Jiping Cheng

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.

A strong magnetoelectric voltage gain effect in magnetostrictive-piezoelectric composite

A magnetoelectric laminate composite consisting of magnetostrictive Terfenol-D (Tb1–xDyxFe2–y) and piezoelectric Pb(Zr,Ti)O3 layers has an extremely high voltage gain effect of ≈300 at its resonant state, offering potential for high-voltage miniature transformer applications.

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

Tunable temperature dependence of electrocaloric effect in ferroelectric relaxor poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene terpolymer

The electrocaloric effect (ECE) was directly measured in a relaxor ferroelectric poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) (P(VDF-TrFE-CFE)) terpolymer. It was observed that the temperature dependence of ECE of the terpolymer films depends critically on the film preparation conditions. While the uniaxially stretched terpolymer films show pronounced temperature dependence of ECE, the non-stretched films exhibit nearly temperature independent ECE from 5 °C to 45 °C. Such a difference is likely caused by the changes in possible polar states and polar-correlation range by film stretching. Besides, large ECE (T > 15 °C) can be induced in both films at 30 °C and 150 MV/m.

Major phase transformations and magnetic property changes caused by electromagnetic fields at microwave frequencies

We demonstrate in this paper that common crystalline phases can be made noncrystalline and hard magnets can be converted to soft magnets in the solid state in several seconds at temperatures far below the melting points. New crystal structures and magnetic structures of ferromagnetic oxides (ferrites such as BaFe 1 2 O 1 9 , CoFe 2 O 4 , Fe 3 O 4 , and ZnFe 2 O 4 , etc.) are formed by reacting either the stoichiometric mixture of oxides or the preformed phase-pure crystalline material in a pure H field (or E field) at microwave (2.45 GHz) frequencies. These major changes in the magnetic properties as well as major structural phase changes are caused by the magnetic field.

Effect of powder reactivity on microwave sintering of alumina

Effect of the reactivity of starting alumina powder of varying crystallinity on the sintering behavior in microwave process was studied. From X-ray amorphous to highly crystalline alumina, powders were obtained by conventional heating of compacts made of the precursor amorphous powder by heating it at different temperatures from 800 to 1500 °C. These samples were then sintered in a multimode microwave field of 2.45 GHz for 10 min at 1500 °C. The microwave effect on densification of the various alumina powders was evaluated by comparing the microwave and conventional sintering data. The results show significant microwave enhancement in the densification of the samples without any pretreatment. This enhancement became less significant as the temperature of the pretreatment increased and finally diminished. Since the pretreatment at elevated temperatures made the powder more stable thermodynamically, this study indicates that the sintering enhancement of a ceramic material in microwave is a metastability-related phenomenon.

Microwave sintering of sulphoaluminate cement with utility wastes

Class C flyash, baghouse dust and scrubber sludge have been successfully used for sulphoaluminate belite (SAB) cement preparation by microwave sintering. For proper raw mix proportioning, the modulus of gehlenite (C2AS)1 formation (MG) and modulus of calcium sulphosilicate (2C2S·CS) formation (MS) were put forward. The results indicated that, when MG and MS were 0.90–1.10, C2AS and 2C2S·CS could be chemically eliminated when microwave-sintered at low temperature; with the 1150°C/10 min sintering condition, microwave-prepared sample had developed into SAB cement clinker with the main phases of C4A3S and β-C2S, while a conventionally fired one had not; with proper gypsum addition, microwave-prepared SAB cement developed strength similar to Type-I cement in 28-day hydration.

Enhancing densification of zirconia-containing ceramic-matrix composites by microwave processing

Various ceramic-matrix composites containing zirconia were sintered using a 2.45 GHz microwave field. The effects of the addition of zirconia and the processing parameters on the sintering and microstructure development were investigated. The results showed that microwave processing enhanced the densification of these composites considerably. The enhancement in sintered density was up to 46% over conventional sintering, depending on the systems.