Manabu Takatsu

Preparation of LaNiO3 powder from coprecipitated lanthanum-nickel oxalates

Coprecipitation of lanthanum and nickel oxalates in a water-alcohol mixed solution of an oxalic acid resulted in a simultaneous and homogeneous deposition of the respective oxalate particles with a desired cation ratio. The coprecipitated oxalate could be readily converted to a fine powder (5 to 6 m2 g−1) of the desired LaNiO3 by heating at 800 to 850° C. Detailed examination of some precipitation conditions established an optimum procedure needed for the powder synthesis of LaNiO3. Thermal analysis showed that La2NiO4 (high temperature form) is transiently produced prior to the formation of LaNiO3. Mixed valency of the nickel ion in the synthesized powders was quantitatively determined by means of the oxidation-reduction titration, suggesting that the chemical formula of the powders might be LaNiO2.85 to 2.90.

Fracture behavior of hardened cement paste incorporating mineral additions

Abstract Various powder materials have been added on the hydration of cement to improve its mechanical and chemical properties. Industrial waste or by-product powder is consumed in large quantities as aggregates or hydraulic reactants. In the present study, hardened cement pastes (HCP) were prepared with ordinary Portland cement containing fine powder consisting of isotropic graphite or glass waste to investigate their microstructure and mechanical properties. Fracture behavior was investigated by compact tension, by which the fracture energy and fracture toughness of a specimen were obtained. The amount of calcium hydroxide and Ca Si molar ratio of CSH were constant in spite of being mixed with graphite. Graphite powder did not take part in the hydration of cement. An improvement in fracture toughness and a remarkable plastic behavior were observed because of the dispersion of graphite particles. On the other hand, glass waste powder reacting with alkaline solution in the pores resulted in decreasing the Ca Si molar ratio of CSH. Similar behavior was observed in a mixture of cement and fine blast furnace slag. Bending and compressive strength increased, but fracture toughness decreased. The lower elastic fracture energy in HCP containing reactive fine powder facilitated crack propagation and enlarged the fracture surface.

Heat transmission during thermal shock testing of ceramics

The thermal shock resistance of ceramics is generally evaluated by the water-quench test, in which it is important and necessary to understand the heat-transmission behaviour. A novel and simple method for measuring the transitional changes of temperatures in ceramics has been proposed. Changes in temperature at two different positions in zirconia ceramics were measured to estimate the temperature distribution. From the analytical results, it was clear that the heat-transmission behaviour changed with the quenching temperature or water temperature. The Biot number also changed remarkably with time or with the surface temperature in this experiment. These results are useful in practice for examining the cooling conditions in the thermal shock test.

Decomposition behaviours of dopant-free and doped solid solutions in the TiO2-SnO2 system

Decomposition behaviours, including phase separation by the spinodal mechanism, were investigated for selected compositions ofx = 0.7, 0.5 and 0.3 in TixSn1−xO2 alloy. Inside the coherent spinodal, an equimolar alloy was found to decompose most rapidly at 1100° C and the decomposition rate decreased as the annealing temperature increased. The difference in the rate of decomposition outside the spinodal betweenx = 0.7 andx = 0.3 alloys suggested that the ionic mobility of diffusing tin was much slower than that of titanium in the alloy system. The effect of some dopants on the decomposition rate was also examined. Doping with tungsten or antimony atoms strongly suppressed the phase separation both inside and outside the spinodal, especially resulting in a prolonged stabilization ofx = 0.7 alloy. The role of the dopants in affecting the decomposition rate is discussed in relation to selective substitution of the doping atoms in the tin or titanium sublattice.

Effect of the temperature dependence of thermal properties on the thermal shock tests of ceramics

Thermal stress generated during a thermal shock is closely related to the fracture of ceramics. An attempt has been made to obtain thermal stress in a specimen by numerical calculation. The temperature dependence of thermal conductivity and diffusivity were introduced to realize the practical thermal conditions. The maximum thermal stress, σmax*, was recognized at the Fourier number, but differed from the temperature dependence. Correlative equations of σmax* and ηmax* with the Biot number, βi, under cooling or heating tests, have been proposed. These equations resulted in the exact σmax* and ηmax* compared with the previous equations, in which temperature dependence was ignored. The thermal shock resistance parameter was expressed by the correlative equations of σmax* in order to suggest adequate experimental conditions and specimen size. A comparison of the measured and calculated time to failure of the specimen led to confirmation of the fracture criterion. The measured time disagreed with the calculated one, if the fracture by thermal shocking was not predominant. The correlative equations were also useful to select the kind of ceramics subjected to thermal shocking.

Cyclic Fatigue of Electrically Poled Piezoelectric Ceramics

Static and cyclic fatigue properties of unpoled lead titanate (PT) and lead zirconate titanate (PZT), were earlier investigated and classified according to the fatigue parameter for the crack propagation charateristics [1] However, the polar direction in ceramics must be obtained by applying high voltage and the orientation may bring about the anisotropic mechanical properties, using these ceramics for piezoelectric material. Actually, Yamamoto et al. [2] found that the crack length introduced by Vicker’s indentation, exhibited anisotropy with the polar direction for PT, while, Mehta et al. [3] obtained the same results for poled PZT under the application of mechanical stress. Bruce et al. [4] measured the crack propagation of poled PZT by DT technique and found the fracture behavior depended sensitively on the state of poling. Mechanical properties of PZT were distinctly different depending on whether the material is poled parallel or perpendicular to the direction of applied stress.

Cyclic fatigue properties of isotropic graphite at elevated temperatures

Abstract Fatigue properties of two kinds of isotropic graphite were investigated at elevated temperatures. After the temperature region applicable to the crack propagation rate equation was estimated, V = A( K I K IC ) r , three-point bend bar test specimens were subjected to cyclic loading at room temperature, 473 K or 673 K. The results under different stress ratios were normalized by the effective lifetime. Reduction of lifetime and weak bonding of particles due to oxidation were observed with the increase of temperature. Reducing the stress ratio caused a drastic decrease in lifetime, in which graphite particles acted as the debris instead of grain-bridging at the fracture surface. These fatigue behaviors were discussed quantitatively with fatigue parameters n and ΔA; the latter was the ratio of A value under cyclic fatigue to that under static fatigue.

Correlative Equation of Temprature Distribution and Calculation of Thermal Stress in Rapid Heating Thermal Fracture Test by Measuring Repulsive Load(Special Issue on Synthesis and Evaluation of Ceramic Materials)

In the thermal fracture testing technique proposed previously by the present authors, a one-side heated slab specimen is held by three points and the repulsive load appearing during heating is measured by the load cell placed at the upper point of the heated side. By this way, the thermal fracture stress of a ceramic specimen can be estimated from the repulsive load without knowing the heat transfer coefficient unlike the water quenching thermal shock test. However, it needs numerical calculation for every specimen at each experimental condition.In this paper, the numerical calculation of non-steady-state temperature distribution in an infinite plate, heated from one side with a constant heat flux were conducted in order to calculate the magnitude of thermal stress from the repulsive load data in the above-mentioned thermal fracture test. The temperature dependences of thermal conductivity, λ*=eAT and thermal diffusivity κ*=eBT were introduced in the calculation to realize a practical heat conductive condition. The time dependent temperature distributions were unified into a correlative equation that includes the coefficients of temperature dependence of material properties A, B, supplied heat flux Qi and Fouriers number ηi=κit/h2. By using this equation, the repulsive load and thermal stress at fracture were calculated for several ceramics, and the influences of material properties were discussed. The correlative equation is also useful to define the experimental conditions in thermal fracture tests.

CYCLIC FATIGUE PROPERTIES OF SINTERED MULLITE CERAMICS

ABSTRACT Static and cyclic fatigue tests were conducted on four types of sintered mullite ceramics, and results were compared and studied. Sintered mullite ceramics differ from sintered alumina in that they are high quality materials that do not exhibit R-curve effect and acceleration effect when stress ratios are changed. A slight acceleration effect has been observed in 25 vol% ZrO 2 dispersed mullite-zirconia ceramics when the stress ratio is 0. 1, however. Use condition and guaranteed fatigue time were studied.

Determination of heating or cooling schedule of brittle materials from the stand point of thermal failure

Thermal stress and failure of ceramic materials during thermal processes are the important factors for selecting heating or cooling schedules of their uses and manufacturingIn this paper, the authors propose the method for calculating transient thermal stresses in elastic bodies during thermal processes with the schedules of the following type.(Constant initial temperatures θi)→(Heating or cooling at constant rates k)→(Final constant temperatures θf)The bodies were assumed to be spheres, infinite cylinders and infinite plates in shape, respectively. Newtons law was applied for the heat transfer between the surfaces of the bodies and the surrounding mediums.The calculations were performed with the computer and the maximum thermal stresses at surfaces or at centers of the bodies were shown graphically in dimensionless form. Refering tensile strength and some other physical properties of a brittle material to the figures, one can determine the maximum rate kmax for faultless heating or cooling of the material.