Sintering of molybdenum metal powder using microwave energy

Sintering of molybdenum metal powder using microwave energy P. Chhillar1, D. Agrawal*2 and J. H. Adair2 Molybdenum is a refractory metal used for high temperature applications. Sintered molybdenum (Mo) with fine microstructures is desired due to its improved mechanical properties. However, the sintering of Mo is not easily achievable by conventional processes. In this work, the author report the results of sintering molybdenum powder to obtain submicron grain size microstructure using microwave energy. As received Mo powder was agglomerated with a mean agglomerate size of 1.6 mm, but equivalent surface area based on N2 adsorption suggests an average particle size of 200 nm. Sintering was carried out using the as received powder. Samples with densities as high as 98% of theoretical density (TD) were obtained with limited grain growth in ,5 min of sintering time in microwaves, compared to 10–20 h in a conventional process. The highlight of this research is achieving 98%TD in 1 min at 1650uC with a submicron grain size. Keywords: Molybdenum, Sintering, Microwave, Powder metals

Background Molybdenum is a typical transition metal element having a high melting point, high mechanical strength, and high modulus of elasticity.1,2 Most of the applications for pure molybdenum metal and its alloys involve high temperatures.1 Some of the important uses are electrodes for electrically heated glass furnaces and forehearths, nuclear energy applications, missile and aircraft parts, thermocouple sheaths, flame and corrosion resistant coatings for other metals, and as an alloying agent in steel. Conventionally the sintering of molybdenum powder is conducted using a resistance or induction sintering furnace in an inert atmosphere (argon) or in a reducing atmosphere (hydrogen).3 High temperatures in the range of 2000uC are employed, resulting in densities of 90–95% of theoretical, depending upon the sintering time.3 Huang et al.4 reported the sintering of molybdenum using vacuum furnaces and obtained densities of 97 to 98.5% at a sintering temperature of 1750uC with times ranging from 10 to 40 h. Microwave sintering of molybdenum metal powder has not been reported in the literature, but various other metals, metal composites and ceramics have been sintered using microwave energy.5–17 Microwaves are electromagnetic radiation with wavelengths ranging from about 1 mm to 1 m in free space, with corresponding frequencies ranging from about 0.3 to 300 GHz. Microwave heating of materials is fundamentally different from conventional heating in that the thermal

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M Cubed Tech., 1 Tralee Industrial Park, Newark, DE 19711, USA Materials Research Institute, Materials Research Laboratory Building, The Pennsylvania State University, University Park, PA 16802, USA

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*Corresponding author, email [email protected]

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ß 2008 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 15 November 2006; accepted 8 January 2007 DOI 10.1179/174329007X178001

energy is internally generated within the material through molecular interactions with the electromagnetic field, instead of originating from an external heating source and subsequent radiative transfer. Microwave heating is a very sensitive function of the material being processed and depends upon such factors as size, geometry and mass of the sample. Sintering of ceramics using microwave energy is widely reported and examined in detail.12,13,18–20 It is much better understood than microwave sintering of metal powders. It is well known that bulk metals do not absorb, but reflect, microwaves. Powder metals, on the other hand, have shown considerable microwave absorption, resulting in high sintered densities.5,6,14,15 The exact mechanism for the heating of metal powders in microwaves is not well understood. Conventionally, the electric field has been identified as the major source of heating by ohmic losses due to local potential differences. Lately, experiments performed in separated electric and magnetic fields have shown the important role played by magnetic field’s interaction with materials, especially in the case of conducting and semiconducting materials such as cobalt, iron, copper and iron oxide (Fe3O4).21–24 For these and other materials it was found that the interaction with the magnetic field, and not the electric field, is the principle sources of heating. Contributions to the magnetic heating can by hystersis, eddy currents, magnetic resonance and domain wall oscillations.

Experimental procedure The molybdenum powder used in this study was received from Climax Molybdenum Co. Particle size distribution (PSD) of molybdenum powder was measured using light scattering (Malvern Mastersizer, Malvern Instruments, UK). The phase identification of

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Sintering of molybdenum metal powder using microwave energy

1 Schematic of experimental setup used for microwave sintering in this study

the as received powder was carried out by X-ray diffraction (XRD, Scintag Inc., Cupertino, CA, Cu Ka with ˚ ). The morphology of the a wavelength of 1.5418 A molybdenum powder was determined by scanning electron microscopy (SEM, Hitachi S-3000H, Tokyo, Japan). Higher resolution images of molybdenum powder were taken with a field emission scanning electron microscope (FESEM, JEOL 6700-F). The specific surface area of the powder was determined by BET surface area measurement (Micromeritics, Gemini 2370, Atlanta, USA). Molybdenum powder sample was also sent to LECO Corp. (St. Joseph, Michigan) for oxygen analysis. Pellets of the as received powder of 0.5 inch diameter (weighing y2 g) were uniaxially pressed with a hydraulic die press (Carver Inc.) at loads from 4 ton (44 ksi/ 30.7 MPa) to 11 ton (110 ksi/76.7 MPa). Green densities of the pellets were calculated by measuring the volume and the weight of the pellets. The microwave sintering experiments were carried out using a 6 kW, 2.45 GHz, multimode microwave furnace. The experimental setup is shown in Fig. 1. A multilayered insulation package was used to provide sufficient insulation to obtain high and uniform temperatures throughout the sample. The outer package was made up of thick ceramic fibre (alumina and silicon carbide) sheets. A mullite tube was placed at the centre of the package, and samples were placed in this mullite tube on a 1 cm thick layer of fused alumina powder, for additional insulation. The entire package was placed on a turntable to ensure uniform exposure of the sample to the microwave field. A reducing atmosphere was maintained during the sintering by first creating a vacuum of 8–10 torr inside the furnace followed by back filling ultra high purity (UHP) hydrogen. Throughout the sintering, hydrogen gas flow was

maintained at 2 L min21. This gas was diluted with nitrogen gas before venting to the atmosphere. Temperature measurement was performed with an optical pyrometer (Leeds & Northrup Co., Philadelphia, PA), through a small quartz window at the top of the furnace. All samples were initially heated at 800uC in hydrogen for 4 h to remove oxygen from the green bodies. After the deoxidation stage, samples were sintered at higher temperatures for different times. Temperatures varied from 1050 to 1700uC, and sintering times varied from a few hours at low temperatures to only a few minutes at high temperatures. After sintering, the microwave power was switched off and samples were allowed to furnace cool. Conventional sintering was done in a Lindberg box furnace. The furnace had a maximum operating temperature of 1400uC. A mixture of hydrogen and dry nitrogen (1 : 9 by volume), was passed through the furnace, at 500 cc min21, to maintain a reducing atmosphere. Alumina boats were used for placing the samples in the furnace. Samples were heated at temperatures from 1150 to 1400uC, with sintering times varying from 30 min to 20 h. The densities of the sintered samples were measured using the ASTM standard procedure.25 The sintered samples were then polished with 180, 320, 400 and 600 grit SiC at 35 psi pressure and at 300 rev min21 wheel speed for 1 min each. The samples were finally polished with 1 mm diamond red felt cloth. After polishing, the samples were etched using Murakami’s reagent (composition: 10 gm K3Fe(CN)6, 10 gm KOH, 100 mL distilled water).26 Microstructures of the polished samples were evaluated with SEM. Grain size measurements were conducted using the Image J software. Statistically large numbers of grains (40–100) were measured individually using the software tools, to calculate the mean grain size.

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2 Particle size distribution of as received molybdenum powder in water, measured by Malvern Mastersizer: mean particle size is 1.6 mm with a mono modal distribution

Results and discussion Characterisation The particle size distribution of the powder in water is shown in Fig. 2. Mean particle size value is 1.6 mm with a monomodal PSD curve. The XRD pattern (not shown) for the powder shows only single phase Mo. The SEM image in Fig. 3 shows the presence of large agglomerate with strongly bonded particles and irregular morphology leading to an atypical large specific surface area. The BET surface area for the powder is 3 m2 g21, corresponding to a 200 nm equivalent spherical diameter for the powder. Compared to the measured mean particle size of 1.6 mm (dispersed in water), the primary particle size of 200 nm verifies the large extent of agglomeration. Oxygen analysis shows the presence of 1.3 wt-% oxygen in the powder. This is not unexpected, as molybdenum in contact with air reacts with oxygen. The fine particle size of the powder ensures that a large surface area of the powder is exposed to air, and it results in a large uptake of oxygen. Oxygen could be present as a thin molybdenum oxide layer or as free oxygen adsorbed at the particle surface. This oxygen is removed during a presintering heat treatment of the green bodies.

3 Image (SEM) of molybdenum powder showing large agglomerate and irregular shape of particles

highest temperature obtained in the conventional furnace used was 1400uC, only samples processed at 1400uC were compared. The results show that to achieve similar sintered densities, microwave processing takes only 10% of the conventional sintering time. The dependence of sintered density on soaking time for conventional and microwave sintered samples at 1400uC is shown in Fig. 4. Sintered density increases very rapidly with sintering time for microwave sintered samples relative to conventionally sintered samples. At 1400uC, microwave sintering takes 30 min to achieve 99% density, whereas conventional sintering takes 10 h to achieve 98% density. This finding justifies the notion that microwave energy produces enhanced sintering of molybdenum powder regardless of the exact kinetic mechanism. Thus, microwave energy is an efficient way to sinter molybdenum powder relative to conventional heating. It should also be noted that the results obtained even in the conventional sintering in this study have much higher sintered densities at much lower temperatures (1300 and 1400uC) than previously reported. A sintered

Sintered density Results of the microwave and conventional sintering are given in Tables 1 and 2 respectively. Samples sintered in the microwave are labelled as SM, and samples sintered in the conventional furnace as SC. With microwave energy, sintered densities greater than 98% theoretical density can be achieved at temperatures as low as 1400uC in 30 min (SM15), and at the higher temperature of 1650uC a similar degree of densification was obtained in just 1 min (SM19). Dependence of sintered density on heating profiles was also observed. Table 3 compares samples sintered at the same temperatures and times but with different heating profiles. Heating rates varied from 10 to 100uC min21. Results show that slower heating rates aid in the final sintering density as compared with very high heating rates. A comparison between conventional sintering and microwave sintering data is shown in Table 4. As the

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4 Comparison of sintering behaviour in conventional furnace with microwave furnace at 1400uC, shows faster sintering with microwave: in microwave furnace, 99%TD could be achieved in 30 min, but in conventional furnace 98%TD could be achieved after 10 h


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Table 1 Microwave sintering data presented in order of increasing sintering temperature* Debinding

Density, %TD Time to sintering Sintering Sintering time, Sample no. Temperature, uC Time, h temperature, min temperature, uC min Green Sintered SM1 SM2 SM3 SM4 SM5 SM6 SM7 SM8 SM9 SM10 SM11 SM12 SM13 SM14 SM15 SM16 SM17 SM18 SM19 SM20 SM21 SM22 SM23 SM24 SM25 SM26 SM27

– 800 800 800 800 800 800 800 800 800 800 800 800 800 800 – – 800 800 – – 800 800 850 800 800 –

– 4 4 4 4 4 4 4 4 4 4 4 4 4 4 – – 4 4 – – 4 4 4 4 4 4

10 10 60 30 10 10 10 45 45 5 50 50 20 30 30 120 120 60 40 30 12 15 30 5 1 10 3

800 1050 1150 1200 1300 1300 1300 1300 1300 1400 1400 1400 1400 1400 1400 1550 1600 1650 1650 1650 1650 1650 1700 1700 1700 1700 1750

240 150 120 30 60 10 60 30 60 10 10 20 10 20 30 20 5 20 1 5 5 1 1 5 2 2 1

44 44 44 58 44 49 54 58 58 49 49 49 58 58 58 44 44 44 54 54 54 54 58 44 49 49 44

52 75 90 90 64 64 87 91 93 64 84 89 94 96 99 97 91 97 98 98 97 96 98 91 88 97 94

Grain size, mm – –

– – – – – – –

5.6 0.6

0.9

2.4 1.7 4.3 7 9.4 7.9 0.6

– – 4 5 32 5.5 7.6 8

*Results show that high densities could be obtained at low temperatures with longer sintering time and at higher temperatures in very short times. Table 2 Results for conventional sintering* Debinding

Density, %TD

Sample no.

Temperature, uC

Time, h

Sintering temperature, uC

Sintering time, min

Green

Sintered

Grain size, mm

SC1 SC2 SC3 SC4 SC5 SC6 SC7 SC8

800 800 800 800 800 800 800 800

4 4 4 4 4 4 4 4

1250 1150 1150 1300 1300 1400 1400 1400

10 600 1200 600 60 30 180 600

49 49 49 58 58 58 58 58

77 87 86 96 86 86 95 98

– 1.8 1.0 4.3 0.6 0.7 1.8 5.4

*Samples were sintered to more than 95%TD at temperatures as low as 1300–1400uC.

Table 3 These results show effect of heating proﬁle on sintered density Green density, %TD

Sintering temperature, uC

Heating time, min

Sintering time, min

Sintered density, %TD

49

1400

49

1700

5 50 1 10

10 10 2 2

64 84 88 97

*Samples that were raised to the sintering temperature slowly show higher density. Table 4 Comparison of microwave sintering results with conventional sintering results* Green density, %TD

Temperature, uC

Sintering mechanism

Sintering time, min


58

1300

C

58

1400

60 60 30 30 600

86 93 86 99 98

*Samples with equal green density, sintered to same temperatures, require much longer sintering times to reach similar densities.

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5 Microstructure of sample SM3: sample microwave sintered at 1150uC for 2 h to sintered density of 90%TD; green density 44%TD; average grain size 5.6 mm

6 Microstructure of sample SM19: sample microwave sintered at 1650oC for 1 min to sintered density of 98%TD; green density 54%TD; average grain size 0.6 mm

density of 98%TD at 1400uC is significant with a sintering temperature y400uC lower than previously reported temperatures3,4,29 required for such high density. The authors attribute higher densities at lower temperatures and times to the smaller mean particle size of the starting powder used in this study.

Effect of green density Density data for samples with different green densities, sintered at the similar temperatures and times, are given in Table 5. As expected, results show that higher green density results in a higher sintered density. In agglomerated powders the intra-agglomerate pores are usually much smaller than the interagglomerate pores. Pressing the powder at high loads results in high green density by plastically deforming the large agglomerates and removing the large interagglomerate pores. Removal of large pores results in much faster densification kinetics with diffusion occurring over shorter distances. At lower sintering temperatures, the difference in sintered densities is more significant than that at higher temperatures. These results are consistent with the higher diffusion rates at higher temperatures, permitting even the large interagglomerate pores to be removed. In contrast, at lower temperatures, the diffusion kinetics is much slower, and removing large pores is difficult, even when sintered for several hours.

Microstructural analysis Grain sizes of selected samples are given in Table 1. Images (SEM) of sintered samples are shown in Figs. 5–8. The grain sizes of samples sintered to more than 90%TD in microwave varied from less than 1 mm (SM19, Fig. 6) to 32 mm (SM24, Fig. 8). SM19, with 98%TD and ,1 mm grain size shows that with careful modifications in the

7 Microstructure of sample SM25: sample microwave sintered at 1700uC for 2 min to sintered density of 88%TD; green density 49%TD; average grain size 5.5 mm

sintering parameters, grain growth can be minimised without compromising sintered density. Grain size distribution curves are shown in Fig. 9 for samples sintered in a microwave furnace to different densities. These results show that the grain size distribution is mono modal, and with increasing sintered density, the distribution shifts to the large sizes. Sample SM9 sintered to near 93%TD had a grain size of y1 mm. The grain size increases to ,2 mm for sample SM14 sintered to 96%TD, and for sample SM23 sintered to 98%TD, the grain size is ,6 mm. Only sample SM19, sintered to 98%TD, has a grain size ,1 mm (not shown in the figure). It is clear that higher sintering times at high temperatures always result in higher grain growth. At

Table 5 Effect of green density on sintered density Temperature, uC

Time, min

Sample ID

Green density, %TD


1300

60

1400

10

1400

20

SM5 SM9 SM11 SM13 SM12 SM14

44 58 49 58 49 58

64 93 84 94 89 96

*Higher green densities result in much improved sintered densities for equal sintering temperature and time.

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8 Microstructure of sample SM24: sample microwave sintered at 1700uC for 5 min to sintered density of 91%TD; green density 44%TD; average grain size 32 mm

1700uC, a sintering time of 2 min results in a grain size of 6–8 mm in sample SM25 (Fig. 7); however three more minutes’ sintering raises the grain size to 32 mm in sample SM24 (Fig. 8). A very careful heating profile from the debinding temperature to the sintering temperature is required for controlling the grain size. Samples heated very rapidly have less heating time, which results in poor sintered densities, as in sample SM25, where the heating from deoxidation stage (at 800uC) to sintering temperature took place in just 1 min. The sintering took place at 1700uC, yet the sintered density is lower than 90%TD. However, just having large heating times is not sufficient. The rate of temperature increase is also critical. Spending too much time at lower temperatures will lead to poor sintered density due to particle coarsening, and spending too much time in the vicinity of sintering temperature will lead to extensive grain growth.

Conclusions Microwave sintering results show that near theoretical densities can be obtained at much reduced temperatures, and with much reduced sintering times, as compared to conventional sintering. Samples were successfully sintered to above 98%TD at 1600–1700uC in less than 5 min. This study shows that with a carefully designed sintering profile, samples can be sintered to submicron grain sizes at 1650uC in y1 min of sintering time. Conventionally sintered samples also showed improved results. Sintering at 1400uC for 10 h resulted in 98%TD. However, using microwave energy 99%TD could be obtained at 1400uC in just 30 min, conclusively showing that microwave sintering is much faster than conventional sintering.

Acknowledgement Financial support for this work from the Particulate Materials Center of Pennsylvania State University under contract number NSF/EEC0002987 is gratefully acknowledged.

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