Due to specific new developments, especially at Penn State and in Japan, microwave processing has become an innovative technology for a wide variety of real materials and is attracting worldwide attention. Initially, successes in microwave heating and sintering were confined to mainly oxide and some nonoxide ceramics. Today, this has been extended to cemented carbides for cutting and drilling applications with improved performance, and has been successfully commercialized. In addition, microwave processing has been found to be applicable for sintering of powder metals effectively and efficiently.
Microwave heating is a sensitive function of the material being processed; only those materials that couple in the microwave field will get heated. Most materials transmit and/or absorb microwaves to varying degrees. While it is well recognized that bulk metals are opaque to microwaves and good reflectors, metallic materials in powder or porous form are very good absorbers of microwaves and can be heated very rapidly.
The microwave process has many advantages over the conventional heating methods including:
- Time and energy saving°'Low cost
- High heating rates (>400°C/minute)°'Minimization of grain growth
- Uniform and volumetric internal°'Uniform microstructures
- Lower sintering time and temperature°'Energy savings
- Fine microstructures°'Improved mechanical properties
- Synthesis of new and special materials°'New market potential
- Improvement in the product performance
- Controlled inert atmosphere°'Environmentally friendly process
Research PSU Microwave Center
Since 1984, the Materials Research Institute at The Pennsylvania State University has been a pioneer institution in the microwave processing of a whole range of ceramics, composites, and metallic materials. The 1980s saw successes in sintering and synthesizing many traditional ceramics such as alumina, zirconia, ZnO, [NZP], hydroxyapatite, zeolites, mullite, silica, etc. The focus in the 1990s aimed at new materials and in new directions. Many electroceramics such as PZT, BaTiO3, Ba(Mg1/3Ta2/3)O3 and transparent ceramics were successfully synthesized, fabricated and sintered in microwave fields. Following this, programs were launched to sinter non-oxides, especially WC/Co based products. The success made in this area led to the innovative approach of continuous microwave sintering, which made it possible to successfully commercialize the developed technology for WC/Co based products applied in the cutting and drilling industry. Another advancement in 1996 was the successful sintering of powder metal parts (steel) with improved performance and better mechanical properties, which opened up completely new avenues of research and commercial exploitation of microwave technology in new applications. Some of these achievements are discussed below.
Traditional and advanced ceramics
For over a decade, a wide variety of such ceramics including a variety of electroceramics, transparent phases and composite materials have been synthesized, reacted and/or sintered in microwave fields. In virtually 100% of all these materials fabricated using microwave processing, it has been possible to obtain step function advances in the sintering parameters and significant improvements in the mechanical properties over conventional product.
Hard metal composites based on tungsten carbide (WC) and cobalt (also known as cemented carbides) are universally used for cutting tools and drilling operations underground. Conventional methods for sintering WC with Co as a binder phase involve high temperature and lengthy sintering cycles on the order of one day. However, WC/Co composites could be sintered in a microwave apparatus in 15 minutes at the sintering temperature. The microwave processed WC/Co parts had much better mechanical properties than their conventional counterparts and a fine, uniform microstructure (~1 um grain size) with very little grain growth, and nearly full density was achieved without adding any grain-growth inhibitors. With the innovative approach of using a continuous microwave sintering system, this technology has been commercialized by several carbide companies to manufacture various cemented carbide based products.
Transparency is a valuable optical property of materials. Main factors that influence the degree of transparency include grain size, density, crystal structure, porosity and grain boundary phase. To achieve transparency in a ceramic, it is necessary to control the grain growth, eliminate porosity and achieve a fully dense material. Conventional methods used to fabricate fully dense, reasonably transparent ceramics involve high temperatures, lengthy sintering conditions and various complex processing steps, such as hot pressing, which makes processing uneconomical, often not achieving desired properties.
Microwave processing, on the other hand, has been successful due to its ability to minimize grain growth and produce a fully dense ceramic in a very short time without using high pressure. Successfully microwave processed materials having reasonable transparency include alumina, mullite, spinel, AlN, hydroxyapatite, MgO and AlON (Fig. 1).
Microwave synthesis and sintering using reduced oxide precursors
By comparison, conventional synthesis methods for these phases require temperatures in the range of 900 to 1600°C (1650 and 2910°F) and several hours of soaking time. Pure stoichiometric metal oxides, such as Ta2O5 and TiO2, do not couple with microwave field efficiently unless heated to temperatures where they become dielectrically lossy (>1000°C, or 1830°F). Partially reducing these phases to oxygen defective states (such as Ta2O5-x and TiO2-x) enhances their ability to absorb microwave energy at low temperatures. Further, rapid sintering of PZT using microwave energy has almost eliminated the loss of PbO, a serious environmental issue.
Processing metallic materials
A recent, surprising development in microwave application is sintering and melting metal powders. Bulk metals reflect microwaves, but reflection by a metal occurs only if it is in a solid nonporous form and is exposed to microwaves at room temperature. Otherwise, the metal in the form of powder will absorb microwaves at room temperature, being effectively and rapidly heated. Also, if a bulk metal is preheated to about 400-500°C (750-930°F), then exposed to microwave field, it effectively couples with the microwave instead of reflecting it and can be heated to high temperatures, even to its melting point.
This technology is being used to sinter various powder metal components, producing useful products including small cylinders, rods, gears and automotive components in 30 to 90 minutes. It also is used melt bulk metals and casting the melt into various shapes. Its application to virtually any powder metal part already in final net shape made of iron and steel, Cu, Al, Ni, Ag, Au, Mo, Co, W, Re, WC, Sn, Ti, etc. and their alloys has been demonstrated (Fig. 2).
Functionally graded materials (FGMs)
FGM type materials are useful in applications requiring property variation. These materials vary in composition, microstructure and function with depth. However, it is very difficult to maintain compositional variation using conventional methods due to the long processing time and temperature involved, which tend to degrade FGMs by inter-diffusion, coarsening and chemical reactions. These materials have been developed using microwave processing. Steel and WC/Co composites are produced using a single step sintering process needing only about 10 minutes of sintering time, thereby ensuring compositional variation (Fig. 3).
Multilayer Capacitors (MLCs)
Multilayer ceramic capacitors (MLCCs) are used in almost all areas of electronics as an important ceramic component. In designing capacitors, dopants are added to a basic BaTiO3 powder to control temperature coefficient of capacitance, magnitude of dielectric permittivity and limit dielectric loss; and to maximize insulation resistance and degradation resistance. More than 80% of modern multilayer capacitors involve cofiring the BaTiO3 dielectric formulations with nickel inner electrodes, which requires conducting firings at a low pO2. Typical firings are done at temperatures between ~1260 and 1300°C (~2300 to 2370°F) and atmospheres with pO2 ª10-10 to 10-12 atm.
Microwave sintering of Ni-electrode MLCCs was conducted in an intermediate reducing atmosphere of ~pO2 x 10-6 atm. The X7R MLCC chips were sintered well at a temperature around 1250°C (2280°F), resulting in dense, uniform parts without any delaminations or cracks. Figure 4 shows a typical microstructure of a microwave-sintered MLCC. The internal electrodes were continuous. Total processing time for microwave sintering was only about 10% that of conventional sintering, and the dense microstructures suggest that densification kinetics of the BME MLCCs was substantially enhanced in microwave sintering.
Materials processing in E and H fields at 2.45 GHz
It is important to understand how the electric and magnetic fields separately interact with solids to understand the science behind microwave-matter interaction. Therefore, PSU researchers are extensively investigating the heating behavior of various materials "essentially" in pure E fields or H fields at 2.45 GHz in a single mode cavity, with astonishing results.
A preliminary survey of a variety of metal, ceramic and composite samples shows remarkable differences in their heating behavior depending on whether they were exposed to E or H field. All metals were very effectively and rapidly heated in the H field, but not in the E field, while the reverse is generally true for insulators and ceramics. Further, only powder metals could be heated fast; a solid metallic rod did not get heated at all in either field.
The most surprising observation was the decrystallization of many magnetic materials and selected oxides. For example, Fe3O4 and a hard ferromagnetic mixed powder of Ba-ferrite composition (which normally takes 12 hours to react completely), is converted to a totally homogeneous, totally amorphous, soft magnetic phase in 30 seconds in the H field. An identical sample in the same cavity only 3-4 cm (~1-1.5 in.) away is converted to a beautifully crystalline ferrite (Fig. 5). Similarly, insulator oxides like TiO2 are made amorphous in seconds without melting. Crystalline silicon metal powder in an E field transforms into an unprecedented 5-mm (~0.2 in.) sphere of amorphous silicon.
These results in the single mode microwave cavity have opened a totally new avenue for future research to make new materials and find clues to explain the causes of the enhancements in materials diffusion and reaction kinetics reported by many researchers working in microwave materials processing field.
Microwave research at PSU involves broad areas and a wide variety of materials and powders. While much research has been conducted in microwave processing of ceramics, not much has been translated into commercial success mainly due to the limitations and difficulties faced in scaling up operations. The success achieved so far in the laboratory should be exploited for commercialization. Several microwave systems have been developed at PSU that can circumvent these problems and can be used for commercialization of the technology. Therefore, the future thrust will be to convert our laboratory microwave technology successes into commercial successes in specialty material synthesis and sintering. IH