Session chairperson and summary author: David Abe
The first session on Materials featured four papers describing recent progress in the development of vacuum-compatible, high thermal conductivity ceramics based on aluminum nitride (AlN) as alternatives to beryllia (BeO) based materials and a paper on process optimization for the manufacture of rare-earth magnet materials.
To start the session, Mikijelj et al. of Ceradyne, Inc. presented several new classes of AlN-based lossy dielectrics with dielectric properties comparable to the 60%-BeO 40%-SiC composites commonly used in high average power tube designs. The new AlN-based dielectrics feature thermal conductivities up to 110 W/m-K (compared with 130 W/m-K for BeO-SiC) and relatively high electrical conductivities at low frequencies to facilitate the dissipation of surface charge.
Abe (U.S. Naval Research Laboratory) presented a paper by Savrun et al. (Sienna Technologies, Inc.) describing the development of pressureless-sintered high purity AlN for rod and window applications with thermal conductivities ranging from 185-200 W/m-K, as well as two new classes of AlN-based lossy dielectrics with dielectric properties comparable to 60%-BeO 40%-SiC. The thermal conductivities of the lossy ceramics ranged from 110 to 150 W/m-K.
The results of RF cold- and hot-test evaluations of the AlN-based lossy dielectrics developed by Ceradyne and Sienna Technologies was presented by Kirshner et al. (Northrop Grumman Electronic Systems). The new materials were shown to have characteristics compatible with high peak and average power applications that were previously the exclusive domain of BeO-SiC composites. In a series of X-Band termination wedge experiments, a Sienna material (ST-100C-58B) was observed to outperform BeO-SiC, successfully handling >1050 Watts of average power. In S-band klystron cavity experiments, Ceradyne loss buttons (137CD-1) demonstrated excellent performance, handling a maximum of 333 kilowatts peak power and an average power of 1164 Watts.
As an alternative to conventional hot press and pressureless sintering techniques, Carmel et al. of the University of Maryland presented an interesting paper on the use of microwave energy at 2.45 GHz to produce fully densified ceramics in which the microstructures (and materials properties) could be controlled by the processing history. Using this technique, several high thermal conductivity materials were produced: single-phase polycrystalline AlN (224 W/m-K) as well as lossy dielectrics based on AlN-TiB2 and AlN-SiC (128 W/m-K).
The Materials I session concluded with a paper given by Willhite et al. (University of Kentucky/Semicon Associates) on the development of optimum milling process parameters for the production of rare-earth samarium cobalt magnets. The work resulted in an improved yield of target Sm-Co grain sizes coupled with a minimization of the required milling time.