Thermophysical Properties of Ceramic Filler-Added BaO–ZnO–B<Subscript>2</Subscript>O<Subscript>3</Subscript>–P<Subscript>2</Subscript>O<Subscript>5</Subscript> Glass Composites for Barrier Ribs of Plasma Display Panels

The capability of BaO–ZnO–B₂O₃–P₂O₅ glass in hosting various ceramic fillers (up to 20 mass% of Al₂O₃, TiO₂, and ZnO) has been investigated. All the investigated filler-added glasses have demonstrated a reasonable densification at 550 °C to form stable ceramic filler-glass composites. Modifications of the thermophysical properties, such as coefficient of thermal expansion (CTE), glass transition temperature, and dilatometric softening temperature, by the addition of the fillers have been investigated and correlated to phase and microstructural evolution. The CTE of the fabricated composites with varying filler addition is well correlated with theoretical predictions based on the Turner equation considering the modification by phase evolution, which indicates the thermal property tuning potential of the BZBP-based glass composites for application to barrier ribs of plasma display panels.     

Effect of doping with Co2O3, TiO2, Fe2O3, and Mn2O3 on the properties of Ce0.8Gd0.2O2?δ

The effects of Co, Fe, Mn, and Ti oxide additions on the sinterability and crystal-chemical, thermal, and electrical properties of Ce_0.8Gd_0.2O_2−δ have been studied. The results indicate that these oxides enhance the sinterability of the mixed oxide, regardless of whether they were introduced before or after synthesis. The most effective sintering aid is Co₂O₃. The lattice parameters of Ce_0.8Gd_0.2O_2−δ samples containing different metal oxide additions (1 mol %) are refined in space group Fm3m. The temperature-dependent thermal expansion data are used to determine the linear thermal expansion coefficients of the samples. Manganese oxide additions reduce the electrical conductivity of Ce_0.8Gd_0.2O_2−δ, whereas the other dopants increase it in the order Ti < Fe < Co. The activation energy for conduction increases in the order Co < Ti < Fe < Mn. Original Russian Text ? E.Yu. Pikalova, A.N. Demina, A.K. Demin, A.A. Murashkina, V.E. Sopemikov, N.O. Esina, 2007, published in Neorganicheskie Material, 2007, Vol. 43, No. 7, pp. 830–837.     

Effects of Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> doping on the microstructure and the dielectric temperature characteristics of barium titanate ceramics

In this work, the effects of Nb₂O₅ addition on the dielectric properties and phase formation of BaTiO₃ were investigated. A core–shell structure was formed for Nb-doped BaTiO₃ resulted from a low diffusivity of Nb⁵⁺ ions into BaTiO₃ when grain growth was inhibited. In the case of 0.3–4.8 mol% Nb₂O₅ additions, two dielectric constant peaks were observed. The Curie dielectric peak was determined by the ferroelectric-paraelectric transition of grain core, whereas the secondary broad peak at lower temperature was due to strong chemical inhomogeneity in Nb-doped BaTiO₃ ceramics. The dielectric constant peak at Curie temperature was markedly depressed with the addition of Nb₂O₅. On the other hand, the secondary dielectric constant peak was enhanced when sintered above 1280 °C for higher Nb₂O₅ concentrations (≥1.2 mol%). The Curie temperature was shifted to higher temperatures, whereas the transition temperature corresponding to the secondary peak moved to lower temperatures as increasing the amount of Nb₂O₅ more than 1.2 mol%. The decrease of this lower transition temperature was assumed to be closely related with the secondary phase formation when Nb concentration greater than 1.2 mol%. From XRD analyses, a large amount of secondary phases was observed when Nb₂O₅ amount exceeded 1.2 mol%. The coefficients of thermal expansion of Nb-doped BaTiO₃ were increased with increasing Nb₂O₅ contents, resulting in large internal stress between cores and shells. Therefore, the shift of Curie temperature to higher temperatures was attributed to internal stress resulting from the formation of a core–shell structure and a large amount of secondary phase grains.     

The optimum contact angle range for metal foam stabilization: an experimental comparison with the theory

Using the powder metallurgy (PM) route, metal foam precursors were produced from pure aluminum, Al–1 Mg, Al–7Cu, and Al–11.5Si containing 1 wt% TiH₂ as blowing agent and 8 vol% Al₂O₃ as stabilizer. Subsequent foaming of these precursors in an expandometer at 90 and 140 °C above the melting point or the liquidus temperature of the metal produced expansion curves for each metal/Al₂O₃ composition. These expansion curves, as well as foaming experiments (interrupted at maximum expansion as well as 5 min after maximum expansion), were used to judge the stability of the foams produced from the different metal/Al₂O₃ compositions. Foam stability and wetting behavior of the same metal/Al₂O₃ combinations were used to evaluate the validity of contact angle measurements during idealized wetting experiments to be used to predict promising metal/stabilizing ceramic particle combinations for foam production based on the model proposed by Kaptay.     

Effect of Nb and Nb2O5 additives on mechano-thermal processing of TiAl/Al2O3 nano-composite

In this research, TiAl matrix nano-composite with Al₂O₃ reinforcement was obtained by mechanical activation of TiO₂ and Al powder mixture and its subsequent heat treatment. Effect of Nb and/or Nb₂O₅ additions on the process was investigated. Structural changes and thermal behavior of the samples were evaluated by X-ray diffraction and differential thermal analysis, respectively. Moreover, the microstructure was characterized by transmission electron microscopy. The results confirmed the partial dissolution of Nb in Al during the milling stage in the Nb-added samples. The reaction mechanism during heat treatment in the sample without any additives was a two-stage process that was quite similar to the sample with Nb addition. However, Nb₂O₅ addition led to the progress of reaction through a single stage and with a higher rate. Both additives promoted formation of the Ti₃Al phase in the final products. The results confirmed the formation of nano-sized Al₂O₃ particles in a nano-crystalline Ti–Al matrix with a mean crystallite size of 30 nm.     

Effect of Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> additions on the power loss of NiZn ferrites

Nickel–zinc ferrite system, Ni_0.65Zn_0.35Fe₂O₄ + x Nb₂O₅ where x varies from 0.0 wt% to 1.5 wt% in steps of 0.3 wt%, has been prepared by conventional ceramic technique. The samples were sintered at 1250 °C for 4 h in air atmosphere followed by natural cooling. The power loss and microstructures of these materials are examined. Microstructures reveal that niobium oxide additions promoted grain growth with an increase in grain size from 4 μm to 13.2 μm with the increase in niobium concentration. The measured power loss at frequencies from 100 kHz to 10 MHz under different exciting flux densities from 5 mT to 30 mT was found to be low up to 3 MHz, thus making the materials suitable for power applications up to this frequency. In the total power loss, hysteresis loss is predominant below 500 kHz and eddy current loss component is much higher at higher frequencies.     

Glasses and glass-ceramics in the SrO–TiO2–Al2O3–SiO2–B2O3 system and the effect of P2O5 additions

The glass formation abilities of various compositions in SrO–TiO₂–Al₂O₃–SiO₂, SrO–TiO₂–B₂O₃–SiO₂, SrO–TiO₂–Al₂O₃–B₂O₃, and SrO–TiO₂–Al₂O₃–SiO₂–B₂O₃ systems were studied. Many new compositions were found to be suitable for the casting of crack-free, optically clear glasses of different color and with glass transition temperatures ranging from 595 to 775 °C. The crystallization behavior, structure, and thermal expansion behavior of selected glasses were analyzed by DTA, XRD, dilatometry, and heat treatment. The effect of P₂O₅ on the glass structure and crystallization behavior was also studied. P₂O₅ played a dual role depending on composition. In some glasses it acted as a nucleating agent while in others it suppressed crystallization. Heat treatment of borate and borosilicate glasses transformed them into glass-ceramics while comparable SrO–TiO₂–Al₂O₃–SiO₂ glasses showed a lower tendency to crystallize and form glass-ceramics under the same conditions.     

Preparation and characterization of Nb2O5-Al2O3 composite oxide formed by cathodic electroplating and anodizing

Al foil was coated with niobium oxide by cathodic electroplating and anodized in a neutral boric acid solution to achieve high capacitance in a thin film capacitor. X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) revealed the niobium oxide layer on Al to be a hydroxide-rich amorphous phase. The film was crystalline and had stoichiometric stability after annealing at temperatures up to 600 °C followed by anodizing at 500 V, and the specific capacitance of the Nb₂O₅-Al₂O₃ composite oxide was approximately 27% higher than that of Al₂O₃ without a Nb₂O₅ layer. The capacitance was quite stable to the resonance frequency. Overall, the Nb₂O₅-Al₂O₃ composite oxide film is a suitable material for thin film capacitors.     

Al doping effect on microstructure, electrical properties, dielectric characteristics, and aging behaviors of ZPCCY-based varistors

The Al doping effect on the microstructure, electrical properties, dielectric characteristics, and aging behaviors of ZPCCY-based varistors was investigated in the range of 0.0–0.1 mol%. The breakdown electric field in the E-J characteristics decreased in a wide range from 4,921 to 475 V/cm with increasing amounts of Al₂O₃. The nonlinear properties were improved by increasing amounts of Al₂O₃ up to 0.005 mol%, whereas the further additions caused it to decrease. The highest nonlinear coefficient (α = 45.2) was obtained when Al₂O₃ concentration is 0.005 mol%. The Al₂O₃ acted as a donor due to the increase of electron concentration in the small range of 0.0–0.1 mol%. On the other hand, an appropriate addition of Al₂O₃ in the range of 0.001–0.005 mol% was found to significantly improve the electrical stability against DC accelerated aging stress.     

Interface microstructure and shear strength in the diffusion brazing joint of TiC–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> ceramic composite with W18Cr4V tool steel

The diffusion brazing of TiC–Al₂O₃ ceramic composite with W18Cr4V tool steel is realized by using Ti and Cu filler metals at 1500°K for 1 h under a pressure of 15 MPa in a vacuum of 10^–5 Pa. Thus, a brazed TiC–Al₂O₃/W18Cr4V joint was obtained with an interface shear strength of up to 105 MPa. The microstructural characteristics of the TiC–Al₂O₃/ W18Cr4V joint are studied by using a scanning electron microscope (SEM) with energy-dispersion spectroscopy (EDS) and X-ray diffractometer (XRD). The results indicated that the interface layer with a thickness of 90 μm was formed between TiC–Al₂O₃ and W18Cr4V steel. The Ti and Cu filler metals were completely fused and diffused to react with Al and C from the substrates, whereas Ti₃Al, Ti₃AlC₂, TiC, and CuTi₂ were produced in the TiC–Al₂O₃/W18Cr4V joint.     

High-temperature glass-ceramics of the BaO-Al2O3-SiO2-Ta2O5 system and molybdenum metal composites

Glass-ceramics which consist of tantalum pentoxide (Ta₂O₅), hexacelsian (BaO·Al₂O₃· 2SiO₂), and aluminium tantalate (Al₂O₃·Ta₂O₃) are described. These glass-ceramics can form refractory composites up to 1400° C with molybdenum metal. The glass-ceramics and metal have compatible physical and chemical properties which allow close thermal expansion and excellent bonding.     

Effect of Nb coating on the sulphidation/oxidation behaviour of Ti and Ti-6Al-4V alloy

The environmental response of Nb-coated Ti and Ti-6Al-4V alloy was studied at 750 °C in an atmosphere of pS₂ ∼ 10⁻¹ Pa and pO₂ ∼ 10 ⁻¹⁸ Pa. By acting as a diffusion barrier and through the formation of a Nb_1−xS scale the Nb coating deposited enhanced the corrosion resistance of both Ti and Ti-6Al-4V alloy. The corrosion products generated on uncoated titanium in the same environment and temperature were characterized by a double layered oxide scale of TiO₂ beneath which a TiS₂ layer was formed. For the Ti-6Al-4V alloy, α-Al₂O₃ was precipitated in the external portion of the outer-layer of TiO₂ whilst a layer containing Al₂S₃, TiS₂ and vanadium sulphide (possibly V₂S₃) was idenitified underlying the inner TiO₂ layer. After prolonged exposure (168 h), the Nb coating deposited on Ti and Ti-6Al-4V alloy was consumed. A scale following the sequence of TiO₂/TiO₂+NbO₂+Nb₂O₅/Nb_1−xS/TiO₂/ TiS₂/(substrate) was observed on the surface of the Nb-coated Ti, whilst a scale with sequence of TiO₂/V₂S₃/TiO₂+NbO₂+Nb₂O₅/Nb_1−xS/TiO₂/Al₂S₃+TiS₂/(substrate) characterized the corrosion products formed on the Nb-coated Ti-6Al-4V alloy.     

Synthesis and characterization of borosilicate glass/β-spodumene/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> composites with low CTE value for LTCC applications

Glass?+?ceramic composites based on low-softening-point borosilicate (BS) glass, β-spodumene and Al₂O₃ were produced in this work. The influence of ceramic filler composition on the microstructure, sintering quality, mechanical properties, thermal properties and dielectric properties of composites were studied. XRD and DSC indicated that both kinds of ceramic filler as well as the BS glass maintained their characteristics after sintering. The addition of β-spodumene would decrease the coefficient of thermal expansion (CTE) value of composites to match with silicon well. The better wetting behavior between β-spodumene and BS glass would lead to better sintering quality, microstructure and dielectric properties for composites containing more β-spodumene. With appropriate Al₂O₃ content, the flexural strength of composites could be enhanced. Composite with 45 wt% BS glass, 30 wt% β-spodumene and 25 wt% Al₂O₃ sintered at 875 °C showed good properties which meet the requirements of low temperature co-fired ceramic applications: dense microstructure with high relative density of 96.27%, proper CTE value of 3.57 ppm/°C, high flexural strength of 156 MPa, low dielectric constant of 6.20 and low dielectric loss of 1.9?×?10^?3.     

Effect of high-intensity light on the micro- and nanostructuring and thermal expansion of Ta2O5 and Nb2O5 ceramics

This paper examines the effect of exposure to high-intensity light on the formation of fractal micro- and nanostructures and the thermal expansion of tantalum pentoxide and niobium pentoxide ceramics. After such processing, the thermal expansion curve of Ta₂O₅ shows anomalous regions of zero or negative expansion, whereas the thermal expansion of Nb₂O₅ ceramics decreases in magnitude in the region of negative values, and their thermal expansion curve becomes more symmetrical.     

Hot-pressed AlN-Cu metal matrix composites and their thermal properties

AlN-Cu metal matrix composites containing AlN volume fractions between 0.1 and 0.5 were fabricated firstly by liquid phase sintering of AlN using Y₂O₃ as a sintering aid and then by hot pressing the powder mixtures of sintered AlN and Cu at 1050°C with a pressure of 40 MPa under flowing nitrogen. With Y₂O₃ additions of 1.5 to 10 wt%, the densification of AlN could be achieved by liquid phase sintering at 1900°C for 3 h and subsequently slow cooling. The sintered AlN showed a maximum thermal conductivity of 166 W/m/K at a Y₂O₃ level of 6 wt%. Dense AlN-Cu composites with AlN contents up to 40 vol% were achieved by hot pressing. The thermal conductivity and the coefficient of the thermal expansion (CTE) of the composites decreased with increasing AlN volume fractions, giving typical values of 235 W/m/K and 12.6 × 10^–6/K at an AlN content of 40 vol%.     

Microwave dielectric properties of Ba2Ti3Nb4O18 ceramic doped with ZnO–B2O3–SiO2 glass and its compatibility with silver electrode

The effects of ZnO–B₂O₃–SiO₂ glass (ZBS) additions on the sintering behavior and microwave dielectric properties of Ba₂Ti₃Nb₄O₁₈ ceramic have been investigated. Due to the liquid phase effects resulting from the additives, the addition of ZBS glass can reduce the sintering temperature of Ba₂Ti₃Nb₄O₁₈ ceramic from 1,250 to 925 °C and induce no obvious degradation of the microwave dielectric properties. Typically, when 3wt% ZBS is added, a dense ceramic can be sintered at 925 °C with a ε_r of 33.2, a high of Q × f of 13,600 GHz and a low τ _f of −5.9 ppm/°C. Moreover, ZBS glass-added Ba₂Ti₃Nb₄O₁₈ ceramic has a chemical compatibility with silver electrode, which suggests that the ceramic could be applied to LTCC devices application.     

Relationship between thermal and sliding wear behavior of Al6061/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> metal matrix composites

Al6061 alloy and Al6061/Al₂O₃ metal matrix composites (MMCs) were fabricated by stir casting. The MMCs were prepared by addition of 5, 10 and 15 wt% Al₂O₃ particulates and the size of particulates was taken as 16 μm. The effect of Al₂O₃ particulate content, thermal properties and stir casting parameters on the dry sliding wear resistance of MMCs were investigated under 50–350 N loads. The dry sliding wear tests were performed to investigate the wear behavior of MMCs against a steel counterface (DIN 5401) in a block-on-ring apparatus. The wear tests were carried out in an incremental manner, i.e., 300 m per increment and 3,000 m in total. It was observed that, the increase in Al₂O₃ vol% decreased both thermal conductivity and friction coefficient and hence increased the transition load and transition temperature for mild to severe wear during dry sliding wear test.     

Enhanced dielectric properties of modified Ta2O5 thin films

 Tantalum oxide (Ta₂O₅) is a promising high dielectric constant material for the DRAM applications because of its ease of integration compared to other complex oxide dielectrics. The dielectric constant and thermal stability characteristics of bulk Ta₂O₅ samples were reported to enhance significantly through small substitutions of Al₂O₃. However, this improvement in the dielectric constant of (1-x)Ta₂O₅-xAl₂O₃ is not clearly understood. The present research attempts to explain the higher dielectric constant of (1-x)Ta₂O₅-xAl₂O₃ by fabricating thin films with enhanced dielectric properties. A higher dielectric constant of 42.8 was obtained for 0.9Ta₂O₅–0.1Al₂O₃ thin films compared to that reported for pure Ta₂O₅ (25–30). This increase was shown to be closely related to a-axis orientation. Pure Ta₂O₅ thin films with similar a-axis orientation also exhibited a high dielectric constant of 51.7, thus confirming the orientation effect. The leakage current properties and the reliability characteristics were also found to be improved with Al₂O₃ addition. Received: 24 November 1998 / Reviewed and accepted: 7 December 1998     

Young’s modulus evolution at high temperature of SiC refractory castables

The evolution of Young’s modulus versus temperature has been evaluated in SiC-based hydraulically bonded refractories used in waste-to-energy (WTE) plants. Two types of low cement castables (LCC) with 60 and 85 wt% of SiC aggregates have been considered. The study was conducted by the way of a high temperature ultrasonic pulse-echo technique which allowed in situ measurement of Young’s modulus during heat treatment starting from the as-cured state up to 1400 °C in air or in neutral atmosphere (Ar) and during thermal cycles at intermediate temperatures (1000 and 1200 °C). For comparison in order to facilitate interpretation, thermal expansion has also been followed by dilatometry performed in the same conditions. Results are discussed in correlation with phase transformations occurring in the oxide matrix (dehydration at low temperature, crystallization of phases in the CaO–Al₂O₃–SiO₂ system) above 800 °C and damage occurring when cooling. The influence of oxidation of SiC aggregates on elastic properties is also discussed.     

Coefficients of thermal expansion of some laminated ceramic composites

The coefficients of thermal expansion (CTE) were determined for several non-fibrous alumina-based laminated ceramic composites. The results were compared with the CTE values predicted by the modified equations of Schapery and Chamis. Both models are identical in the longitudinal direction and showed differences from experimental CTE of 2.3 and 4.1% for alumina/barium zirconate (Al₂O₃/BaZrO₃) and alumina/tin dioxide (Al₂O₃/SnO₂) composites, respectively. For Al₂O₃/BaZrO₃ in the transverse direction, Schapery's model showed a 1.9% difference while Chamis’ model showed a 7.5% difference. For the Al₂O₃/SnO₂ in the transverse direction, Schapery's model had an 8.2% difference while Chamis’ model showed only a 2.3% difference. Results for alumina/calcium titanate (Al₂O₃/CaTiO₃) laminates showed larger differences, 27.6% in longitudinal CTE and differences of 9.6 and 19.8% transverse CTE for the Schapery and Chamis models, respectively. These differences were attributed to the formation of cracks that occurred in this composite system during processing because of the large CTE mismatch between Al₂O₃ and CaTiO₃. Results for Al₂O₃/BaZrO₃ and Al₂O₃/SnO₂ composites showed that for continuous laminae both the Schapery and Chamis models were adequate predictors of CTE for the systems.