Microwave Sintering Of Alumina Research Articles

Microwave Sintering of Alumina at 915 MHz: Modeling, Process Control, and Microstructure Distribution.

Microwave energy can be advantageously used for materials processing as it provides high heating rates and homogeneous temperature field distribution. These features are partly due to the large microwave penetration depth into dielectric materials which is, at room temperature, a few centimeters in most dielectric materials. However, up to now, this technology is not widely spread for high-temperature material processing applications (>1200 °C), because its reproducibly and ability to sinter large size samples (>30 cm3) still needs to be improved. In this context, this paper describes both an empirically designed 915 MHz single-mode cavity made from SiC susceptors and refractory thermal insulation, and the 3D modeling of the process in order to improve our understanding of it. Different susceptors geometries and coupling slit position were numerically tested in order to better understand how these parameters impact the field homogeneity and the process stability. It was found that positioning the largest surface of the susceptors parallel to the electrical field allows a very uniform and hybrid heating of the material, while avoiding plasma or thermal instabilities. This was correlated to the 3D modeling results. Finally, thanks to a fully-automatized system this apparatus was used to sinter large size (~30 cm3) low-loss dielectric alumina samples. The sintered materials were subsequently characterized in terms of density, grain size distribution, and homogeneity. The reproducibility was also discussed, demonstrating the process efficiency and reliability.

Effect of Speed on Tribological Properties of Graphene Toughned Alumina (Al-G) Composite

This research work discusses the influence of different wt% of Graphene addition to Alumina matrix and its impact on mechanical properties such as hardness and wear properties of prepared composites (Al-G). The Al-G composites were prepared using powder metallurgy technique with the help of microwave sintering. The hardness of prepared specimens was tested using the Vickers hardness test for various combinations of Graphene in alumina matrix. The wear behaviour of material is explored using Pin on disk apparatus at various speeds and sliding distances for different wt% of graphene added alumina samples. The weight percentages of graphene vary from 0.1 to 0.4wt% with the interval of 0.1%. It was observed that the presence of graphene in alumina and processed with microwave sintering has a significant effect on wear properties of the Al-G composite. The present work focussed on the improvement in wear resistance through microwave sintering of alumina with the addition of various wt% of graphene. The main aim is to explore best optimum AL-G composition for increased wear resistance.

Combined Processes: Laser Assisted Microwave Sintering of Alumina

The engineering applications of technical ceramics are generally related to microstructure-controlled properties. The main objective of processing is to produce the desired microstructure, which frequently involves obtaining high density and small grain size. In the present study hybrid heating technique, which combine direct microwave heating with laser heat sources was proposed to sinter alumina which is a poor absorber of microwaves at room temperature. With increasing temperature by absorbing laser radiation, alumina can absorb microwave energy. The aim of this work is to explore numerically the hybridization of laser and microwaves energies to achieve rapid sintering with controlled grains size, density and free cracks sample by minimizing the temperature gradient between the core and sample surface. The simulation use finites volumes method in FORTRAN program.

Effects of the Susceptor Dielectric Properties on the Microwave Sintering of Alumina

Microwave sintering of alumina has been carried out using SiC and Y‐ZrO2 based susceptors. The microstructure of the sintered samples has been rigorously compared and correlated to the heating behaviour of the susceptor used. It was found that the nature of the susceptor highly influences the sintering behaviour of alumina. The results show that at high temperatures, the electrical conductivity of the SiC susceptor tends to screen the incident electric field, thus resulting in a surface heating of the alumina sample material. When using ZrO2 susceptor, the microstructure analysis of the sintered alumina samples reveals a volumetric heating, which is a signature of the microwave dielectric loss mechanism. This could be explained by the lower ZrO2 electrical conductivity compared to the SiC one. The simulation results confirm this behaviour, particularly showing that in the presence of ZrO2, the intensity of the electric field within the sample is higher than the one when SiC susceptor is used. Basically, the results of the simulation data are in good agreement with our experimental results. Although the SiC based susceptor is usually used in the microwave heating of materials, the ZrO2 based susceptor presents numerous advantages over the SiC one, especially in term of direct microwave heating contributions.

Microwave Sintering of Alumina at 2.45 GHz

The sintering kinetics and microstructural evolution of alumina tubes (∼17 mm length, ∼9 mm inner diameter, and ∼11 mm outer diameter) were studied by conventional and microwave heating at 2.45 GHz. Temperature during microwave heating was measured with an infrared pyrometer and was calibrated to ±10°C. With no hold at sintering temperature, microwave‐sintered samples reached 95% density at 1350°C versus 1600°C for conventionally heated samples. The activation energy for microwave sintering was 85 ± 10 kJ/mol, whereas the activation energy for conventionally sintered samples was 520 ± 14 kJ/mol. Despite the difference in temperature, grains grew from ∼1.0 μm at 86% density to ∼2.6 μm at 98% density for both conventionally sintered and microwave‐sintered samples. The grain size/density trajectory was independent of the heating source. It is concluded that the enhanced densification with microwave heating is not a consequence of fast‐firing and therefore is not a result in the change in the relative rates of surface and grain boundary diffusion in the presence of microwave energy.

Effect of powder reactivity on microwave sintering of alumina

Effect of the reactivity of starting alumina powder of varying crystallinity on the sintering behavior in microwave process was studied. From X-ray amorphous to highly crystalline alumina, powders were obtained by conventional heating of compacts made of the precursor amorphous powder by heating it at different temperatures from 800 to 1500 °C. These samples were then sintered in a multimode microwave field of 2.45 GHz for 10 min at 1500 °C. The microwave effect on densification of the various alumina powders was evaluated by comparing the microwave and conventional sintering data. The results show significant microwave enhancement in the densification of the samples without any pretreatment. This enhancement became less significant as the temperature of the pretreatment increased and finally diminished. Since the pretreatment at elevated temperatures made the powder more stable thermodynamically, this study indicates that the sintering enhancement of a ceramic material in microwave is a metastability-related phenomenon.

Effect of Green Microstructure and Processing Variables on the Microwave Sintering of Alumina

ABSTRACTThe concept of 'hybrid heating with microwave (MW) energy at 2.45 GHZ.'for ultra rapid sintering of alumina is being introduced. This technique is a combination of MW - materual interaction as well as conventional radiant/conduction mechanisms, and facilitates the attainment of perhaps, the highest possible heating rates in a multimode MW cavity at 2.45 GJZz. (1500ºC in 120 sees.). Rapid sintering of pure.undoped alumina with this novel techniQue culminates in uniform, homogeneous microstructures and mechanical property enhancements vis-a-vis conventional fast firing.The role of green microstructure (particle size) on MW(hybrid) heating and processing variables (temperature, time) on the MW (hybrid) heating phenomena vs. conventional fast firing were investigated. Hybrid heated samples showed accelerated densification with comparable grain sizes when compared with the conventionally fast fired samples. The effectof particle size on the microwave (hybrid) heating phenomena was found to be analogous to conventional sintering.