fabrication method of electrical conductive alumina/carbon composite ceramics has been investigated. An advantage of this material is good electrical conductivity based on the uniformity of carbon network generated from polymer in the gelcasted green body. In this study, we discussed the graphitization behavior of the polymer into a gelcasted alumina. As a result of the graphitization, the density, electrical conductivity and degree of graphitization were increasing at a higher temperature. Especially, those values were remarkably increased from 1500°C. The sample sintered at 1700°C, it has an electrical conductivity of 4.11 S/cm and high graphitization similar to pyrolytic graphite. It is thought that the presence and sintering of alumina particle has an important factor in this behavior.

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The partial oxidation reforming method is advantageous to develop highly effective and low-cost products of H₂ gas used for fuel cells. For the partial oxidation, however, large amount of pure O₂ gas are desirable for acceleration of the H₂ production. Electronic and oxygen ionic conductive (mixed-conductive) oxide ceramics can be used to obtain pure O₂ gas, because the mixed conductivity contributes to permeate pure O₂ gas from air at the elevated temperatures. However, high permeation temperature of above 900°C is a problem. Higher temperature for the permeation incurs deterioration of both the ceramics and the steel to support the ceramics. We intend to find out a new mixed-conductive oxide with high oxygen permeation at lower temperatures than 900°C. In this study, we focus the Sr-La-Fe-Co oxides characterized by the intergrowth structures. We have investigated an appropriate condition to prepare Sr_3-xLa_xFeCoO_7-δ ceramics with intergrowth perovskite structure. Then we have investigated microstructures as well as oxygen permeation properties of a dense Sr_2.45La_0.55FeCoO_7-δ ceramics. The oxygen permeation flux of the Sr_2.45La_0.55FeCoO_7-δ ceramics was as high as that of cubic perovskite Sr_0.8La_0.2 Fe_0.8Ga_0.2O_3-δceramics.

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Mechanochemical redox reactions of manganese dioxide with organic vapors have been studied to control the chemical valence (oxidation number) of manganese ions under grinding. Electrolytic-manganese dioxide (EMD) including double chains of edge-sharing MnO₆-octahedra transforms into β-MnO₂ (Pyrolusite) consisting of the single chains under grinding in Ar. The mechanical dissociation of the double chains generates active sites to adsorb the atmospheric (CH₃)₂CO vapor and the strongly adsorbed (CH₃)₂CO molecules pull out the oxide ions from MnO₂ to reduce Mn⁴⁺ to Mn³⁺ ions, while (CH₃)₂CO is oxidized to CO₂ and H₂O via the intermediate oxidation products, HCOH and CH₃COH. One (CH₃)₂CO molecule can reduce 16 MnO₂ units to trivalent MnO_1.5 (Bixbyite). However, the decomposition products, CO₂ and H₂O, occupying the adsorption sites dominantly to (CH₃)₂CO disturb the redox process. This mechanochemical reduction of MnO₂ can be applied to the synthesis of LiMn^IIIMn^IVO₄ powder from both combinations of the starting mixtures of MnO₂-LiOH and MnO₂-Li₂CO₃. Some alcohols and ketones also provide the similar reduction ability.

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Polycrystalline Dy-doped SrTiO₃ thin films deposited by pulsed laser deposition (PLD) on CeO₂-buffered yttria-stabilized zirconia (YSZ) single crystal substrates showed oxygen sensing characteristics at room temperature. The oxygen gas sensing characteristics depended on the amount of Dy-doping, and 1.5 mol% Dy-doping was most effective. The oxygen gas sensing characteristics were closely related to leakage current characteristics of Dy-doped SrTiO₃ thin film deposited on ZnIn₂O₄/YSZ/Si(001) substrates. The suppression of leakage current by Dy-doping suggests that Ti⁴⁺ is substituted by Dy³⁺, and the Dy³⁺ acts as an acceptor. Further doping of Dy³⁺ brought about the increase of the leakage current, and it lowered the oxygen gas sensitivity.

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SiC ceramics were prepared from β-SiC starting powder with AlN and Y₂O₃ sintering additives by spark plasma sintering. Densification, microstructure and flexural strength of the liquid-phase sintered SiC ceramics were investigated. The relative density of SiC sintered bodies reached over 95% at total amount of 10 vol% AlN and Y₂O₃ additives and at 1900°C for 600 s in N₂ at 30 MPa. When the content of AlN additive increased, the size of SiC grains decreased. The SiC body almost without the grain growth was successfully prepared using 95 mol% AlN and 5 mol% Y₂O₃ additives. The flexural strength of the SiC body was 1150 MPa.

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With the aim to reduce the permittivity of a dielectric substrate, closed pores have been introduced into alumina based ceramics by our innovated superplastisically foaming method. Affect of the ambient humidity on the dielectric constant was compared to those for conventional porous ceramics as well as full densified one. In the dry atmosphere, the dielectric constant (k) decreased with the porosity similar to the conventional porous ceramics. Affect of the ambient humidity on the dielectric property was almost identical to that of fully densified one. The degradation of mechanical strength depended on the pore size introduced.

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Lithium transition metal oxide cathode materials such as LiAl_0.05Mn_1.95O₄, LiNi_0.5Mn_1.5O₄, LiCo_1/3Ni_1/3Mn_1/3O₂ were successfully produced at a rate of 2 kg/h by large flame type spray pyrolysis. The continuous spray pyrolysis of 250 h exhibited the good stability for powder production. The powder characteristics of them were determined by SEM, XRD, BET and AAS. Cathode materials had spherical morphology with a particle size of about 8 μm and porous microstructure with specific surface area of 10 m²/g. Their geometrical standard deviation of particle size ranged from 1.2 to 1.5. The chemical composition of them was in good agreement with that of the starting solution. The electrochemical measurement revealed that these cathode materials exhibited the higher rechargeable capacity, good cycle performance and thermal stability.

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Studies on the synthesis of nitrides, nitride-related compounds and silicon carbide by using Na as a flux and reaction enhanced medium are reviewed. Using a Na melt, ternary and quaternary nitrides and nitride-related compounds containing Ba and Sr were prepared at around 1000 K and N₂ pressure below about 10 MPa. Crystal growth of a binary nitride GaN by heating Ga and N₂ with Na was also found during the studies of the multinary compounds. AlN powder was prepared above the Al melting temperature of 932 K. Single crystals of CrN and Mn₂N were obtained by heating Cr and Mn with a Na-Ga flux and a Na-In flux, respectively. β-type SiC powder and porous bulks were prepared by reaction of Si and fullerene with Na or Si and amorphous carbon with Na at 1000 K. The role of Na in the formation of the nitrides and silicon carbide is herein discussed.

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AlN-hBN ceramic composites with 3 mass% CaF₂ were fabricated by spark plasma sintering at 1800°C for 10 min. The effects of hBN content on mechanical properties, microstructure, and machinability of AlN-hBN composites were investigated. With increasing hBN content, relative density, flexural strength, and Young's modulus decreased, whereas fracture toughness increased slightly. Vickers' indentation cracks are largely deflected near hBN platelets, implying a weak interfacial bond between the AlN and the hBN. The fracture surfaces of AlN-hBN composites showed a higher tendency of intergranular fracture than monolithic AlN. During end-milling processes, the cutting resistance decreased with hBN content, demonstrating the machinability of AlN ceramic can be improved significantly by the addition of hBN.

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The metal-ferroelectric-metal-insulator-semiconductor field-effect transistor (MFMIS-FET) using the (Bi,La)₄Ti₃O₁₂ (BLT) as a ferroelectric and SrTa₂O₆ (STA) as a buffer layer is prepared. The Au/STA/Si structure shows about 1 nF/cm² of the accumulation capacitance value which is equivalent to about 6.2 nm of SiO₂. The leakage current density is lower than 10⁷ A/cm² under 5 V. The remanent polarization of the 420 nm-thick BLT film was 35.2 μC/cm² at 450 kV/cm. The MFMIS-FET was fabricated with different area ratio (A_I/A_F) from 1 to 8. From the drain current-gate voltage characteristics at the drain voltage of 0.2 V, the memory window is only 0.5 V for the device with A_I/A_F = 1 but it is increased to 1.8 V as the A_I/A_F is increased to 8. For the A_I/A_F ratio of 8, the "on" state of the drain current of 1.12 × 10^-5 A rapidly drops after 10⁵ s to 2 × 10^-6 A and the "off" state current increase from 10^-7 A to 10^-6 A after 10⁵ s. The on/off current ratio decrease from 3 × 10² to 8.

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Non-volatile memories using ferroelectric-gate field-effect transistors (Fe-FETs) with metal/ferroelectric/semiconductor gate stack (MFS-FETs) have superior advantages such as non-destructive read operation and high density. However, the interfacial reactions between ferroelectric materials and Si substrates make it difficult to obtain good electrical properties of MFS-FETs. As an alternative solution, Fe-FETs with a metal/ferroelectric/insulator/insulator/semiconductor gate stack (MFIS-FETs) have been proposed. We prepared SrBi₂Ta₂O₉ (SBT) film as a ferroelectric layer and LaZrO_x (LZO) film as the insulating buffer layer, and then fabricated the n-channel Fe-FETs with the Au/SBT/LZO/Si structure. The thickness of the LZO buffer layer and SBT film deposited by a sol-gel method were about 35 nm and 450 nm, respectively. From the electrical properties of the LZO film on Si, no hysteresis was observed in the C-V curve and the leakage current density was about 1.4 × 10^-7 A/cm² at 5 V. SBT film on the LZO/Si structure was crystallized in a polycrystalline phase with a highly preferred (115) orientation. The C-V characteristics of Au/SBT/LZO/Si structure showed a clockwise hysteresis loop and the memory window width increased as the bias voltage increased. The fabricated Fe-FETs showed typical n-channel MFIS-FETs C-V characteristics and the current on/off ratio was about 10³. Also, the memory window width was about 0.7 V.

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To increase the film quality of the nickel electrode in a multi-layer ceramic capacitor (MLCC), the BaTiO₃ particles are usually used as an additive material in the electrode paste. In this report, we investigated the effects of particle size (30-100 nm) and the amount (10-20% mass ratio to Ni) of BaTiO₃ nanoparticle additives on the quality of thin-film Ni electrodes, stated by a dry film density and Ni film coverage after a firing process. The use of 30 nm BaTiO₃ particles resulted in a high Ni film coverage larger than 75% using only 10 mass% of BaTiO₃ additive in the paste. Using larger particle sizes, a greater amount of BaTiO₃ additive was needed in order to obtain the same Ni film coverage. The use of a smaller particle size of BaTiO₃ nanoparticles leads to the increase of particle numbers for the same mass, resulting in effective BaTiO₃ nanoparticles surrounded on Ni particles. In addition, by decreasing the amount of BaTiO₃ additives, the BaTiO₃ absorbance by the dielectric layer was reduced, and, consequently, the increasing of dielectric layer thickness could also be minimized.

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The effect of barium ions on K₂O-TiO₂-SiO₂ (KTS) glass soaked in molten Ba(NO₃)₂ is reported. In the soaking process, the glass surface crystallized into Ba₂TiSi₂O₈ with barium ions derived from the molten solution. The orientation and crystalline phase of the soaked glasses were studied by X-ray diffraction (XRD) for various soaking times and additional heat treatment. The penetration of barium ions into the glass was verified by the direct methods of X-ray photoelectron spectroscopy (XPS) depth profiling and Scanning electron microscopy-energy dispersive X-ray spectrometer (SEM-EDS). It is suggested that the formation of the surface crystalline layer occurred by the reaction between the barium ions and glass components and the soaking temperature provides enough energy for the crystallization of Ba₂TiSi₂O₈ on the surface.

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The metal nitrates were completely dissolved in a mixed solvent (total 100 ml) of ethylene glycol (EG) (0, 5, 10, 15, 25, and 50 ml) and water, and then dried at 120°C to make precursors for the fine LaFeO₃ (0.05 mol) material. The main product was not LaFeO₃ for the precursor prepared using the nitrate solution containing a small amount of EG (less than 5 ml). The mixed solution of the 10-15 ml EG with water was suitable for the preparation of 0.05 mol LaFeO₃. In this case, clear XRD peaks of LaFeO₃ single phase were obtained by the calcination even at 350°C using this method. The higher calcination temperature for the formation of the LaFeO₃ phase was needed due to the increased EG amount in the solvent. The particle size was ca. 40 nm for the (EG 10 ml) samples calcined at 350-600°C and it increased with an increase in the calcining temperature.

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Translucent alumina with anisotropic-grained microstructure was fabricated utilizing a small amount of glass-forming impurities and the hot isostatic pressing (HIP) method. Its total forward transmission at 600 nm is 72% (1 mm thickness), and bending strength and fracture toughness are estimated to be 649 MPa and 9.4 MPa·m^0.5, respectively. Bimodal structure composed of elongated anisotropic-grain and fine equi-axis grain is responsible for the enhancement of mechanical properties.

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