SYNERGISTIC EFFECTS IN MICROWAVE-LASER HYBRIDIZATION AND ITS APPLICATION TO CERAMICS SINTERING

SYNERGISTIC EFFECTS IN MICROWAVE-LASER HYBRIDIZATION AND ITS APPLICATION TO CERAMICS SINTERING P.D.Ramesh 1, Rustum Roy and Andrzej Badzian Materials Research Institute The Pennsylvania State University University Park, PA 16802 Stephen Copley Department of Mechanical and Nuclear Engineering Applied Research Laboratory The Pennsylvania State University State College, PA 16804 ABSTRACT Sintering of zirconia (3Y-TZP) and ZnO pellets using laser-microwave hybrid technique has been achieved using a Nd-YAG laser and 1 kw kitchen microwave facility. Extensive timetemperature studies reveal that laser and microwave energies synergize to generate an intense plasma on the pellet surface only beyond the critical preheat temperature (T c ). In the case of 3Y- TZP and ZnO, the T c value lie around 1100 o C. Best conditions for sintering in these materials have been achieved only after several trial and error procedures and the conditions are 90W-10J/p-10Hz for 3Y-TZP and 35W-1J/p-50Hz for ZnO for a microwave preheat temperature of 1200 o C. Laser powers higher than this have led to severe internal crackings while lower laser powers did not produce a significant synergistic effect. The microstructure obtained from the cross-sectional area of the laser-microwave hybrid sintered 3Y-TZP pellet revealed a finely distributed nanograin microstructure with almost no grain growth. In the case of ZnO, top surface microstructure showed grain size variation with laser beam distribution. This ultra-rapid sintering procedure could be beneficial for sintering applications wherein a minimal grain growth is required. INTRODUCTION Laser sintering (LS) of powder metals is a well-known process. While this method is successfully used to selectively sinter various powder metal components, only limited publications are available on sintering ceramic materials. In 1982, Mitsuichi et al [1] reported sintering of VO 2 film using a ruby laser. In their study, they irradiated the material for 2 ms by using a maximum beam intensity of 18.9 J/cm 2. From sintering point of view, this was only a preliminary study and the densification reported was in terms of reduction in film thickness. About 10 years later, Sugihara [2] attempted reaction sintering of Pb(Zr 0.53 Ti 0.47 )O 3 bulk using a CO 2 laser. His work involved irradiation of pellet containing stoichiometric mixture PbO + ZrO 2 + TiO 2 powders by a CO 2 laser in an O 2 /Ar gas environment. Gureev et al [3] studied the conditions for laser layer sintering of the same material. Laser sintering of sol-gel coated SnO 2 : Sb was reported by Ganz et al [4] and 1 Corresponding author, email: pdramesh@psu.edu

sintering of optical coatings by Swarnalatha et al [5]. Macedo and Hernandes [6] have showed that bulk ferroelectric Bi 4 Ti 3 O 12 can be sintered using a CO 2 laser. They used a ~6 mm dia laser beam with a maximum power of 1.06 W/mm 2 for 5 minutes to achieve a relative density of ~97%. In a related work [7], the same authors have reported a reduction in sintering time reduction by one order compared to conventional sintering. They have also reported sintering of 2 mm thick Bi 4 Ge 3 O 12 using a CO 2 laser. To sinter these materials, they have used laser power densities of < 0.25 W/mm 2 for up to 25 minutes. They reported a maximum density of 95% for this material. More recently, Tolochko et al [8] have reported sintering of SiO 2 using CO 2 laser. Enhanced processing effects have been reported by combination of 2 or more lasers. QQC, Inc., a materials division of Turchan Technologies Inc., Dearborn, Michigan developed a novel technique using multiple lasers for surface sintering [9]. This process has also been referred as simultaneous multiple pulsed laser process (SIMPLE) in which, 3 lasers have been used simultaneously to process the material surface. This combination of lasers has been reported to produce staggering effects. In all the above-mentioned works one thing is quite clear. Despite the laser sintering has been proved to be somewhat successful, the use of laser alone has been more effective for surface processing and the sintering effects are substantial only when the sample thickness is small. When a laser irradiates a material, the radiation attenuates rapidly and the penetration depth is limited to several microns from the irradiated surface. The absorbed light energy is transformed to heat and the surface temperature increases multifold. If the material is highly conductive, the simultaneous heat conduction process tends to increase the thickness of the heated layer. This would generally result in an inhomogeneous material. Recently, Peelamedu et al have proposed the idea of combining laser with microwave heating for 3Y-TZP sintering [10]. The idea of hybridizing these two energies can potentially be very useful to process materials for the following reasons: (i) The laser-microwave hybrid process can offer both volumetric and surface heating together (Figure 1) ; (ii) The involvement of plasma may produce extreme heating rates that could increase processing rates several times compared to any conventional or microwave processes and (iii) the development of atypical microstructures. Figure 1 : Pellet exposed to microwave and laser energies will undergo surface and volumetric heating simultaneously

In 2001, Gerk and Willert-Porada have reported Laser Assisted Microwave Plasma (LAMP) process [11]. Their investigations have yielded important results. They have observed a gas breakdown phenomenon for a number of ceramic materials, and determined their threshold temperatures. For example, in the case of NiO, pure microwaves could cause a gas breakdown around 1200 o C and for ZnO this value drops down to 850 o C and for MgO it ascends to 1450 o C. They used a 2.45 GHz single mode cavity with a Nd-YAG cw laser beam to melt NiO/ZrSiO 4 (m.p. ~ 1600 o C) coating deposited on a yttria stabilized zirconia substrate (YSZ). Recently, we have developed a laser-microwave hybrid system for the sintering of bulk ceramics. In the following, we discuss the development of a multimode microwave-laser hybrid setup for sintering studies, the synergistic interaction between laser and microwave heating for 3Y-TZP and ZnO, their sintering studies, and microstructure characterization. EXPERIMENTAL A 1100 W kitchen microwave oven was modified by drilling two 25 mm holes in its top. The first hole was directly above the pellet. This is the hole through which laser beam irradiates the pellet. The second hole was drilled such that its axis formed a 20 o angle with the center hole. The IR pyrometer is focused through the second hole to the middle of the pellet. The pyrometer model used in these experiments is model MAS2C (Raytheon Technologies) type, capable of recording temperatures for every 1 sec interval and can measure up to a maximum of 2000 o C. The temperature data has been digitized and recorded using a data converter. The laser type used in these experiments was a Nd-YAG laser (Lumonics Model JK-701) which was focused through the first hole using an objective lens set-up. By defocusing, the beam diameter was expanded to match the size of the pellet. The block diagram of the microwave-laser hybrid unit is shown in Figure 2. Yttria Stabilized Zirconia (3Y-TZP, Tosoh Corporation, Yamaguchi, Japan) and ZnO were chosen for synergistic studies because of their technological significance. Figure 2: Schematic of Microwave-Laser Hybrid heating set up

The average particle size of 3Y-TZP powder was 210 nm and that of ZnO powder was ~20 μm. The powders were uniaxially pressed followed by cold isostatic pressing (CIP) to form cylindrical shaped pellets. The CIPing process has resulted in approximately 10 15% size reduction of the pellet. The microwave-laser experiments were carried out in two phases; (i) synergistic interaction studies followed by (ii) sintering studies. The sintered samples were characterized using x-ray diffraction (XRD), scanning electron microscopy (SEM) and Archimedes density measurements. RESULTS & DISCUSSION (a) Susceptor Interaction 3Y-TZP and ZnO are insulators at room temperature and absorb microwaves efficiently only at elevated temperatures. Hence, in order to achieve uniform sintering, it is necessary to surround these pellets by SiC, a well-accepted room temperature susceptor. Janney et al [12] and Ramesh et al [13] have suggested different SiC susceptor configurations for ZrO 2. Modeling work by Lasri et al [14] using published ε ' r and tan δ values calculated the heating trend of these materials when surrounded by a SiC preheater. In this case, the total power in the system is defined as sys abs abs HL P = P + P ( P + P The absorbed microwave power/unit volume is defined as tot SiC ZrO SiC HL ZrO ) P abs and power loss by heat transfer process ' ' = 2π f [ ε ε tanδ E + μ μ tan H o r 2 o r 2 ] HL P = PCOND + PCONV + P RAD The conduction and convection processes are directly proportional to T while the radiation process varies with T 4. Ramesh et al [13] have also shown that the radiation losses start to dominate only beyond the critical point, T c = 800 o C. Beyond this, the ZrO 2 temperature exceeds that of SiC and the flow of energy is reversed, i.e. energy flows primarily by radiation from ZrO 2 to the SiC. b) Cooperative interactions of 3Y-TZP and ZnO ceramics Prior to sintering studies, it is important to understand the cooperative interaction between laser and microwave heating. Laser alone could not elevate the 3Y-TZP-pellet temperature. Even laser power levels of 300 W and beyond were not sufficient to elevate pellet temperature to 350 o C. But the laser interaction became quite strong beyond microwave heating of 1000 o C and above. The typical interaction curve is shown in Figure 3. The actual cooperative interaction graph for laser conditions of 90W-94Hz-1J/p for varying laser power levels is shown in Figure 4. The kick-off temperature dropped down only slightly when the laser power value was tripled. A view through the pyrometer window clearly revealed formation of plasma above the sample surface. The size of the plasma was slightly bigger than the sample diameter. In contrast, no visible plasma formation

was observed during sole microwave heating irrespective of the surface temperature. Also, no plasma was observed when only laser irradiated the pellet. Since the pyrometer was placed in such a way to obtain temperature values from the middle of the sample surface, after the plasma creation the measured temperature values corresponded to plasma rather than the real surface temperature. Figure 3 : Typical hybrid heating curve Figure 4: Cooperative interaction curves obtained on a 3Y-TZP pellet for various laser power levels

Cooperative interaction studies were performed for ZnO samples similar to 3Y-TZP but at a much lower power of 50 W. At this power level, even with microwave preheat temperatures of 450 o C, 800 o C and 1000 o C the laser interaction was not very strong. But the same preheating conditions, a higher laser power namely, 75W showed a much stronger interaction beyond 1000 o C. The cooperative interaction became further strong beyond 1100 o C. The Laser-MW interaction studies carried out on ZnO for a laser conditions of 75W-50Hz-1J/p-180s and microwave preheat temperatures of 420 o C, 600 o C, 750 o C and 1000 o C is given in figure 5. The threshold temperature for ZnO was ~1000 o C which was quite close to the kick-off temperature of ZrO 2. This similarity is just a mere coincidence. Figure 5: Laser-MW interaction studies carried out on ZnO for a laser power of 75 W/50 Hz/ 1 J/p/180s and microwave preheat temperatures of 420 o C, 600 o C, 750 o C and 1000 o C. c) Sintering and Microstructure Laser power values higher than 90 W were found to be detrimental for 3Y-TZP sintering. At very high laser power levels, the sample surface showed evidence of melting and presence of numerous cracks throughout the sample volume whereas; moderately high laser power levels (~200W) create ablation and surface cracking. The shrinkage, density values and the microstructure have indicated that the 3Y-TZP pellets sintered quite well. The density values obtained were generally above 90% after just 3 minutes of laser irradiation. In the case of microwave sintering, to achieve similar density values it took several minutes. A comparison between microstructures of the microwave-sintered and laser-microwave sintered 3Y-TZP are illustrated in Figures 6a and 6b. A well-sintered, crack-free sample has resulted from laser-microwave sintered sample whereas; the microwave-sintered specimen had plenty of nano and micro cracks spread throughout the microstructure. This difference is quite interesting because the heating rate obtained in laser microwave sintering was much higher than the microwave-sintered sample. These differences could have been caused by the shockwaves generated by laser pulses [15]. Another astounding effect is the

grain size. The average grain size measured from the microstructure of laser-microwave-sintered sample was 20 nm. This value is very close to the grain size of 3Y-TZP quoted by the manufacturer. Comparing both the grain sizes, it is clear that in the laser-microwave process, a zero grain growth has taken place. a b Figure 6: Microstructure obtained at the cross-section of 3Y-TZP pellet sintered by (a) laser-microwave hybrid method and (b) microwave method Table I: Sintering of ZnO for various microwave preheating temperatures T SAMPLE NO LASER CONDITIONS MW PREHEAT TEMP. DENSITY BEFORE (G/CC) Zn75-4 75 W/50Hz/1J/p 240 seconds Zn60-4-1 60 W/50 Hz/ 1J/p, 240s Zn60-4-2 60 W/50 Hz/1J/p, 240 s Zn50-4-3 50 W/50 Hz/1 J/p, 240 s Zn40-4-4 40 W/50Hz/1 J/p 240s Zn35-4-5 35 W/50Hz/1 J/p, 240s Zn35-4-6 35 W/50 Hz/1J/p, 240s Zn35-6-7 35 W/50 Hz/1J/p, 360s Zn35-2-8 35 W/50 Hz/1J/p, 120s Zn35-0-9 35W/50Hz/1J/p, 0s 1200 o C 3.516 (62.7%) DENSITY AFTER (G/CC) 1000 o C 3.904 (70%) 4.984 (89%) 1000 o C 3.628 4.814 (65%) (86%) 1000 o C 3.561 4.878 (63.6%) (87%) 1200 o C 3.418 4.684 (61%) (83.6%) 1200 o C 3.708 5.038 (66.2%) (90%) 1200 o C 3.636 5.087 (65%) (91%) 1200 o C 3.600 5.046 (64.2%) (90%) 1200 o C 3.514 5.102 (62.7%) (91%) 1200 o C 3.621 4.880 (64.6%) (87%) 4.916 (88%) SHRINKAGE (%) able 1 shows ZnO samples laser-microwave sintered for various microwave and laser conditions. Unlike the 3Y-TZP case, the hybrid condition used for ZnO sintering was 35W-1J/p-50Hz-repetition rate and a microwave preheating temperature of 1200 o C. As discussed earlier, microwave preheat temperatures lower than this has resulted in poor interparticle bonding. The surface microstructure 21.6 24.6 26.9 27.0 26.3 28.5 28.6 31.1 25.7 28.4

of 35 W sintered ZnO sample displayed a complex grain size distribution. The average grain size obtained from the middle region was larger compared to other regions of the surface (Figure 7). The micrograph also revealed that well-connected grains with pores located at the boundary between two neighboring grains. Figure 7: Microstructure obtained from the surface of ZnO pellet sintered by laser-microwave hybrid procedure CONCLUSIONS In this work, it was demonstrated that combination of two different electromagnetic energies namely pulsed Nd-YAG laser and microwaves produced a synergistic effect with extraordinary characteristics. This combination was successfully utilized to sinter technologically valuable ceramics such as 3Y-TZP and ZnO. Time dependent synergistic studies have showed that for both 3Y-TZP and ZnO primarily the cooperative interaction was more effective only beyond 1100 o C. After several trial and error procedures, the chosen sintering conditions were 90W-10J/p-10Hz for 3Y-TZP and 35W-1J/p-50Hz for ZnO. In the case of 3Y-TZP, the laser-microwave hybrid sintering offered a zero grain growth microstructure and the sintered material was superior to the microwave-sintered product in terms of quality. In both the cases, the cooperative effect produced by bi-energy synergy showed a reduction in sintering time by several orders. This novel technology can offer altogether different sintering effects in materials that are useful for high performance applications. ACKNOWLEDGEMENT: The funding support by Office of Naval Research, Washington D.C. (Contract # N00014-02-1-0658) is gratefully acknowledged.

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