In Situ Determination of Shrinkage under Microwave Conditions

Abstract

Using a dilatometer it is possible to correlate sintering kinetics with the high heating rates applied under microwave conditions. A rapid temperature increase indicates effective microwave absorption. In situ measurements of shrinkage are suitable to distinguish, whether pure microwave sintering takes place or if sintering is mainly caused by radiation-heating from preheating media. Standard measuring equipment was used, therefore all information normally available from dilatometer investigations can be obtained. Introduction The sintering process is generally linked to a reduction of specimen volume, resulting from a reduction of porosity, driven by surface energy reduction. Grain growth, caused by the driving force of grain boundary length reduction, often accompanies the sintering process. Whereas the reduction of porosity is desired, grain growth is unwanted in most ceramic materials, because of reduced mechanical properties of coarse grained ceramics. Furthermore, fast and inhomogeneous shrinkage may cause macroscopic deformation and crack formation due to inhomogeneous mechanical stresses. Sinter kinetics and grain-growth are correlated with temperature, therefore heating and sintering should be stopped when no further shrinkage takes place. The determination of shrinkage behaviour depending on temperature is therefore one of the most important investigations to control the sinter-process. Shrinkage rate as well as densification rate are experimental data used for investigating sinter mechanisms [1, 2]. The use of dilatometer investigation is well established for this purpose. However, for dilatometer-measurements when comparing microwave heating and sintering with conventional heating some major differences have to be In Situ Determination of Shrinkage Under Microwave Conditions 515 taken into account, particularly with respect to certain measurement techniques as well as to materials. Thermocouples, generally used in conventional dilatometers for temperature measurements as well as ceramic sample holders are a source of problems, when microwave radiation with a frequency of 2.45 GHz is applied for heating. Uncontrolled absorption of microwave energy and change of field distribution in the applicator may occur or even generation of a plasma, which can destroy the sample holder. Therefore, the material of the sample holder must be able to withstand a high thermal gradient developing between the sample and the thermal insulation or an cooling system as well as be insensitive to thermal shock caused by an accidental microwave plasma. Most technical problems of a dilatometer suitable for use in a microwave sintering apparatus are therefore linked to the sample holder and the integration into the microwave applicator. Experimental A standard dilatometer system (NETZSCH, Selb, Germany) was used as measurement unit. The conventional tube-like sample holder was replaced by thin alumina rods. The advantage of using thin rods is the better thermal shock resistance because of the small dimension and the much lower mass for unwanted interaction with the microwave filed. Furthermore, inside a tube a microwave plasma can ignite, which would destroy the sample holder. The dilatometer was mounted on a slide in order to adjust the sample in the applicator. A multi-mode cavity fed with microwave radiation of 2.45 GHz frequency was used as microwave applicator. The experiments were carried out at a constant power level of 800 W. In this cavity an alumina-fiber casket was used for thermal insulation of the material to be sintered, and SiC-plates were used for preheating. A pyrometer (KTR 1085 Maurer Optoelektronik, Germany) was used for temperature measurement. In order to avoid large temeparture gradient between the sintered sample and the cavity interior, a high frequency heating system (Linn Highterm) with a carbon susceptor can be installed instead of the alumina-fiber casket system. The system is described in more detail elsewhere [3]. The powder samples were pre-compacted in an uniaxial press at 15 MPa and additionally compacted in an cold isostatic press at 600 MPa (KIP 300, WeberPressen, Germany). The samples had a cylindrical shape (15mm in length and 1213 mm in diameter). The ratio of surface to volume was about 0.46, which was found to be favourable for the microwave sintering experiments [4]. A hole, 5 mm in diameter, was drilled in the centre of the green body in order to measure the accurate temperature. The principle of the measuring system can be seen in Figure 1. In order to vary the sintering activity of the ceramic compacts, titania powder prepared by different hydrolysis technique and further modified by different calcination temperatures, from 600°C to 950°C, was employed [3, 5]. The powders were 516 Bossert characterised by specific surface area measurements (BET, Gemini Micromeritics, Germany).