Microwave joining of 48% alumina±32% zirconia±20% silica ceramics Ammar Ahmed * , Elias Siores Industrial Research Institute Swinburne, Swinburne University of Technology, John Street, Hawthorn 3122, Australia Abstract Microwave heating leads to generation of an inverted temperature pro®le and provides selective heating within a ceramic. However, most ceramics are almost transparent to microwaves at room temperatures and frequencies reserved for industrial usage. With rising temperature, however, most ceramics became increasingly susceptible to microwave energy. Impurities within ceramics make them susceptible more to microwave heating, when compared with high-purity ceramics. Microwave joining techniques attempt to raise ceramics to fusion temperatures and provide effective joining at targeted regions. Thorough studies have been conducted to characterise zirconia±alumina± silica ceramics at high temperatures. Time±temperature behaviour of alumina±zirconia±silica ceramics at different powers has also been studied. On-line load matching techniques using a six-port impedance analyser coupled to a motorised three-stub tuner, have been utilised to optimise power transmission and energy deposition rates to the material. Microwave joining trials of alumina±zirconia±silica and high- purity alumina ceramics have yielded joint strengths in excess of the base material strength. Moreover, impure interlayers between mating surfaces, which tend to decrease joint strengths, have been totally eliminated. These results are discussed in this paper. # 2001 Published by Elsevier Science B.V. Keywords: Microwave joining; Alumina; Zirconia ceramics 1. Introduction An important property of any dielectric material is its complexpermittivity.Thecomplexpermittivityisameasure of the ability of a dielectric material to absorb and store electrical energy. The real part of permittivity e 0 charac- terises penetration of microwaves into the material, and the loss factor e 00 indicates the material's ability to store energy. The loss tangent tan d indicates the ability of a material to convert absorbed energy into heat. For effective coupling, a balanced combination of e 0 to permit adequate penetration and high loss tangent maximum e 00 and tan d) are required. Materials that are susceptible to microwave heating contain dipoles that reorient rapidly in response to changing electric ®eld strength [1,2]. There are primarily three types of charges arising due to an electric ®eld on a material; these are space charges due to electronic conduction; ionic polarisation; and rota- tion of electric dipoles [3]. If the electric conduction is s c , ionic conduction is s i , and complex permittivity is e 0 je 00 , current ¯ows in the material in the presence of an electric ®eld, E e jot V=m The Maxwell equations give the current density J, J js c s i joe 0 e 0 e 00 jE On simplification, this equation leads to J joe 0 e 0 E oe 0 e 0 tan dE 1) where tan d s c s i =oe 0 e 00 e 0 2) The first term on the right-hand side of Eq. 1) is the component of the current 908 out of phase with the electric field. The second term on the right-hand side is the compo- nentofthecurrentthatisinphasewiththeelectricfield.This term indicates the quantity microwave energy that is con- verted into heat during material processing. The phase angle d relates to the phase lag involved in polarising the material. The quantity tan d is the loss tangent. The displacement current stores electric energy in the material and the average electric energy stored per unit volume is W ave 1 2 e 0 e 0 E 2 J=m 3 3) Theaveragepowerconvertedintotheheatenergyisgivenby the equation Q gen 1 2 oe 0 e 0 tan dE 2 W=m 3 4) Journal of Materials Processing Technology 118 2001) 88±95 * Corresponding author. Tel.: 613-9214-8600; fax: 613-9214-5050. E-mail address: iris@swin.edu.au A. Ahmed). 0924-0136/01/$ ± see front matter # 2001 Published by Elsevier Science B.V. PII:S0924-013601)00892-5