Dinesh K. Agrawal

Full sintering of powdered-metal bodies in a microwave field

The use of microwaves to process absorbing materials was studied intensively in the 1970s and 1980s, and has now been applied to a wide variety of materials. Initially, success in microwave heating and sintering was confined mainly to oxide and some non-oxide ceramics; but recently the technique has been extended to carbide semimetals used in cutting tools. Here we describe the microwave sintering of powdered metals to full density. We are able to sinter a wide range of standard powdered metals from commercial sources using a 2.45-GHz microwave field, yielding dense products with better mechanical properties than those obtained by conventional heating. These findings are surprising in view of the reflectivity of bulk metals at microwave frequencies. The ability to sinter metals with microwaves should assist in the preparation of high-performance metal parts needed in many industries, for example, in the automotive industry.

Microwave processing of ceramics

Abstract Microwave processing of ceramics is fast emerging as a new field of ceramic processing and material synthesis. The past year has witnessed significant progress in the aspect of commercialization and application of the technology to new areas. The most significant developments have been the use of microwaves in the sintering of non-oxides, such as tungsten carbide-based components and powdered metals, fabrication of transparent ceramics, and the design of continuous microwave systems.

[CTP]: A new structural family of near-zero expansion ceramics

Abstract In this paper we report the discovery of a new structural family for use in ultra-low ‘α’ ceramics. The prototype composition is Ca 0.5 Ti 2 P 3 O 12 abbreviated [CTP] after which the structural family is named. The importance of this family in contrast to the eucryptite-spodumene family is the enormous range of substitutions which are possible, permitting one to control ‘α’ continuously over a wide range of temperature. Different compositions exhibit near zero thermal expansion in different temperature ranges.

Sodium zirconium phosphate (NZP) as a host structure for nuclear waste immobilization: A review

Sodium zirconium phosphate [NZP] structural family, of which NaZr[sub 2]P[sub 3]O[sub 12] is the parent composition, has been reviewed as a host ceramic waste form for nuclear waste immobilization. NZP compounds are characterized for their ionic conductivity, low thermal expansion and structural flexibility to accommodate a large number of multivalent ions. This latter property of the [NZP] structure allows the incorporation of almost all 42 nuclides present in a typical commercial nuclear waste. The leach studies of simulated waste forms based on NZP have shown reasonable resistance for the release of its constituents. The calculation of dissolution rates of NZP structure has demonstrated that it would take 20,000 times longer to dissolved NZP than quartz.

Microwave sintering of transparent alumina

Transparent alumina samples have been successfully prepared by microwave sintering processing. In comparison to the conventional sintering processing, microwave sintering to transparent alumina can be achieved at lower sintering temperature and shorter sintering time. It was also found that the microwave heating could substantially increase the conversion rate of polycrystalline alumina to single crystalline sapphire, to improve the transparency and other properties of the transparent alumina samples.

Radically different effects on materials by separated microwave electric and magnetic fields

Abstract Using a 2.45 GHz wave-guided cavity, in a single mode TE103 excitation, we were able to physically locate compacted 5 mm pellets of samples separately at the H (magnetic) node (where the E field is nearly zero), or the E (electric) node (where H field is nearly zero). A preliminary survey of a variety of metals, (Cu, Fe, Co..) ceramics (ZnO, etc.), and composites, (WC-Co, ZnO-Co) showed remarkable differences in their heating behaviors. The results establish conclusively that the magnetic field interaction contributes greatly to microwave heating of common materials in a manner, previously neglected in most theories of microwave heating, albeit still to be understood.

Experimental proof of major role of magnetic field losses in microwave heating of metal and metallic composites

In 1999 we found that powdered metal samples including very complex shaped and large size (100 mm diameter, 1 kilograms) could be fully sintered in 30 min in a 2.45 GHz multi-mode microwave cavity [1]. Moreover, these samples had properties at least as good as, and usually better than, those sintered in conventional furnaces. This finding was outside the experience of a very large number of scientists whose extensive work has been covered in many reviews [2–4]. This achievement was as puzzling to us as to colleagues and efforts to explain this by skin depth absorption etc. did not work. The well known extensive theoretical treatment of microwave-material interaction by many workers (see e.g. Varadan and Varadan [5], Booske et al. [6] and others) have in common that they always treat the energy absorption mechanism as due to the dielectric loss factor. In 1994 Cherradi et al. [7] reported their preliminary work in which they showed that the magnetic field must make substantial contributions to the heating of alumina (at high temperature) and semiconductors, and metallic copper. But in their work, the experimental design of using samples of 120 mm length, where in some cases, the sample was exposed to both magnetic and electric field simultaneously, caused a complicated interplay of the different absorption. In present work, a finely tuned microwave cavity with a cross section dimension of 86 mm by 43 mm which works in TE103 single mode was used to investigate the microwave heating behaviors of various materials in different microwave fields. Fig. 1 shows the scheme of the microwave system, and the distribution of the microwave field within the cavity is sketched in Fig. 2. In the L/2 location along the length of the cavity, the maximum electric (E) field is in the center of the cross section, where the magnetic (H ) field is minimum; and the maximum magnetic field is near the wall, where the electric field is minimum. A quartz tube was introduced in this location to hold the sample and also to enable us to control the atmosphere. A 2.45 GHz, 1.2 kW microwave generator (Toshiba, Japan) with power monitor was used as microwave source. A small cylindrical sample (5 mm diameter and 3 mm thick) was placed inside at two different locations, the maximum electric field area where the magnetic field is minimum, and the maximum magnetic field area where the electric field is minimum, respectively. Sample temperatures were measured using an infrared pyrometer (Mikron Instrument Co., Model M90-BT, temperature range −50 ◦C–1000 ◦C). During the experiments, atmospheric pressure nitrogen gas was passed through the quartz tube to avoid oxidation of metal samples at high temperature. Initially, we tried to use a fixed microwave power for all samples during heating, but for some samples, the temperature increase was too fast and the highest temperature exceeded the measuring range of the pyrometer, and in some cases, discharging and arcing occurred. So we set different microwave powers for different samples to get more stable heating results. Fig. 3a shows the heating observed for a typical commercial powdered metal sample (Keystone Powdered-metal Company, Saint Marys, PA, USA. The

Microwave sintering and mechanical properties of PM copper steel

Abstract Microwave processing has gained worldwide acceptance as a novel method for heating and sintering a variety of materials from food to rubber to specialty ceramics, as it offers specific advantages in terms of speed, energy efficiency, process simplicity, novel and improved properties, finer microstructures, and lower environmental hazards. In the present paper, microwave sintering of modulus of rupture (MOR) bar samples of PM copper steel (MPIF FC-0208 composition) and the comparative evaluation of the mechanical properties using both microwave and conventional sintering techniques has been reported. The starting powder characteristics and the processing details of copper steel bar samples sintered in a conventional furnace and in an in house modified commercial microwave oven has been covered at length. In this study, the sintering temperature used typically ranged between 1100 and 1300°C, soaking time ranged from 5 to 20 min, and the atmosphere was controlled using flowing forming gas (mixture of 95% N2 + 5% H2). Microwave sintering resulted in higher sintered density, higher Rockwell hardness (HRB), and higher flexural strength as compared with conventional sintering. The improved mechanical properties of microwave sintered samples can be mainly attributed to the evolution of distinct porosity distribution, primarily consisting of small, rounded, and uniformly distributed pores as against large, angular and non-uniformly distributed pores observed in the case of conventional sintering.

MICROWAVE SINTERING OF HYDROXYAPATITE CERAMICS

Hydroxyapatite ceramics have been fabricated by microwave sintering in a 500 W microwave oven. Circular-plate specimens of various green densities were sintered in the oven at 1200 and 1300 °C, for 5, 10, and 20 min, respectively. Ceramics with density up to 97% of the theoretical were obtained. Density, grain size, microstructure, and strength of the ceramics sintered by microwave and by conventional methods were compared. The results show that microwave sintering of hydroxyapatite is not only highly efficient in saving time and energy, but can also improve the microstructure and thus enhance mechanical strength of the ceramics.

Structural model for thermal expansion in MZr2P3O12 (M=Li, Na, K, Rb, Cs)

A structural model is proposed to describe the highly anisotropic thermal expansion in the sodium zirconium phosphate NaZr2P3O12 structure as a result of the thermal motion of the polyhedra in the structure. In the proposed model the rotations of the phosphate tetrahedra are coupled to the rotation of the zirconium octahedra. Of the two versions considered, the first one allows angular distortions to occur only in the ZrO6 octahedra; the second one permits all polyhedra to be distorted.