Journal of Alloys and Compounds 627 (2015) 231–237
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The role of the native oxide shell on the microwave sintering of copper metal powder compacts Morsi M. Mahmoud a,b,⇑, Guido Link a, Manfred Thumm a a
Karlsruhe Institute of Technology (KIT), Institute for Pulsed Power and Microwave Technology (IHM), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany City for Scientiﬁc Research and Technological Applications (SRTA), Advanced Technology and New Materials Research Institute (ATNMRI), New Borg Al-Arab City, P.O. Box: 21934, Alexandria, Egypt b
a r t i c l e
i n f o
Article history: Received 25 October 2014 Received in revised form 27 November 2014 Accepted 30 November 2014 Available online 26 December 2014 Keywords: Microwave processing Sintering Copper metal powder Copper oxide
a b s t r a c t Successful microwave sintering of several metal powders had been reported by many researchers with remarkable improvements in the materials properties and/or in the overall process. However, the concept behind microwave heating of metal powders has not been fully understood till now, as it is well known that bulk metals reﬂect microwaves. The progress of microwave sintering of copper metal powder compacts was investigated via combining both in-situ electrical resistivity and dilatometry measurements that give important information about microstructural changes with respect to the inter-particle electrical contacts during sintering. The sintering behavior of copper metal powders was depending on the type of the gas used, particle size, the initial green density, the soaking sintering time and the thin oxide layer on the particles surfaces. The thin copper oxide native layer (ceramics) that thermodynamically formed on the particles surfaces under normal handling and ambient environmental conditions had a very critical and important role in the microwave absorption and interaction, the sintering behavior and the microstructural changes. This ﬁnding could help to have a fundamental understanding of why MW’s interact with copper metal powder in a different way than its bulk at room temperature, i.e. why a given metal powder could be heated using microwaves while its bulk reﬂects it. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Microwave (MW) energy provides a powerful and signiﬁcantly different tool to process materials and in most cases to improve the performance characteristics of the resulting materials [1,2]. In recent years, there has been a lot of interest to use MW energy for sintering of metal powder compacts in order to take advantage of certain features such as volumetric and selective heating that allows for reduced cycle times and increased energy efﬁciency of MW process . It has been reported in the 1990s that MW’s can sinter metal powders as well as provide improved mechanical properties along with ﬁner microstructures . This ﬁnding opened this new ﬁeld of research due to these surprising results. However, the concept behind the MW sintering of metal powder compacts has not been fully understood till now, as it is well known that bulk metals reﬂect MW’s at room temperature. ⇑ Corresponding author at: Karlsruhe Institute of Technology (KIT), Institute for Pulsed Power and Microwave Technology (IHM), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany. Tel.: +49 72160822919. E-mail addresses: [email protected]
(M.M. Mahmoud), [email protected]
(G. Link), [email protected]
(M. Thumm). http://dx.doi.org/10.1016/j.jallcom.2014.11.180 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
Furthermore, the exact nature of the interaction of the MW with metal powders was not clear, as different parameters such as the size and position of the sample, the presence of extra susceptor rods and the insulation around the sample under test had inﬂuenced the MW sintering mechanism in a complicated manner. Further studies were done in this direction [5–9], however, most of these studies were empirical in nature, and hence they do not provide any physical insight into the observed heating pattern. A correlation between the absorbed MW power and the DC conductivity of stainless steel metal powder compacts has been investigated  but a precise correlation could not be accomplished. A digital high resolution optical microscope was developed to investigate the changes in the iron oxide microstructure during the MW iron making process . A selective heating of the iron oxide compacts was observed under the inﬂuence of MW’s depending on the iron oxide modiﬁcation resulting in huge temperature gradients. So, a fundamental understanding of MW-material interactions is crucial in applying MW technology to materials processing. For example, MW absorption can be affected by many factors such as moisture content, changes in dielectric properties during processing, particle size, sample size and geometry, and the position of the sample with respect to the microwave ﬁeld [3,12,13]. Furthermore, the
M.M. Mahmoud et al. / Journal of Alloys and Compounds 627 (2015) 231–237
MW absorption mechanisms and losses in materials are highly dependent on frequency and temperature [14–16]. The goal of this work is to study the high frequency MW sintering behavior of a commonly used metal such as copper metal powder and to have a fundamental understanding of why microwaves interact with copper metal powder in a different way than its bulk.
2. Material and experimental work 2.1. Materials Three (3) commercial gas atomized spherical copper (Cu) metal powders (Alfa Aesar) with different particle sizes were investigated. The speciﬁcations of these powders are shown in Table 1. Copper compacts with different green densities and different particles size were prepared using a uni-axial mechanical press machine before the prepared compacts were sintered using the MW sintering setup. The prepared compacts samples were cylindrical in shape with an average diameter of 6.4 mm and a length range 8–11 mm depending on the initial green density of a given compact.
2.2. Microwave in-situ sintering setup and experiments The progress of the MW sintering process of Cu die-pressed metal powder compacts was investigated via combining both in-situ electrical resistivity measurements using the four-wire method and in-situ dilatometry measurements using a modiﬁed dilatometer set-up for monitoring the MW sintering kinetics as a function of temperature and using different types of processing atmospheres.
Table 1 Cu metal powder speciﬁcations.
All experiments were performed in a controlled atmosphere at ambient pressure. Three (3) different types of gasses (Argon (Ar), Nitrogen (N2) and Forming ((FR), (92% N2 + 8% H2)) were used during the MW sintering experiments. The compacts were heated with a rate of 10 °C/min up to the desired sintering temperature (1000 °C) then soaked for different times at this temperature before free cooling of the treated sample occurs. The millimeter-wave sintering was performed in a compact 30 GHz gyrotron system, as shown in Fig. 1. This system was used in combination with a special dilatometer setup so that information on sintering kinetics of the studied samples as a function of temperature could be obtained. This setup gives important information about microstructural changes with respect to the inter-particle electrical contacts during the sintering process. A commercial dilatometer (type L75 from Linseis Company, Germany) was adapted to the millimeter-wave applicator. This dilatometer setup was modiﬁed so that the DC electric resistance could be measured during the sintering process parallel to the measurement of changes in the sample length. The sample resistance was measured using a Keithley digital multimeter Model 2002 in combination with the four-wire method where two platinum wires were in contact with the top and another two wires with the bottom surfaces of the cylindrical shape sample, respectively. This sample electrode arrangement was assembled into the dilatometer setup whereas the spring force of the dilatometer, which clamps the sample in-between the sample holder end-plate and the dilatometer sensor rod, makes sufﬁcient contact to the sample. So, the dilatometer results will give qualitative information rather than precise quantitative data about the sintering process. The electrical resistivity q is calculated from the measured resistance R using Eq. (1), where d is the diameter of the cylindrical sample and l its length. 2
The heating process was controlled along a preset temperature–time program using the temperature signal of a S-type sheath thermocouple placed touching the sample surface.
2.3. Microstructure and materials characterizations
Particles size (lm)
Cu spherical, 100 mesh (149 lm)
Cu spherical, 170 + 270 mesh (88 + 53 lm) Cu spherical, APS 10 lm
53.8 24.0 21.9 0.3 0 <10 >90 10 50 90
<44 <74 >44 <149 >74 >149 >88 <44 >44 <88 <7.39 <9.78 <14.05
The as-received Cu powders were characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Thermogravimetric (TGA), and scanning electron microscopy (SEM). The microstructure of the sintered Cu compacts was investigated using Olympus BX60H optical microscopy and scanning electron microscopy (SEM), Hitachi S800 and Philips XL40, which were equipped with energy dispersive X-ray spectroscopy (EDX) system. X-ray diffraction was done for the as-received and the sintered samples using a Seyfert C3000 powder diffractometer (Cu Ka radiation). The XPS were performed using a K-Alpha XPS spectrometer (Thermo Fisher Scientiﬁc). The TGA analysis was done using SETARAM Setsys 16/18 device with a gas ﬂow of 4 L/h and was performed at 600 °C for 48 h with a heating rate of 5 °C/min using (95% Ar–5% H2) gas mixture.
Fig. 1. Gyrotron system, 30 GHz, with a schematic of both the in-situ dilatometer and the electrical resistance measurements.
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3. Results and discussions Fig. 2 shows the chemical analysis, XRD and SEM for the spherical Cu powders with particle size less than 149 lm. Fig. 3 shows the MW in-situ resistance and dilatometry measurements of Cu compacts with different particle sizes and different green densities sintered at 1000 °C with different soaking times using the three different process gases. It is clear that the particle size, initial green density, soaking times and the type of the processing environment have a signiﬁcant effect on the MW sintering behavior of the Cu powder. A signiﬁcant expansion behavior was observed in all the Cu compacts processed using the hydrogen containing gas (FR). This abnormal expansion behavior was not observed when using the other two gases (N2 and Ar). Furthermore, a sharp decrease in the electrical resistivity of the Cu compacts was observed at around 190–210 °C when using FR gas while a gradual steady decrease trend on the electrical resistivity of similar samples occurred when using the other gases (N2 and Ar), reaching its minimum around 600 °C. These different observations in the electrical resistivity trend could be explained by either H2 induced reduction, in case of FR gas, or by thermal induced reduction, in case of N2 and Ar gases, of the thin Cu oxide layer on the metal powder surface and according to the following equations:
FR gas ð 210 CÞ
N2 and Ar gases
2Cu2 O þ 2H2 ! 4Cu þ 2H2 O 4CuO ! 2Cu2 O þ O2 2CuO þ 2H2 ! 2Cu þ 2H2 O
2Cu2 O ! 4Cu þ O2 2CuO ! 2Cu þ O2
When this Cu oxide layer was reduced by these two reduction mechanisms, the electrical resistivity of the sample was decreased due to the better contact occurred between the Cu particles. In the FR gas case, a water vapor (H2O) was formed as one of the byproducts in this reduction reaction. This vapor was formed and trapped at the grain boundaries and at the closed pores between the Cu particles. Furthermore, this trapped water vapor will be further heated and hence its vapor pressure will increase and consequently the sample will expand. This expansion was signiﬁcant in the 350–800 °C range with a plateau in the range from 450 °C to 700 °C. This unique expansion was repeatable in all of the Cu powders possessing different particles size and different green densities. Even in a very slow heating rate (1 °C/min) MW sintering experiment (Fig. 3f), this expansion behavior still occurs.
Table 2 shows the effect of different MW and conventional sintering parameters on the achieved ﬁnal densities for some sintered Cu compacts. Fig. 3 and Table 2 indicate that the Cu compacts processed using Ar gas had higher sintered densities when compared with similar samples processed using either FR or N2 gases. On the other hand, the resistivity of the samples processed using FR gas was lower than similar ones processed using either Ar or N2 gases. From this table, it was concluded that for a given Cu compact, the longer the sintering soaking time, the higher the achieved sintered density. Furthermore, the higher the initial green density of a given sample, the higher the achieved ﬁnal density. The Cu compacts with the smallest particle size (APS 10 lm) had the highest sintered densities and in much shorter times when compared with similar Cu samples that had bigger particle sizes samples either (149 lm) or (88 + 55 lm). So the ﬁner the particles, the higher the ﬁnal sintered density and the shorter the sintering soaking time. Finally, the MW processed Cu samples had achieved higher densities than similar ones processed using conventional heating, as in the samples processed using either Ar or FR gases. So, microwaves are believed to enhance the sintering of the Cu compacts via enhancing the reduction process and hence higher ﬁnal sintered density values for the MW sintered samples were achieved. This enhancements was so signiﬁcant in the Cu compacts with smaller particles size, especially (APS 10 lm) and (88 + 55 lm) compacts, compared to similar ones with relatively bigger particles size (149 lm). The MW processed Cu Compacts with smaller particle size (APS 10 lm) has achieved a higher ﬁnal sintered density in a relatively shorter soaking time (10 min) when compared to similar Cu compacts conventionally processed for longer soaking time (2 h) or even compared to similar conventionally processed Cu compacts having higher initial green density. Figs. 4 and 5 show the XRD and the optical micrographs for some MW sintered Cu compacts (149 lm), respectively. The XRD revealed that the samples processed under either N2 or Ar gases had developed some minor CuO and Cu2O phases on the sample surface in addition to the major FCC Cu phase. The formation of such oxide phases on the samples surfaces is believed due to some leakage in the controlled atmosphere MW cavity setup. In the case of the FR gas, this leak was compensated by the H2 reduction effect and hence formation of such oxides was suppressed. The optical micrographs showed that the Cu compacts (149 lm) sintered in FR gas atmosphere using either MW or even conventional heating (Fig. 5a and b) had more and larger pores and
Fig. 2. XRD, SEM and chemical analysis of Cu (149 lm) metal powder.
M.M. Mahmoud et al. / Journal of Alloys and Compounds 627 (2015) 231–237
550oC-10oC/min. - 5 min.
0,00 -0,02 -0,04 (a) Cu (-149μm) FR - (77.5-79.3%)# N2 - (76-79.8%) # Ar - (76.7-81.5%) #
-0,06 -0,08 -0,10 0
(b) Cu (-149μm) FR - (77.5-79.3%)# N2 - (76-79.8%) # Ar - (76.7-81.5%)#
0,00 -0,02 -0,04
(c) Cu - FR - 1000oC - 10min. (-149 μm) - (77.5-79.3%)# (-88+53 μm) - (81.8-86.6%)# APS 10 μm - (72.1-96.8%)#
-0,08 -0,10 -0,12 0
(d) Cu (-88+53 μm) - FR - 1000oC 10 min. - (81.8-86.7%)# 120 min. - (73.3-86.4%)#
-0,08 -0,10 -0,12 800
0,02 0,00 (e) Cu (-149 μm) - 1000°C 10°C/min.- 10 min. - (77.5-79.3%)# 10°C/min.-120 min. - (76.2-83.7%)#
-0,02 -0,04 -0,06 0
100000 10000 1000 100 10 1 0,1 0,01 1E-3 1E-4 1E-5 1E-6 1E-7
(f) Cu (-149 μm) - 1000°C 1°C/min.- 1 min. - (77.8-86.3%)# 10°C/min.-10 min. - (77.5-79.3%)#
0,06 0,04 0,02 0,00 -0,02 -0,04 -0,06 -0,08 -0,10 -0,12 -0,14 -0,16 -0,18
100000 10000 1000 100 10 1 0,1 0,01 1E-3 1E-4 1E-5 1E-6 1E-7
Fig. 3. MW in-situ dilatometry and resistivity measurements of Cu compacts sintered at 1000 °C (a) and (b) Cu (149 lm) (10 °C/min10 min) using different gases. (c) Different particle sizes in FR gases (10 °C/min10 min). (d) and (e) Cu (88 + 53 lm) and (149 lm) sintered using (10 °C/min) at 10 and 120 min soaking times, respectively. (f) Cu (149 lm) sintered using (1 °C/min10 min) in comparison with (10 °C/min10 min) sample. (XY%)#: percentage of the initial theoretical green density (X) and the ﬁnal sintered theoretical density (Y).
voids than similar samples sintered using Ar gas (Fig. 5c). These voids were formed due to the formation and the expansion of the H2O vapor in the closed pores between the Cu particles during the H2 induced reduction process. So, the thin Cu oxide layer has a great inﬂuence on the microstructure of the Cu sintered compacts. Characterization of the thin Cu oxide layer on the Cu particles were done using XPS and thermogravimetric (TGA) analyses of the as-received Cu powder (149 lm) (Fig 6). The XPS data showed that both types of the Cu oxides (Cu2O and CuO) do exist on the Cu particles surfaces under normal handling and at ambient conditions. The XPS depth proﬁling was performed to determine the thickness of this thin oxide layer but an accurate value could not be achieved due to the induced external reduction effect that caused by the XPS depth proﬁling process. TGA of the as-received Cu powder was done to estimate the amount of oxygen in these powders. The onset of the weight loss was started temperatures around 230 °C which matches the H2 induced reduction process. The TGA analysis indicated that 0.49% weight loss occurred (i.e. The O2 content of the thin oxide layer on the Cu powder surface is 0.49 wt.%).
In order to understand this abnormal expansion when using H2 containing gas, a series of MW step-heating in-situ dilatometry and electrical resistivity measurements experiments within the temperature range (350–800 °C) were conducted. These experiments were designed in order to freeze and to slowly investigate the microstructure at this temperature range so that it could be carefully studied (Fig. 7e). On the other hand, another set of experiments were performed to heat the as-received Cu powder (149 lm) using MW at 550 °C (10 °C/min) for 5 min, i.e. in the plateau area of this expansion behavior (Fig. 3a), using the 3 different gases followed by microstructure investigation (Fig. 7a–d) of these samples. Fig. 7 shows the SEM micrographs for some of these experiments. These micrographs indicated that only when using FR gas, the Cu particles in either the powder or in the compacts forms were suffered from cracks while no cracks were observed when using the other two gases. It is believed that these observed cracks were caused because of the high vapor pressure of the superheated H2O vapor that was formed as a byproduct during the previously discussed reduction reaction, between the thin Cu oxide layer
M.M. Mahmoud et al. / Journal of Alloys and Compounds 627 (2015) 231–237 Table 2 Sintering parameters and densities of some Cu compacts sintered at 1000 °C with a heating rate of 10 °C/min. Powder/heating method
Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu
(149 lm)/MW (149 lm)/Conv. (149 lm)/MW (149 lm)/MW (149 lm)/Conv. (149 lm)/MW (149 lm)/Conv. (149 lm)/MW (149 lm)/MW (149 lm)/Conv. (149 lm)/MW (149 lm)/Conv. (149 lm)/MW (149 lm)/MW (149 lm)/Conv. (149 lm)/MW (149 lm)/Conv. (149 lm)/MW (149 lm)/MW (149 lm)/Conv. (88 + 53 lm)/MW (88 + 53 lm)/Conv. (88 + 53 lm)/MW (88 + 53 lm)/MW (APS 10 lm)/MW (APS 10 lm)/Conv. (APS 10 lm)/MW (APS 10 lm)/Conv. (APS 10 lm)/Conv.
Green density (%th.d)⁄
Sintered density (%th.d)⁄
Soaking time (min)
77.5 76.3 76.0 76.7 76.9 76.2 76.4 76.2 76.9 76.7 85.2 86.2 85.3 85.6 84.1 88.6 89.3 87.8 85.8 87.7 73.3 74.6 79.4 75.6 72.1 72.1 78.4 79.0 76.5
79.3 79.0 79.8 81.5 80.5 83.7 83.5 83.5 86.7 83.9 88.4 87.3 88.7 90.7 87.5 92.8 91.6 92.2 92.4 91.5 86.4 83.0 86.6 90.2 96.8 91.0 96.1 90.6 92.8
10 10 10 10 10 120 120 120 120 120 10 10 10 10 10 120 120 120 120 120 120 120 120 120 10 10 10 10 120
FR FR N2 Ar Ar FR FR N2 Ar Ar FR FR N2 Ar Ar FR FR N2 Ar Ar FR FR N2 Ar FR FR FR FR FR
(%th.d)⁄: percentage of the theoretical density.
Fig. 4. XRD of MW sintered Cu compacts (149 lm) with N2, FR and Ar gases environment.
and the H2 gas, at the Cu particles surface and along the grain boundaries during this expansion range. Therefore, this observed expansion was due to the plastic deformation as well as the (a)MW-FR gas- 79.5%(*)
cracking of the Cu particles caused by the high H2O vapor pressure. On the other hand, this expansion and cracks observed on the Cu particles were healed and disappeared at the end of this expansion range as well as closer to the sintering temperature (1000 °C), so that no cracks were observed at the end of the sintering process for any sintered Cu samples processed using any of the 3 different gases. So this in-situ MW setup used along with these designed experiments were so useful to carefully monitor and to understand this abnormal expansion behavior that happened as a results of the formation and the high H2O vapor pressure that later on caused the plastic deformation followed by the cracks of the Cu particles when using H2 containing processing gas. So the thin Cu oxide layer formed on the Cu particles surface under ambient condition and regular handling conditions have a critical role in the sintering behavior and in the microstructural changes in the Cu powder and compacts sintered by either MW or conventional heating. It is well known that MW heating of any material is depending on the dielectric properties of this material. CuO possess relatively high values of dielectric loss tangent (tan d) over wide ranges of frequency and temperature which makes it a good MW absorber material. CuO ceramics has been reported as a high dielectric response material with a giant dielectric constant and has been used in the microelectronic materials applications [17–19]. Furthermore, the effect of the CuO on the MW heating of Cu powder at 2.45 GHz was investigated by mixing different ratios of Cu/ CuO compacts and concluded that CuO enhanced the MW heating and worked as a catalyst . Another study had investigated the MW heating of Cu compacts under different oxygen partial pressure atmosphere and had concluded that the MW heating depend greatly on the type and the thickness of the Cu oxide layer . Furthermore, the absorbed MW power and distribution in a given sample was greatly depend on the native oxide shell . So due to the shielding and the isolating effect of the formed thin Cu oxides shell that exist on the Cu metal particles, eddy currents are suppressed to some extend which will lead to an increased MW penetration depth and a more efﬁcient MW volumetric heating till the reduction process occurs either at 210 °C (H2 containing gas) or at 600 °C (Ar and N2 gases). As discussed earlier in the H2 induced reduction, when this oxide layer will be removed at 210 °C, the MW penetration into the powder compact will be minimized. The H2O vapor that was formed as a byproduct was experimentally proved by revealing this unique expansion behavior that was occurred as a result of the plastic deformation and the cracks of the Cu particles during the sintering process. Therefore, this oxide layer on the surface of the Cu metal particles is playing a very important and critical role on the MW absorption and interaction from room temperature until the reduction process occurs and even after the reduction, as in the H2 containing gas case. These experimental observations and ﬁndings could explain why metal powder MW absorption and behavior was different from its bulk metal at room temperature. It also could explain why metal powder compacts could be MW sintered and it highlights the role of ceramics (oxides) in the MW sintering of metals. (c)MW Ar gas- 81.5%(*)
(b)Conv.-FR gas- 79.3% (*)
Fig. 5. Optical micrographs of Cu (149 lm) compacts sintered at 1000 °C (10 °C/min)10 min with (a) MW-FR gas, (b) conventional-FR gas, and (c) MW-Ar gas. (⁄): percentage of the theoretical density.
M.M. Mahmoud et al. / Journal of Alloys and Compounds 627 (2015) 231–237
Cu 2p 3/2
normalized intensity [a.u.]
TGA- 600 ° C(5%H 2 -95%Ar) gas.
Binding energy [eV] Fig. 6. XPS and TGA of the as-received Cu powder (149 lm).
(a) As-received Cu powder (149µm)
(b) Cu powder (-149µm) - MW550°C-10 °C/min - 5min. - Ar gas
(d) Cu powder (-149µm) - MW-550 °C -5 min. FR gas
(c) Cu powder (-149µm) - MW550° C-10 °C/min- 5min. - N2 gas
(e) Cu compact (-149µm) MW- 450 °C-1 min - FR gas polished
Fig. 7. SEM of the as-received Cu powder (149 lm), some MW Cu treated powder and compacts using Ar, N2 and FR gases.
4. Conclusions High frequency microwave heating was successfully used for the sintering of metal powder compacts indicating that this is a promising candidate technology in powder metallurgy. The employed in-situ electrical resistivity and dilatometry measurement setup gives important information about the MW sintering behavior and microstructural change of the materials. The sintering behavior of metal powders is depending on the type of the gas used, particle size, the initial green density, the soaking sintering time and the oxygen content or the thin oxide layer formed on the particles surfaces. Argon sintered Cu samples had the highest achieved sintered densities when compared with similar samples sintered using either Forming gas or N2 gas atmospheres. The smaller the particle size, the higher the achieved sintered density of the metal powder. MW processing of metal powders results in higher density samples in a relatively shorter soaking time when compared with similar samples processed using conventional processing. The unique expansion behavior of Cu metal powder compacts occurs only when using H2 containing gas. This is due to the
plastic deformation and crack formation of the Cu particles. Finally, it has been experimentally veriﬁed that at ambient conditions, the thin Cu oxide layer on the metal powder surface plays an important role in the MW absorption and interaction, the sintering behavior and the microstructural changes. These experimental observations and ﬁndings could explain why metal powders could be MW heated while bulk metal could not and why the MW absorption and behavior of metal powders is different from their bulk metals at room temperature. Acknowledgement We acknowledge support of this project by Deutsche Forschungsgemeinschaft (DFG) under the reference number TH 656/ 3-1 within the special Indo-German (DST-DFG) research program. References  E.D. Clark, C.D. Folz, E.C. Folgar, M.M. Mahmoud, Microwave Solutions for Ceramic Engineers, The American Ceramic Society, Westerville, Ohio, 2005.
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