Effect of frequency on MW assisted sintering: 2.45 GHz versus 5.8 GHz

1. Introduction

In the field of biomaterials, the design and development of innovative technologies has been motivated by the increasing demand for materials capable of supporting new specifications and the need to reduce process times and costs. Zirconia-based composites are commonly used for developing metal-free restorations and dental implants due to their superb mechanical properties, biocompatibility and aesthetics [3,4,14].

In this work, tetragonal zirconia polycrystalline (TZP) is stabilized with cerium oxide (CeO₂), a less known dopant, since the most commonly used for the stabilization of zirconia is yttrium oxide. With the use of CeO₂ as a stabilizer, it is possible to improve fracture toughness and reduce the low hydrothermal degradation (LTD) of the TZP sample [11]. Low hydrothermal degradation is considered a major problem for biomaterials, especially zirconia is prone to ageing in presence of water [5].

In addition to choosing the appropriate material, another way to improve its properties is to modify the sintering mechanisms. The adoption of non-conventional sintering methods, such as microwave technique, which has certain advantages over conventional sintering [7].

Microwave processing eliminates the need to spend energy to heat the walls of furnace or reactors, their massive components and heat carriers. Hence, the use of microwave heating significantly reduces energy consumption, especially in high temperature processes, as heat losses increase considerably as processing temperatures rise. The heating processes involved in conventional and microwave sintering are completely different, as in conventional sintering the heat flows from the surface of the material to its interior. In microwave sintering, heat is generated within the core of the material, which flows from the inside to the surface; this is known as volumetric heating [1,6,12]. One of the most important parameters within microwave sintering are the dielectric properties of the material, since the amount of absorption of the sample is influenced by them. However, the dielectric properties are not constant and vary according to the frequency used [10].

In short, the main objective of this study is to obtain highly densified samples of 10 mol% CeO₂ doped TZP/Al₂O₃ (10Ce-TZP/Al₂O₃) by microwave sintering at two different frequencies: 2.45 and 5.8 GHz.

2. Materials and methods

2.2. Microwave cavities

Microwave sintering (MW) was performed in two microwave systems operating at two different magnetron frequencies, 2.45 and 5.8 GHz, respectively. The first one is a single-mode cylindrical cavity operating in the TE₁₁₁ mode with a resonant frequency of 2.45 GHz (Fig. 1a). In this case, the temperature has been measured with an optical pyrometer, previously calibrated in this temperature range. The dimensions of the circular cavity are 100 mm in diameter and 80 mm in height. The cavity has one 30-mm-diameter hole on the top that allows the access for a quartz tube containing the specimen (radius = 10 mm, height = 3 mm) and a 12-mm-diameter hole in lateral wall for a temperature sensor. The dimension and position of these holes were designed to ensure that there was no microwave leakage from the cavity and there was negligible perturbation of the resonant mode. The E field vectors are perpendicular to the cavity axis with the maximum electric field magnitude at the center, where the sample is located. A moving short circuit at the bottom of the cavity allows to track the cavity heating mode resonant variations caused by changes in the dielectric properties of the heated test sample during the sintering process [2].

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Fig. 1.

Microwave systems at (a) 2.45 GHz and (b) 5.8 GHz.

The second microwave cavity is a 5.8 GHz single-mode applicator based on the rectangular waveguide geometry (WR159) short circuited by a moving plunger (Fig. 1b). Microwave furnace was equipped with directional couplers, which allowed for the measurement in real time of the emitted and reflected power and hence, by difference of the power dissipated into the load. The temperature was monitored during the microwave processing using a sapphire fiber that directly touched the free upper surface of the sample and connected to a signal conditioner (MIKRON M680 Infraducer, Mikron Infrared Inc., CA, USA). A specific temperature detection procedure was applied to reach correct monitoring of the sintering cycle, optimized in the Microwave Application Group at University of Modena and Reggio Emilia, Italy [15].

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Fig. 2.

Hardness values of 10Ce-TZP/Al₂O₃ composites sintered at different temperatures by conventional and microwave as a function of frequency.

3. Results and discussion

All relative densities values of MW and CS sintered samples are above 98%, as can be seen in Table 1. For the estimation of density, 5.06 g/cm³ has been taken as theoretical density for the studied composite 10Ce-TZP/Al₂O₃. From these values, it can be decuded that the relative densities increase steadily from 98.5 to 99.5% with the rise of sintering temperature.

The sintered composites at 2.45 GHz show a slightly lower density than those sintered at 5.8 GHz. An increase in microwave frequency can lead to an improvement in the microwave absorption by the ceramic material, therefore more energy is transmitted to the material. Comparing both sintering technologies, microwave and conventional sintering, the density of microwave-sintered materials is higher (from 98.7% at 1200 °C-10 min with 2.45 GHz to 99.5% at 1300 °C-10 min with 5.8 GHz) even if they were processed at lower temperature and sintering times than the conventionally densified material. Therefore, a remarkable result is that microwave sintering allows to obtain highly sintered samples at considerably lower temperature and with less processing time than conventional sintering method.

As far as the mechanical properties are concerned, the results are consistent with the calculate relative density values. The hardness values obtained as a function of heating mode and frequency are shown in Fig. 2.

By microwave sintering at 2.45 GHz, the hardness of the composites at 1200 and 1300 °C is slightly lower (∼13 GPa) than the values obtained at 5.8 GHz frequency (14.2 and 14.6 GPa for 1200 and 1300 °C, respectively). The latter is closed to the maximum hardness value obtained in this work (15.1 GPa) which belongs to the sample prepared by conventional sintering at 1500 °C. Then, it is observed that MW-sintered samples show Hv values comparable to conventionally sintered samples, even though they have been sintered for shorter times at a much lower temperature.

Another mechanical property, which plays a very important role in bioceramics, is fracture toughness, K _IC . In Fig. 3, it can be seen the K _IC values for 10Ce-TZP/Al₂O₃ sintered at different temperatures by conventional and microwave depending on the frequency.

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Fig. 3.

Fracture toughness values of 10Ce-TZP/Al₂O₃ composites sintered at different temperatures by conventional and microwave as a function of frequency.

In the case of 10Ce-TZP/Al₂O₃ composite, the addition of ceria as a stabilizer induces a considerable increase in its fracture toughness compared to yttria doped zirconia (Y-TZP) materials [8,13], resulting in a much more fracture resistant material, which is of vital importance when it is used for implant and prosthesis.

In this study, sintering behavior of 10Ce-TZP/Al₂O₃ composite is investigated through conventional and microwave sintering applying two different frequencies: 2.45 GHz and 5.8 GHz. To the best knowledge, no studies have been found in the literature to investigate and compare behavior of sintering in ceramic materials at high temperature is investigated and compared by using these frequencies.

The most notable result is that material sintered by MW has resulted in H and K _IC high values at least comparable to those obtained by conventional furnace. In addition, our composites present values within the admissible range for applications such as implants and prosthesis.

Overall, microwave sintering proved to be an exceptional alternative for sintering 10Ce-TZP/Al₂O₃ composite, due to the good mechanical properties and fine microstructure of resulting materials. Moreover, this technology requires lower sintering temperatures and dwell time than conventional sintering, being able to reduce processing times and energy consumption and, consequently, microwave technique has a lower environmental impact.