MICROWAVE SINTERING OF NANO PARTICLES

3.4. MICROWAVE SINTERING OF NANO PARTICLES

The process of “Microwave Sintering” of nano-sized alumina powders is helpful to obtain a high density (~ 99% of theoretical density) with MgO doping. Under the microwave sintering process, a dense microstructure is also observed by transmission electron microscopy (TEM), which is known to increase the fracture strength of sintered alumina. Such a dense microstructure can be observed due to the increase in density for alumina materials sintered at a higher temperature. Some details of the proc- ess of microwave sintering of alumina is presented in this section.

The last half of 1980’s showed a great deal of interest in the area of microwave sintering. Despite this surge of recent activity, the first pioneering experiments on microwave sintering were performed as early as 1968 by Tinga and Voss [24]. The real activity began to accelerate by Sutton [25] for drying and firing of alumina castables, and by Schubring [26] for sintering of ceramics. In the late 1970’s and 1980’s, the microwave heating and sintering of uranium oxide was reported by Hass [27], for ferrites by Krage [28] and for ‘Alumina-Silicon Carbide Composites’ by Meek et al [29]. But a very few literature existed on alumina sintering by the microwave route [3].

Alumina ceramics are normally densified by a conventional process of sintering, which requires higher temperatures (~ 1600°C) and longer duration. Thus, the manufacturing cost naturally becomes very high, since the energy cost is increasing by the day. In order to make the production of alumina economically viable, i.e. lesser cost, the microwave sintering process should be very useful. In 1989, Katz and Roger [30] reported microwave sintering of alumina. Compared to conventional methods, this technique has proved to offer several advantages like attainment of the desired phase and fine-grained microstructure, which are the ultimate ‘goals’ of sintering any ceramic material.

It is quite plausible to mention a few lines on the considerable amount of developmental work, which has been in progress for microwave furnaces, now that it is clear that microwave heating is an effective method for sintering ceramics. The companies with manufacturing experience in furnaces and/ or microwave equipment are developing research and industrial microwave furnaces to meet the de- mand. As the availability and affordability of microwave furnaces improve, this in turn would stimulate the commercialization of microwave sintering, especially for advanced ceramics. It is important to un- derstand the different features available in microwave furnaces and to make comparisons, so that the appropriate system can be used in materials research, product development, and production.

Early microwave materials research was performed using systems built ‘in house’ and/or de- signed with little consideration for ‘manufacturing expenses’. It is difficult to compare the features of these systems, or to determine the feasibility of scaling up microwave processes from the literature. Recently, microwave system designs have improved, becoming more user friendly and cost effective. In any comparative work, different ‘approaches’ to microwave furnace design should be discussed with the description of the important ‘operating parameters’, and then a direct comparison should be made by

NANO MATERIALS

sintering known ceramic ‘standards’ in different microwave furnaces, and also commercially available microwave furnaces should be used.

Shulman and Walker [31] investigated different types of microwave systems which included :

1. Several sizes of multimode 2.45 GHz,

2. Hybrid microwave + electric,

3. Hybrid microwave with 2 frequencies, and

4. Millimeter-wave. It was found that microwave firing parameters were readily transferred between different micro-

wave systems, if similar refractory packages were used. The equivalent properties and microstructures could be achieved, by using different microwave systems. However, temperature measurements may not

be comparable. Energy savings could vary between the systems and depend on the (a) Insulation, (b) Load size, and (c) Type of ceramic. There is now an excellent selection of microwave furnaces available for research and production. It is anticipated that for material scientists, the research focus will shift from fabricating furnaces to developing useful processes. Some companies are currently develop- ing microwave processes for many types of advanced ceramics and exploring the feasibility and chal- lenges of scaling up [31].

Shulman et al [32] also studied the ‘scaling-up’ of microwave processes for production, which requires the systematic study of the following :

1. Uniformity of microwave sintered materials, and

2. Reproducibility of material properties for given processing parameters. In this study, a microwave power versus time recipe for ‘sintering zirconia ceramics’ to full

density was successfully transferred from a 1.1 kW modified ‘kitchen microwave’ to a large 3 kW chamber (35 ′′ dia × 54′′ length). The effects of increasing the load and refractory box volume, as well as the box position were investigated. The uniformity and reproducibility of the density, hardness, and microstructure were explored [32].

Finally, Shulman’s group [33] investigated alumina and zirconia materials, which were sintered to > 99% theoretical density in less than 1 hour total cycle time. The study investigates the effects of input power and cycle time on heating rates, microstructure, density and mechanical properties, i.e. hardness and MOR. The properties are compared to conventional sintering conditions, which resulted in similar densities. In addition, the effects of various sintering conditions, e.g. (a) Location of sample within the microwave chamber, i.e. hot spots, (b) Number of samples within a given insulation box, (c) Number, shape and position of various SiC susceptors, and (d) Aging effects of the insulation as well as susceptors, were also investigated. Additionally, some materials from a rapid prototyping process, i.e. gelation casting, were investigated and interesting results obtained [33].

3.4.1. Microwave Sintering Route

The merits of microwave sintering of ceramics are : (a) Very Rapid Heating, and (b) High Densification achieved at relatively low temperature. In this process, microwave energy interacts with the material at the molecular level. The ability

of the molecules to follow the rapid reversal of the 'electric field' results in the conversion of the 'electro- magnetic' energy into heat within the irradiated material. The dissipation of power by a ceramic material in an electric field is given by the following equation :

NANO PARTICLES OF ALUMINA AND ZIRCONIA

P = (2 πf σ)(E 2 /2) tan δ

where, P is the power dissipated, f is the frequency of microwave generator, σ is the dc conductivity, E is the electric field strength, and tan δ is the ‘loss tangent’ of the dielectric material.

It is readily apparent from the above equation that the microwave energy absorbed within a given material depends on its ‘effective dielectric loss’ and the distribution of electric field within the material due to its quadratic dependence. Here, the depth of the microwave penetration into the material is quan- tified by defining the ‘skin depth’, which is the distance at which the electric field falls to 37%. The skin depth is calculated by the following formula :

Skin Depth =

( f ) π µσ 1/ 2

where, µ is the dielectric permittivity. The microwave heating is fundamentally different than conventional heating for sintering of

ceramics, where heat energy flows from outside the material towards the interior of the matrix. This may cause some type of inhomogeneity of heat distribution (i.e. thermal gradients inside and outside the material) resulting in differential density arising out of the inadequate distribution of the required phase for strength development. However, in the microwave heating, the heat energy is generated ‘internally’ within the core of the material instead of coming from the external heating source.

As a result, an ‘inverse’ temperature profile is developed from centre or core, i.e. to the outside surface of the material, in microwave heating in comparison with the conventional sintering. Due to this ‘reverse’ flow of heat, the microwave heating makes it possible to sinter small and large shapes very rapidly and uniformly, and also to reduce the 'thermal stresses' due to 'thermal gradients' that cause cracks during the fabrication process.

The microwave heating is sensitive to the amount and the type of impurities present in the mate- rial. If the material contains some ‘impurities’, which reasonably absorb in the microwave region of the electro-magnetic spectrum, then the heating will be impaired. However, high purity alumina, which normally requires higher temperatures in conventional sintering, is highly transparent to 2.45 GHz mi- crowave frequency region at room temperature that is usually used in the sintering process through ‘microwave route’.

At the above frequency domain, the ‘skin depth’ is of the order of one meter, which is much more than the usual shapes being sintered in special ceramics. In contrast, for ‘lossy’ material with dispersion behaviour, such as silicon carbide, the ‘skin depth’ is of the order of microns, and in most cases, the microwaves are absorbed before they can fully penetrate the ‘platelets’ used in this type of study.

The details of the material preparation with alumina from Alcoa (USA) as a starting material is already given in section 3.2.1. with the details of Attrition Milling. The dopants used were magnesium oxide from E-Merck (Germany) of grade - Gr., as a sintering aid during the milling. The details of the ‘grinding parameters’ on the final particle size of 50 nm alumina particles are also given in section

3.2.1. After measuring the particle size in an Autosizer 2C, the particles were also checked under a transmission electron microscope, as described in the section 2.6.3. The electron micrograph is shown in Figure 3.5 to show the distribution of nano-particles.

NANO MATERIALS

Figure 3.5: TEM photo of nano particles of alumina powders.

3.4.2. Sample Preparation from Nano Particles

The milled nano-crystalline alumina powder was mixed in acetone, with 1% Oelic acid in solu- tion as a ‘pressing aid’, in order to form a slurry. After drying the slurry at 80°C, the mixture was passed through a sieve of mesh size 30 to form a ‘free flowing’ granules, which could be easily compacted under isostatic pressure. These granules are then packed into a rubber mould usually needed in isostatic pressing and compacted at an isostatic pressure of 210 MPa. The tubes were formed of length of 45 mm with inner diameter of 10 mm and ouside diameter of 20 mm. These compacts were processed in such a way in order to have a high green density, i.e. 64% in this case, which is fundamentally important in order to achieve a high sintered density.

3.4.3. Sintering Procedures of Nano Particles

The sintering of this compacted alumina tube is carried out in a microwave oven at 1400°C and 1500°C from 30 to 120 minutes. The microwave energy is supplied by using an optimised power of 1350 ± 10% Watts magnetron operating at a single frequency of 2.45 GHz. In order to increase the microwave energy absorption by the alumina compacts and also to ensure an uniform temperature, a furnace cavity is designed from ceramic wool boards (formed under vacuum) after giving it a special treatment to withstand high temperature. The green alumina tubes are loaded in alumina tray and packed with α-silicon carbide powder, and kept in the furnace cavity in the microwave oven.

After sintering, the tubes are characterized by different measurements like density by simple Archemedes principle, microstructure by scanning electron microscope (SEM) and fracture strength by Universal Testing Machine in order to see the effect of microwave sintering on different physical prop- erties of nano-sized alumina paricles.

NANO PARTICLES OF ALUMINA AND ZIRCONIA

3.4.4. Sintering Data of Nano Particles of Alumina

The nano-crystallline alumina particles have been prepared by optimising different grinding pa- rameters having average particle size of 50 nm, as shown in the TEM micrograph in Figure 3.5. The microwave sintering of these nano-crystalline alumina particles show interesting densification behav- iour as a function time of sintering between 30 and 120 minutes at 1400°C and 1500°C, as shown in Figure 3.6. It is seen that the densification at 1500°C is much faster than at 1400°C, and thereby leading to a maximum density of 3.93 g/cc for 120 min [i.e. 99% of theoretical density (TD)] for the former case. The maximum density achieved at lower temperature of 1400°C for 120 min is only 3.57 g/cc (i.e. 90% of TD), indicating a reduction of 9.2%. While the rate of densification is quite steady till 60 minutes for 1500°C, that for 1400°C it is constant upto 75 minutes, showing both the effects of temperature and time of sintering of 50 nm nano-crystalline alumina particles. After these times, the sintering rate decreases with time showing a sort of saturation near 120 minutes, i.e. the highest time used in this work [3].

TIME (nm)

Figure 3.6 : Densification behaviour of nano particles of alumina at 1400°C and 1500°C.

The diametral fracture strength of the samples sintered at 1500°C is 390 MPa, whereas that for 1400°C is only 210 MPa, indicating a reduction of 46.2%. This is where the ‘quality improvement aspect’ of nano-crystalline particles is clearly observed with respect to microwave sintering, i. e. a den- sity reduction of 9.2% corresponds to a reduction of fracture strength of 46.2%. This is ascribed to the formation of dense microstructure with increasing density from 90 to 99% by increasing the sintering temperature from 1400 to 1500°C [3].