Dielectric properties of nanocrystalline TiO2 prepared using spark plasma sintering

Ultra fine rutile powders (below 50 nm) were prepared via the sol-gel process and bulk type TiO₂ specimens were fabricated using spark plasma sintering (SPS). The TiO₂ specimen sintered at a low temperature (720^∘C) exhibited a highly relative density (97%) and a nano-sized grain structure (200 nm). Dielectric properties of spark plasma sintered TiO₂ specimens including dielectric constants (k) and losses (tan δ) were measured. The TiO₂ specimen, obtained by SPS, showed a high dielectric constant (∼780) and a low tan δ (∼0.005), and a relaxation behavior at 1 MHz. After the subsequent annealing process of the TiO₂ specimen in O₂ flow, the dielectric constants remarkably decreased (k = 100s). These dielectric properties of nanocrystalline TiO₂ specimens prepared by SPS were discussed in terms of space charges produced by the reduction of Ti⁴⁺ ions and crystallographic orientations of grains.     

Calculation of the intrinsic dead layers thicknesses from Au/Ba0.5Sr0.5TiO3/Pt thin film capacitors

Ferroelectric Ba_0.5Sr_0.5TiO₃ (BST) films were prepared on Pt/Ti/SiO₂/Si substrates by the sol-gel process. The films were spin-coated at 2000 rpm for 30 secs and then pyrolysed for 5 mins at the temperature of 350^∘C. This coating procedure was repeated for 3, 4, 5 and 6 times to obtain BST films with different thicknesses. After coating the films with the desired repetition times, the films were finally annealed in a conventional furnace at temperatures ranging from 600^∘C to 800^∘C with a 50^∘C interval in between. The films obtained with an annealing procedure of 750^∘C were polycrystalline with the presence of an impurity BaCO₃ phase. The capacitance and leakage current were measured and used to extract information on the metal-BST interface. With the series capacitance model and modified Schottky emission equation, the thickness of the dead layers for Au/BST and Pt/BST interfaces were calculated to be less than 6 nm and 5 nm, respectively.     

The Rutile Phase of TiO2/Graphene Films: Sol–Gel Method,Phase Transition,Photocatalytic Properties

Abstract

TiO₂ materials are of great interest in different applications because of the controllable preparation of different structures. In this paper, the TiO₂ and TiO₂/graphene films with different phase structure and inserted graphene layer between Cu substrate and TiO₂ film were prepared by sol–gel method at different annealing temperatures, and then used as catalysts for photo-degradation of methylene blue (MB) dye solution under ultraviolet irradiation using 15?W of ultraviolet lamp. The effects of annealing temperature and graphene layer on the phase structure, morphology, chemical composition, binding energy level, and photocatalytic performance of TiO₂ films were studied in details. XRD results reveal that the anatase phase of TiO₂ films transfers to rutile phase with the increase in annealing temperature, and the introducing of graphene film layer can accelerate the phase transformation of anatase to rutile and improve the crystallization quality of TiO₂ films. It is found that the MB degradation efficiency of TiO₂ and TiO₂/graphene films is enhanced with the increasing annealed temperature, which shows that the existence of rutile phase, well crystalline quality and the better dispersion of the TiO₂ particles are helpful in photocatalytic behavior. In addition, compared to the rutile phase TiO₂ film, the rutile phase TiO₂/graphene films at annealed temperatures of 900?°C exhibit much higher photocatalytic activity due to the introduction of graphene films.     

Preparation and electrical properties of 0.4Pb(Zn1/3Nb2/3)O3-0.6Pb(Zr0.4Ti0.6)O3 thin films by 2-step annealing method

Films of (1−x)Pb(Zn_1/3Nb_2/3)O₃-xPb(Zr_0.4Ti_0.6) O₃ (x = 0.6, 40PZN-60PZT) were deposited on Pt/TiO₂/ SiO₂/Si substrate through spin coating. Using a combination of homogeneous precursor solution preparation and two-step pyrolysis process, we were able to obtain the 40PZN-60PZT thin films of perovskite phase virtually without pyrochlore phase precipitation after annealing above 650^∘C. But since annealing done at the high temperatures for extended time can cause diffusion of Pt, TiO₂ and Si, and precipitation of nonstoichiometric PbO, we adopted 2-step annealing method to circumvent these problems. The 2-step annealed films show dense microstructure than the 1-step films annealed at higher temperature. Furthermore, the root-mean-square surface roughness of 220 nm thick films which are annealed at 720^∘C for 1 min and then annealed at 650^∘C for 5 min was found to be 3.9 nm by atomic force microscopy as compared to the 12 nm surface roughness of the film annealed only at 720^∘C for 5 min. The electrical properties of 2-step annealed films are virtually same and those of the 1-step annealed films annealed at high temperature. The film 2-step annealed at 720^∘C for brief 1 min and with subsequent annealing at 650^∘C for 5 min showed a saturated hysteresis loop at an applied voltage of 5 V with remanent polarization (P _r) and coercive voltage (V _c) of 25.3 μC/cm² and 0.66 V respectively. The leakage current density was lower than 10⁻⁵A/cm² at an applied voltage of 5 V.     

Characteristics of Bi4Ti3O12 thin films on ITO/glass and Pt/Si substrates prepared by R.F. sputtering and rapid thermal annealing

Bi₄Ti₃O₁₂ thin films are deposited on ITO/glass and Pt/Ti/Si(100) substrates by R.F. magnetron sputtering at room temperature. The films are then heated by a rapid thermal annealing (RTA) process conducted in oxygen atmosphere at temperatures ranging from 550–700^∘C. X-ray diffraction examination reveals that the crystalinity of the films grown on Pt/Ti/Si is better than that of the films grown on ITO/glass under the same fabrication conditions. SEM observation shows that the films grown on Pt/Ti/Si are denser than those grown on ITO/glass substrates. Interactive diffusion between the Bi₄Ti₃O₁₂ film and the ITO film increases with the increase of annealing temperature. The optical transmittance of the thin film annealed at 650^∘C is found to be almost 100% when the effect of the ITO film is excluded. The relative dielectric constants, leakage currents and polarization characteristics of the two films are compared and discussed.     

ZnGa2O4 thin films fabricated by Sol-gel spinning coating process

ZnGa₂O₄ thin film phosphors have been synthesized on ITO coated glass and soda-lime glass at a firing temperature of 500^∘C and an annealing temperature of 500^∘C and 600^∘C via a chemical solution method using Zinc acetate dihydrate, Gallium nitrate hydrate and 2-methoxiethanol as a solution. XRD patterns of the film phosphors synthesized showed the peaks of ZnGa₂O₄ crystalline phases. AFM surface morphologies of the ZnGa₂O₄ thin film phosphors revealed marked differences according to an annealing temperature of 500^∘C and 600^∘C under an annealing atmosphere (3% H₂/Ar). On the other hand, the sheet resistance of ZnGa₂O₄ thin film phosphors, which were measured by four-point probe instrument, was approximately 5.76 Ω /square and 7.86 Ω /square with annealing temperature, respectively. The ZnGa₂O₄ thin film phosphors exhibited blue emission spectra with peak wavelength of 434 nm and 436 nm by ultra-violet excitation around 230 nm.     

Photocatalytic property of SiO2/TiO2 nanoparticles prepared by sol-hydrothermal process

Silica-embedded nanocrystalline TiO₂ powders were synthesized by sol-hydrothermal process. The influence of the composition of the solvent and the embedded silica content on the phase transition, grain growth and subsequently, on the photoactivity of TiO₂ were investigated. The volume ratio of ethanol to water for the solvent composition was varied from 0.125 to 8 and the mole fraction of silica content was changed from 0 to 0.4, while the condition for hydrothermal reaction was fixed at 250^∘C for 2 hour. With an increase in ethanol content in solvent composition, the crystallite sizes of pure TiO₂ particles decreased from 15 nm to 6 nm and crystal phase changed from rutile/anatase mixed phase to pure anatase phase. Addition of silica to TiO₂ brought about an increase in the photocatalytic activity by suppressing the phase transition from anatase to rutile. Judging from the result of the decomposition of 1, 4-dichlorobenzene, the most efficient catalyst was found to be 0.2 mole fraction SiO₂ embedded TiO₂ prepared with ethanol-rich solvent (the volume ratio of ethanol to water is 4).     

Synthesis and properties of ferroelectric Si-doped (Bi, Nd)4Ti3O12 thin films by chemical solution deposition

Ferroelectric Si-doped (Bi,Nd)₄Ti₃O₁₂ thin films have been prepared on Pt/TiO_x/SiO₂/Si substrates through metal-organic compounds by the chemical solution deposition. The Bi_3.25Nd_0.75Ti_2.9Si_0.1O₁₂ (BNTS) precursor films were found to crystallize into the Bi-layered perovskite Bi₄Ti₃O₁₂ single-phase above 600^∘C. The synthesized BNTS films revealed a random orientation having a strong 117 reflection. The BNTS thin films prepared between 600^∘C and 700^∘C showed well-saturated P-E hysteresis loops with P _r of 13–14 μ C/cm² and E _c of 100–110 kV/cm at an applied voltage of 5 V. The surface roughness of the BNTS thin films was improved by Si doping compared with that of undoped Bi_3.35Nd_0.75Ti₃O₁₂ films.     

Study of nanocrystal TiO2 thin films by thermal annealing

The amorphous films were annealed in a wide temperature range (250–1000^∘C) and film properties of TiO₂ thin films were studied. Nano-sized anatase polycrystallites had been induced by thermal annealing for the films annealed at and above 300^∘C as confirmed by X-ray diffraction. Strong LO-phonon Raman modes, especially B_1g (395 cm⁻¹) and E _g (636 cm⁻¹) in Raman spectra and the absorption peak at 436 cm⁻¹ in absorbance spectra by Fourier transform infrared spectroscopy also indicated the existence of anatase phase in crystalline thin films. In addition, with the increase of the annealing temperature, the wettability of the film surface was enhanced as shown by the decrease of water contact angle from over 90^∘ to less than 40^∘. Moreover, upon UV laser irradiation on film surface, the water contact angle saturated at 10^∘ indicative of a highly hydrophilic surface for all the films, which arose from the dissociative adsorption of water molecules on the defect sits of the surface generated by the photocatalysis reactions of TiO₂. This behavior makes the film a good potential candidate for self-clean coatings.     

Crystallinity and photocatalytic activity of liquid phase deposited TiO2 films

Anatase TiO₂ films were deposited on glass substrates at 50 and 200^∘C to investigate the effect of growth temperature on the photocatalytic acitivity of the films. It was observed that the films grown at 200^∘C were composed of columnar crystallites and were more porous than the films grown at 50^∘C which had more compact structures. Also, the film crystallinity increased from 75 to 90% if the higher growth temperature was used. Despite the higher crystallinity, it was observed that for crystallinities between 60 and 90%, the photocatalytic behavior of the films was more significantly affected by changes in the surface area.     

Influence of anatase-rutile phase transformation on dielectric properties of sol-gel derived TiO2 thin films

The phase transformation behavior and resulting dielectric properties of sol-gel derived TiO₂ thin films were investigated. Thin films showed a typical behavior of mixture systems during the phase transformation; a kinetic investigation on the isothermal curve of pre-crystallized thin films revealed that the phase transformation was a first-order reaction with an Avrami time component of 1. Dielectric constants of TiO₂ thin films increased with the increasing amount of the rutile phase while the dielectric losses showed the opposite relationship. From a fitting process using the parallel mixing rule, dielectric constants of two end members of the mixture system were calculated to be 41.4 and 145.2 for the pure anatase and rutile phase thin films, respectively.     

Control of Microwave Dielectric Properties in the System BaO ⋅ Nd<Subscript>2</Subscript>O<Subscript>3</Subscript> ⋅ 4TiO<Subscript>2</Subscript>—BaO ⋅ Al<Subscript>2</Subscript>O<Subscript>3</Subscript> ⋅ 4TiO<Subscript>2</Subscript>

BaO ⋅ Nd₂O₃ ⋅ 4TiO₂—based ceramics were prepared by the mixed oxide route. Specimens were sintered at temperatures in the range 1200–1450^∘C. Microstructures were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM); microwave dielectric properties were determined at 3 GHz by the Hakki and Coleman method. Product densities were at least 95% theoretical. The addition of up to 1 wt% Al₂O₃ to the starting mixtures reduced the sintering temperatures by at least 100^∘C. Incorporation of small levels of Al into the structure (initially Ti sites) led to an increase in Q × f values, from 6200 to 7000 GHz, a decrease in relative permittivity (ε_r) from 88 to 78, and moved the temperature coefficient of resonant frequency (τ_f) towards zero. The addition of 0.5 wt% Al₂O₃ with 8 wt% Bi₂O₃ improved densification, increased both ε_r (to 88) and Q× f (to 8000 GHz) and moved τ_f closer to zero. Ceramics in the system (1 − x)BaO ⋅ Nd₂O₃ ⋅ 4TiO₂ + xBaO ⋅ Al₂O₃ ⋅ 4TiO₂ exhibited very limited solid solubility. The end member BaO ⋅ Al₂O₃ ⋅ 4TiO₂ was tetragonal in structure with the following dielectric properties: ε_r = 35; Q× f = 5000 GHz; τ_f = −15ppm/^∘C. Microstructures of the mixed Nd-Al compositions contained two distinct phases, Nd-rich needle-like grains and large Al-rich, lath-shaped grains. Products with near zero τ_f were achieved at compositions of approximately 0.14BaO ⋅ Nd₂O₃ ⋅ 4TiO₂ + 0.86BaO ⋅ Al₂O₃ ⋅ 4TiO₂, where Q× f = 8200 GHz and ε_r = 71.     

Optical Properties of LiTaO3 Thin films crystallized by RTA processes

High-performance pyroelectric infrared detectors have been fabricated using Lithium tantalite (LiTaO₃) thin films deposited on Pt(111)/Ti/SiO₂/Si(100) substrates by diol-based sol-gel method and rapid thermal annealing (RTA) technique. The dielectric and pyroelectric properties of IR detectors of LiTaO₃ thin films crystallized by conventional and RTA processes are investigated. Experimental results reveal that the heating rate will influence strongly on dielectricity and pyroelectricity of LiTaO₃ thin films. The voltage responsivities (Rv) measured at 80 Hz increase from 5496 to 8455 V/W and the specific detecivities (D^∗) measured at 300 Hz increase from 1.94 × 10⁸ to 2.38 × 10⁸ cmHz^1/2/W with an increase of heating rate from 600 to 1800^∘C/min. However, the voltage responsivity and the specific detecivity decrease with heating rate in excess of 1800^∘C/min. The results show that the LiTaO₃ thin film detector with a heating rate of 1800^∘C/min exists both the maximums of voltage responsivity and specific detecivity.     

Control of 0.2CaTiO3-0.8Li0.5Nd0.5TiO3 microwave dielectric ceramics by additions of Bi2Ti2O7

Ceramics of 0.2CaTiO₃-0.8Li_0.5Nd_0.5TiO₃) have been prepared by the mixed oxide route using additions of Bi₂O₃-2TiO₂ (up to 15 wt%). Powders were calcined 1100^∘C; cylindrical specimens were fired at temperatures in the range 1250–1325^∘C. Sintered products were typically 95% dense. The microstructures were dominated by angular grains 1–2 μm in size. With increasing levels of Bi₂O₃-2TiO₂ additions, needle and lath shaped second phases developed. For Bi₂Ti₂O₇ additions up to 5 wt%, the relative permittivity increased from 95 to 131, the product of dielectric Q value and measurement frequency increased from 2150 to 2450 GHz and the temperature coefficient of resonant frequency (τ _f ) increased from −28pp/^∘C to +22pp/^∘C. A product with temperature stable τ _f could be obtained at ∼2 wt% Bi₂Ti₂O₇ additions. For high levels of additives, there is minimal change in relative permittivity, the Qxf values degrade and τ _f becomes increasingly negative.     

Microwave Dielectric Properties of CuO-V<Subscript>2</Subscript>O<Subscript>5</Subscript>-Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-Doped ZnNb<Subscript>2</Subscript>O<Subscript>6</Subscript> Ceramics with Low Sintering Temperature

Microwave dielectric properties of low temperature sintering ZnNb₂O₆ ceramics doped with CuO-V₂O₅-Bi₂O₃ additions were investigated systematically. The co-doping of CuO, V₂O₅ and Bi₂O₃ can significantly lower the sintering temperature of ZnNb₂O₆ ceramics from 1150 to 870C. The secondary phase containing Cu, V, Bi and Zn was observed at grain boundary junctions, and the amount of secondary phase increased with increasing CuO-V₂O₅-Bi₂O₃ content. The dielectric properties at microwave frequencies (7–9 GHz) in this system exhibited a significant dependence on the relative density, content of additives and microstructure of the ceramics. The dielectric constant ( _r) of ZnNb₂O₆ ceramics increased from 21.95 to 24.18 with increasing CuO-V₂O₅-Bi₂O₃ additions from 1.5 to 4.0 wt%. The quality factors (Q× f) of this system decreased with increasing CuO-V₂O₅-Bi₂O₃ content and ranged from 36118 to 67100 GHz for sintered ceramics, furthermore, all Q× f values of samples with CuO-V₂O₅-Bi₂O₃ additions are lower than that of un-doped ZnNb₂O₆ ceramics sintered at 1150C for 2 h. The temperature coefficient of resonant frequency ( _f) changed from –33.16 to –25.96 ppm/C with increasing CuO-V₂O₅-Bi₂O₃ from 1.5 to 4.0 wt%     

Effect of B2O3 and CuO on the sintering temperature and microwave dielectric properties of the BaTi4O9 ceramics

The effect of B₂O₃ and CuO on the sintering temperature and microwave dielectric properties of BaTi₄O₉ ceramics was investigated. The BaTi₄O₉ ceramics were able to be sintered at 975^∘C when B₂O₃ was added. This decrease in the sintering temperature of the BaTi₄O₉ ceramics upon the addition of B₂O₃ is attributed to the formation of BaB₂O₄ second phase whose melting temperature is around 900^∘C. The B₂O₃ added BaTi₄O₉ ceramics alone were not sintered below 975^∘C, but were sintered at 875^∘C when CuO was added. The formation of BaCu(B₂O₅) second phase could be responsible for the decrease in the sintering temperature of the CuO and B₂O₃ added BaTi₄O₉ ceramics. The BaTi₄O₉ ceramics containing 2.0 mol% B₂O₃ and 5.0 mol% CuO sintered at 900^∘C for 2 h have good microwave dielectric properties of ε_r = 36.3, Q× f = 30,500 GHz and τ_f = 28.1 ppm/^∘C     

Structure and dielectric properties of BaTi4O9 thin films for RF-MIM capacitor applications

BaTi₄O₉ thin films were grown on a Pt/Ti/SiO₂/Si substrate using RF magnetron sputtering. A homogeneous BaTi₄O₉ crystalline phase developed in the films deposited at 550^∘C and annealed above 850^∘C. When the thickness of the film was reduced, the capacitance density and leakage current density increased. Furthermore, the dielectric constant was observed to decrease with decreasing film thickness. The BaTi₄O₉ film with a thickness of 62 nm exhibited excellent dielectric and electrical properties, with a capacitance density of 4.612 fF/μm² and a dissipation factor of 0.26% at 100 kHz. Similar results were also obtained in the RF frequency range (1–6 GHz). A low leakage current density of 1.0 × 10⁻⁹ A/cm² was achieved at ± 2 V, as well as small voltage and temperature coefficients of capacitance of 40.05 ppm/V² and –92.157 ppm/^∘C, respectively, at 100 kHz.     

Ferroelectric Properties of Bi3.25Nd0.75Ti3O12 Thin Films Prepared by MOD Process

Ferroelectric Bi_{4 – x}Nd_xTi₃O₁₂(BNdT) thin films with the composition (x = 0.75) were prepared on Pt/Ti/SiO₂/Si(100) substrate by metal-organic deposition. The films were annealed by various temperatures from 550 to 650C and then the electrical and structural characteristics were investigated for the application of FRAM. Electrical properties such as dielectric constant, 2Pr and capacitance were quite dependent on the thermal heat treatment. The measured 2Pr value on the BNdT capacitor annealed at 650C was 56 C/cm² at an applied voltage of 5 V. No fatigue was observed up to 8 × 10¹⁰ read/write switching cycles at a frequency of 1 MHz regardless of annealing temperatures.     

Thermal and dielectric properties of ZnO-B2O3-MO3glasses (M = W, Mo)

Glasses in the ZnO-B₂O₃-MO₃(M = W, Mo) ternary were examined as potential replacements to PbO-B₂O₃-SiO₂-ZnO glass frits with the low firing temperature (500–600^∘C) for the dielectric layer of a plasma display panels (PDPs). Glasses were melted in air at 950–1150^∘C in a narrow region of the ternary using standard reagent grade materials. The glasses were evaluated for glass transition temperature (T _g ), softening temperature (T _d ), the coefficient of thermal expansion (CTE), dielectric constant (ε _r ), and optical property. The glass transition temperature of the glasses varied between 470 and 560^∘C. The coefficient of thermal expansion and the dielectric constant of the glasses were in the range of 5–8 × 10^{− 6}/^∘C and 8–10, respectively. The addition of MO₃to ZnO-B₂O₃binary could induce the expansion of glass forming region, the reduction of T _g and the increase in the CTE and the dielectric constant of the glasses. Also, the effect of the addition of MO₃to ZnO-B₂O₃binary on the transmittance in the visible-light region (350–700 nm) was investigated.     

Electrical properties of Ga2O3-based dielectric thin films prepared by plasma enhanced atomic layer deposition (PEALD)

Ga₂O₃ and Ga₂O₃-TiO₂ (GTO) nano-mixed thin films were prepared by plasma enhanced atomic layer deposition with an alternating supply of reactant sources, [(CH₃)₂GaNH₂]₃, Ti(N(CH₃)₂)₄ and oxygen plasma. The uniform and smooth Ga₂O₃ and GTO thin films were successfully deposited. Excellent step coverage of these films was obtained by chemisorbed chemical reactions with oxygen plasma on the surface. The dielectric constant of GTO thin film definitely increased compared to Ga₂O₃ film, and the leakage currents of GTO films were comparable to Ga₂O₃ films. The leakage current density of a 40-nm-GTO film annealed at 600^∘C was approximately 1×10⁻⁷ A/cm² up to about 600 kV/cm.