Systematic study of microwave absorption, heating, and microstructure evolution of porous copper powder metal compacts

Abstract

We present a systematic study of the absorption, heating behavior, and microstructure evolution of porous copper powder metal compacts subjected to 2.45 GHz microwave radiation and explain our observations using known physical mechanisms. Using a single-mode microwave system, we place the compacts in pure electric (E) or magnetic (H) fields and compare the heating trends. We also investigate the effect of particle size on the same. The observed trends and the differences between E- and H-field heating are reflected in the dramatic changes in the conductivity, permittivity, and permeability of the samples. These property changes are effected by the microstructure evolution during heating in the two types of fields. We also find that the observed dependence of the initial microwave heating on particle size is suggestive of single-particle behavior.

A Unit Cell Approach to Model and Characterize the Metal Powders and Metal-Dielectric Composites at Microwave Frequencies

“…It can be observed from the above table that the effective constitutive properties of the Cu powder is significantly high, which would result in much lower penetration depth of microwaves thereby not allowing the effect of volumetric heating [3] for these powders. Hence, the presence of significant thickness of native oxide layer between the metal particle looks more appropriate, which actually increases the penetration of these the microwaves inside the sample thereby enhancing the overall microwave heating inside the sample [7,8]. It may be noted here that the value of the effective material properties given in Table 5 is quite large as compared to that reported in [30].…”

“…Hence, it looks more appropriate to insert some native oxide layer between the metal particles to model the metal powder compact using the proposed numerical unit cell approach, as has been done earlier using the classical models [7]. It is mainly due to this reason that the gap between neighbouring copper particles is varied from 10 nm to 1 µm in this section by keeping the radius of the particle to be constant at 0.1 mm, which is akin to the presence of a varying oxide layer between particles, to model the effects of these changes.…”

“…The host medium for simulating the Cu metal compact is taken as the Cu oxide rather than air, which provides the numerical stability and is also close to the real situation [7]. The relative permittivity of Cu oxide is taken as r = 9.483 and r = 0.0755 at 30 GHz [30].…”

“…The microwave energy has traditionally been used to heat lossy dielectrics such as food items for home appliances, and certain class of polymers and composites for various industrial applications [3][4][5]. However, recently, there has also been lot of interest to explore the use of microwave technology for sintering of metal powders after a study showing the absorption of microwaves by metal powders under certain conditions [6][7][8], and it has been reported that the microwave sintering could provide improved mechanical properties along with the finer micro structures [9] as compared to the conventional sintering. As the bulk metals reflect microwaves, hence the basis of microwave heating of metal powders can only be understood by realization of the basic fact that the metal powders should be treated as lossy dielectrics having mixtures of metal particles embedded in a host dielectric medium which may be considered as air for a very simple case.…”

“…A detailed study about the use of various classical and coreshell mixing formulas for the computation of effective permittivity of various metal composites has recently been presented in [18], where it is concluded that most of these formulas provide an error of the order of 10% when compared with some experimental data. However, the experimental data of the complex permittivity of the metal powders employed for comparison in [18] have mainly been obtained using the cavity perturbation technique [7], which is usually valid for low loss samples [19] as it is based on the assumption that the test specimen does not change the field configuration significantly inside the cavity. It looks rather more appropriate to make use of the reflection-transmission approach [20,21] instead of the cavity resonant technique to estimate the effective dielectric and magnetic properties of metal powders.…”