Guido Link

Sintering of advanced ceramics using a 30-GHz, 10-kW, CW industrial gyrotron

At the Forschungszentrum Karlsruhe, Germany, a compact gyrotron system was established in 1994 to investigate technological applications in the field of high-temperature materials processing by means of millimeter-wave (mm-wave) radiation. Besides the improvement of the system design, research activities are mainly engaged in studies on debindering and sintering of various types of advanced structural and functional ceramics. Due to volumetric heating and enhanced sintering kinetics, the application of microwaves allows one to shorten the processing time and therefore reduce energy consumption. Besides these effects, microwave technology gives the unique possibility of influencing the microstructure and physical properties of the ceramic materials. This paper will discuss the benefits of the mm-wave technology with respect to sintering of structural ceramics, such as TiO/sub 2/-ZrO/sub 2/-MgO multicomponent ceramics, nanocrystalline oxide ceramics, and Si/sub 3/N/sub 4/, as well as lead-zirconate-titanate piezoceramics as one of the most interesting classes of functional ceramics.

Theory and experiments of electromagnetic loss mechanism for microwave heating of powdered metals

The physical explanation of electromagnetic loss mechanism for microwave heating of powdered metals is presented in this letter. A theoretical model is proposed to simulate and analyze the heating process. An experiment has been performed to prove the theory.

Millimetre wave effects on sintering behaviour of metal powder compacts

Abstract Experimental investigations on millimetre wave sintering of copper and mixed elemental metal powder compacts in the system iron–copper–carbon have been performed in nitrogen atmosphere. Therefore, a compact gyrotron system operating at a frequency of 30 GHz and with a maximum power level of 15 kW was used. With metal powder compacts of different green densities, the temperature gradients within the powder metal compacts were measured during sintering using two shielded K type thermocouples. By this means, for the first time objective evidence for millimetre wave volumetric heating in powder metal compacts could be provided. With samples of low green density and at temperatures below ∼400°C, which means before the active stage of sintering starts, volumetric heating was found. Dilatometer measurements, microstructure analysis and mechanical testing supported these results.

Preferred orientation of pores in ceramics under heating by a linearly polarized microwave field

A spherical pore in the ionic crystalline material subjected to a linearly polarized microwave radiation is shown to flatten along the electric field vector. The deformation of the pore occurs due to rectification of high-frequency flows of charged vacancies in the course of their nonlinear interaction with the electric field near the pore surface. The estimates show that the effect is most pronounced in the materials with a significant contribution of mobile vacancies into microwave absorption. Preferred orientation of pores has been observed experimentally in a zirconia ceramic sample sintered under heating by a linearly polarized microwave radiation.

Pb系強誘電体セラミックスのマイクロ波(2.45GHz,30GHz)焼結

Microwave sintering of Pb(Zr, Ti)O3, PZT, based ceramics was investigated using multimode heating at 2.45 GHz and 30 GHz and compared to conventional thermal processing. For both PZT-5 and PZT-5H “soft” piezoelectric ceramics, microwave processing resulted in a reduction of sintering temperature by more than 150K. Of significance to the production of high surface area lead-based components, minimal PbO volatility was detected for samples processed as high as 1200°C. In contrast to conventionally processed PZTs, minimal grain size dependency of the dielectric and piezoelectric properties was observed.

Potential advantages for millimeter-wave heating of powdered metals

Based on electromagnetic induction heating, an interaction model of microwaves with powdered metal has been established. The formulae of attenuation coefficient, heating rate and heating conversion efficiency are obtained and analyzed. The results of calculations show that millimeter waves may be able to overcome the obstacle caused by smaller skin depth and have strong advantages for the microwave heating of powdered metals. Experiments are done for comparison with present theory.

Microwave cavity perturbation technique for high-temperature dielectric measurements

The present work is motivated by the use of microwaves for the catalytic conversion of carbon dioxide to carbon monoxide. Chemical reactions like this are running at catalyst temperatures of more than 800 °C. The appropriate design of the microwave assisted reactor requires the exact knowledge of the dielectric properties of the catalyst and catalyst support materials up to those high temperatures. For this purpose a measurements system based on the cavity perturbation method was built. The system is using a rectangular TE104 cavity with a resonance frequency at 2.45 GHz and a quality factor Q ≈ 12000. A novel synthetic calibration method based on numerical full-wave modeling has been developed as an alternative to traditional calibration methods. The results of high temperature dielectric measurements for polyether ether ketone (PEEK), MACOR glass ceramics and zirconia are presented.

Microwave Crystallization of Lithium Aluminum Germanium Phosphate Solid-State Electrolyte

Lithium aluminum germanium phosphate (LAGP) glass-ceramics are considered as promising solid-state electrolytes for Li-ion batteries. LAGP glass was prepared via the regular conventional melt-quenching method. Thermal, chemical analyses and X-ray diffraction (XRD) were performed to characterize the prepared glass. The crystallization of the prepared LAGP glass was done using conventional heating and high frequency microwave (MW) processing. Thirty GHz microwave (MW) processing setup were used to convert the prepared LAGP glass into glass-ceramics and compared with the conventionally crystallized LAGP glass-ceramics that were heat-treated in an electric conventional furnace. The ionic conductivities of the LAGP samples obtained from the two different routes were measured using impedance spectroscopy. These samples were also characterized using XRD and scanning electron microscopy (SEM). Microwave processing was successfully used to crystallize LAGP glass into glass-ceramic without the aid of susceptors. The MW treated sample showed higher total, grains and grain boundary ionic conductivities values, lower activation energy and relatively larger-grained microstructure with less porosity compared to the corresponding conventionally treated sample at the same optimized heat-treatment conditions. The enhanced total, grains and grain boundary ionic conductivities values along with the reduced activation energy that were observed in the MW treated sample was considered as an experimental evidence for the existence of the microwave effect in LAGP crystallization process. MW processing is a promising candidate technology for the production of solid-state electrolytes for Li-ion battery.

Determination of Effective Constitutive Properties of Metal Powders at 2.45 GHz for Microwave Processing Applications

Abstract A reflection-transmission based rectangular waveguide approach for measuring the effective constitutive properties of metal powders at 2.45 GHz is presented. The measured effective dielectric properties of these metal powders are quite important for sintering of metal powders using microwaves, which is a new area of research. The proposed method is based on placing the metal powders of varying density into a standard glass tube, which is then positioned inside a specially designed waveguide holder for measuring the scattering coefficients at 2.45 GHz using a vector network analyser. The effective constitutive properties of the metal powders are obtained in terms of the measured scattering coefficients using the proposed two step approach. In order to validate the accuracy of the measured material properties, an electromagnetic core-shell model of the metal powders is developed. Finally, the effective dielectric properties of Cu and stainless steel powders are measured and compared with the values obtained using the electromagnetic model.

MICRO- AND MILLIMETER-WAVE PROCESSING OF ADVANCED MATERIALS AT KARLSRUHE RESEARCH CENTER

At the Karlsruhe Research Center research and development on micro- and millimeter-wave processing of advanced materials has been established. As a spin-off technology from thermonuclear fusion research a compact 30 GHz gyrotron system is in use to investigate technological applications in the field of high temperature millimeter-wave (mm-wave) processing of materials such as sintering of advanced ceramics. In parallel the development of industrial microwave technology at 2.45 GHz is a growing task for applications, such as curing of fiber reinforced polymers. Both experimental systems are introduced and recent results will be reported.