S.-O. Chen

Large E-field tunability of microwave ferromagnetic properties in Fe50Co50-Hf/lead zinc niobate–lead titanate multiferroic laminates

Fe50Co50-Hf films were deposited on the (011)-cut single crystal lead zinc niobate–lead titanate (PZN-PT) substrates by a composition gradient sputtering (CGS) method. Strong converse magnetoelectric (ME) coupling was observed in the multiferroic laminates of CGS Fe50Co50-Hf/PZN-PT, which exhibited a large electric field (E-field) tunability of microwave magnetic properties. With the increase of E-field strength from 0 to 8 kV/cm, the ferromagnetic resonance (FMR) fields Hr shifted upwards by 270.2 Oe and downwards by 237.7 Oe along hard axis and easy axis directions, being equivalent to 33.8 and 29.7 Oe cm/kV, respectively. Accordingly, the self-biased ferromagnetic resonance frequency fFMR significantly enhanced from 4.0 to 6.5 GHz with an increment of ΔfFMR = 2.5 GHz under a zero-bias magnetic field, and the magnetic damping constant α decreases from 0.0280 to 0.0185. The strong ME coupling between CGS Fe50Co50-Hf film and PZN-PT substrate not only enhanced the fFMR but also reduced the magnetic loss a...

Preparation of dense nanostructured titania ceramic using two step sintering

Abstract TiO2 ceramic was sintered by two types of processes: a conventional isothermal sintering and a two step sintering. A full densification without notable grain growth was achieved when the specimen was first heated to a higher temperature (T1=1300°C) and then cooled immediately for a long treatment at a lower temperature (T2=1000°C). Nanostructured TiO2 ceramics with a grain size of 110 nm and relative density 97% has been prepared using a two step sintering procedure. The results show that it is possibe to prepare dense and fine size grain nanostructured titania ceramics under an optimum sintering condition. The two step sintering method can successfully suppress the accelerated grain growth that usually occurs during the final sintering stage and make it possible to apply titania ceremics as structure materials and tissue engineering materials.

Construction and validation of master sintering curve for TiO2 for pressureless sintering

Abstract One of the ultimate objectives for sintering research is to predict densification results under different thermal profiles for a given processing method. This paper studies the construction and validation of the master sintering curve (MSC) for rutile TiO2 for pressureless sintering. The MSC was constructed using dilatometry data at two heating rates and was then validated using isothermal holds at three different temperatures. The scanning electron microscopy (SEM) observation shows that the partially sintered samples have the same density under different heating procedures, which demonstrates that the assumptions of the model are reliable. The concept of the MSC could be used to predict the sintering shrinkage and final density and calculate the activation energy. A value of 105 kJ mol-1 for TiO2 was obtained. The MSC could be applied to predict the sintering profile to prepare ceramics with required density and a minimum of grain growth.

Apparent sintering activation energy for densification of nanosized TiO2 ceramic powders

Abstract TiO2 powder samples with an average particle size of 50 nm were sintered in air at 1200°C at different heating rates of 1, 3, 5 K min–1. The apparent sintering activation energy was determined from dilatometer shrinkage data using the Arrhenius theory. The results give an activation energy of 115 ± 10 kJ mol–1 for the present material, which is in good agreement with values reported in the literature. It is concluded that the constant rate of heating method can be used to measure activation energy.

Relationship of densification rate and relative density for submicron α-Al2O3 green compacts during low heating rate sintering

Abstract The sintering of α-Al2O3 powder compacts with the mean particle size of 350 nm has been investigated at constant heating rates of 0˙5–5°C min–1. Curves of the instantaneous relative density differences and the temperature for different heating rates were constructed, the changing trend of which was consistent with that of densification rate v. temperature. The correlation between densification rate and the relative density was also constructed from the beginning to the end of the sintering process. Maximum densification rates occur at the same relative density of ∼73% and are independent of heating rates. For samples with the same initial relative density, the logarithm of temperature derivative of the densification rate v. the relative density fits within a single, relatively narrow band.

Prediction of densification during low heating rate sintering of microcrystalline alumina ceramics based on master sintering curve theory

Abstract The master sintering curve (MSC) approach is useful for analysing the shrinkage behaviour of ceramics. In this study, the shrinkage behaviour of α-Al2O3 of mean particle size 2·5 μm during constant heating rate sintering was evaluated. An MSC has been constructed using dilatometry data containing lower heating rates only with the help of a combined stage sintering model. The validity of the MSC has been verified from experimental data. A comparison between the predicted and the Archimedes densities showed good agreement, suggesting that it is possible to control shrinkage behaviour during processing using the MSC. The concept of the MSC has been used to evaluate the apparent activation energy for the above powder; a high value of 1148 kJ mol−1 was obtained.

Phase analysis of Al2o3-ZrO2 composite powders prepared by co-precipitation method

Abstract Al2O3–ZrO2 composite powders with different molar proportions were prepared by the co-precipitation method. The powders obtained were calcined at 1200°C for 1˙5 h. The phase composition in the Al2O3–ZrO2 system was analysed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). It was found that only a fraction of ZrO2 retains the tetragonal phase owing to the size effect when the Al2O3 content is lower than 40 mol-%. When the Al2O3 content is higher than 50 mol-%, metastable t-ZrO2 can be obtained owing to the restriction effect of Al2O3 on ZrO2, although its average crystallite size is larger than the critical size. Meanwhile, Al2O3 retains the α phase and the diffraction intensity of α-Al2O3 becomes weaker with the decrease in the Al2O3 molar content.

Densification behaviour and microstructure evolution of γ-Al2O3 nanoceramics during pressureless sintering

Abstract A high purity γ-Al2O3 sample with an average particle size of 30 nm was sintered in air at temperature 960–1650°C at a heating rate of 5 K min–1 using a dilatometer. The instantaneous density determined by shrinkage was compared with that measured by the Archimedes method at room temperature. The microstructure evolution during sintering was observed by scanning electron microscope. The shrinkage shows two stages. The first stage, ∼8% linear shrinkage, occurs between 950 and 1150°C. The second starts at 1150°C and extends to higher temperatures. There are two shrinkage rate peaks respectively at 1110 and 1550°C. The two stages in the shrinkage curve indicate that the phase transformation has occurred. The α-Al2O3 ceramics were prepared with grain size of 1˙32 µm, relative density of 92˙3%, open porosity of 5˙81% and closed porosity of 1˙89% at 1650°C.

On the Sintering Activation Energy of α-Al2O3

The sintering activation energy of high-purity alumina powders with different particle sizes was evaluated under non-isothermal condition. It was found that, during sintering, the activation energy for the lower temperature stage is higher than that for higher temperature stage. The value of the activation energies for the powder compact with larger particle size was higher than that for the powder compact with smaller particle size. If the selected temperature interval for calculation was narrow enough, the evaluated activation energy values varied with the increasing temperature continuously.

Infrared spectral studies on relationship between phase transition and grain size of nanometre size γ-Al2O3 powder

Abstract A high purity γ-Al2O3 nanopowder with an average particle size of 30 nm was calcined for 2 h at temperature range 200–1600°C, using a heating rate of 5 K min–1. Over this range, γ-Al2O3 transforms to α-Al2O3 via the intermediate phases δ and θ. Dynamic laser scattering and Fourier transform infrared spectroscopy were used to investigate the relationship between the phase composition and grain size of the powder obtained. Particle size generally increased with increasing calcining temperature, but a decrease was observed in the temperature ranges of 400–600, 600–800 and 950–1000°C, which correspond to the γ→δ, δ→θ and θ→α phase transitions respectively. The degree of the decrease in particle size at 950–1000°C is the greatest, reflecting the fact that the θ→α transformation is reconstructive as opposed to the displacive phase transitions in the lower temperature ranges. A mechanism for the phase transitions based on progressive dehydroxylation is proposed; this process results in a decrease in specific volume and thus a decrease in particle size. Kinetically, the fcc→hcp transition of θ to α can be divided into three steps: growth of θ-Al2O3 grains to critical size at 800–950°C; generation of α phase nuclei at 950–1000°C; and growth of the nuclei followed by particle impingement and rapid coarsening at 1000–1200°C.