WO2008077224A2

WO2008077224A2 - Compositions for external susceptors and external susceptors for the sintering of ceramics by microwaves

Info

Publication number: WO2008077224A2
Application number: PCT/BR2007/000361
Authority: WO
Inventors: Ruth Herta Goldschmidt Kiminami; Romualdo Rodrigues Menezes; Pollyane Márcia de SOUTO
Original assignee: Fundação Universidade Federal De São Carlos - Ufscar
Priority date: 2006-12-27
Filing date: 2007-12-27
Publication date: 2008-07-03
Also published as: WO2008077224A3; BRPI0605383A

Abstract

Compositions and external susceptores used as auxiliary heating agents for sintering ceramics by microwaves at the frequencies of 2.45 GHz and 912 MHz, with said susceptors being based on mixtures of calcium aluminates and/or calcium aluminates and phases of SiC and/or ZrO2 and/or AlN, in both cases in specific proportions, being used after the curing of the cementing material or after curing and firing at determined temperatures and times and presenting the main characteristics of high heating capacity when exposed to microwaves, high structural stability at high temperatures and resistance to degradation of the heating properties when exposed to microwaves even subsequent to several heating cycles. The use of said susceptors allows obtaining ceramics with densities exceeding 98% of the theoretical density of the material in firing cycles of less than 30 minutes.

Description

COMPOSITIONS FOR EXTERNAL SUSCEPTORS AND EXTERNAL SUSCEPTORS FOR THE SINTERING OF CERAMICS BY MICROWAVES

FIELD OF THE INVENTION

[001] The present invention relates to external susceptors for the sintering of ceramic materials by the use of microwave energy at frequencies of 2.45 GHz and 912 MHz.

BACKGROUND OF THE INVENTION

[002] Susceptors are auxiliary heating agents for processing ceramics by microwaves. Most ceramics are transparent to microwaves at ambient temperature, which prevents them from efficiently absorbing the microwaves and thus heating when exposed to this type of radiation. At high temperatures however, they do absorb microwave energy efficiently and therefore heat accordingly.

[003] Thus, auxiliary heating systems are used to enable the rapid and efficient sintering of ceramics using microwave energy. These systems heat the ceramic material at low temperatures while at higher temperatures the ceramic material heats through the interaction and absorption of the microwaves.

[004] The use of microwave energy for the sintering of ceramic materials allows lower firing temperatures than those used in conventional processes and/or significantly shorter firing cycles. As an example, the sintering of alumina has been successfully achieved in cycles of 30 minutes instead of the 380 minutes more commonly required when processing using conventional heating methods and this has direct implications in the reduction of processing energy costs and increased productivity.

[005] The possibility of sintering within shorter time spans is associated to the characteristics inherent to heating by microwave, which is volumetric and is caused by the interaction of the atoms and molecules of the material with the waves emitted by the microwave oven.

[006] The advantages of processing with microwaves are difficult to obtain if the material to be processed does not efficiently absorb microwave energy at low temperatures, which makes it necessary to use auxiliary heating systems in order to achieve successful processing.

[007] The auxiliary heating systems used for the processing of ceramics by microwaves may be of three different types: conventional heating systems, e.g. resistances, gas burners, etc.: internal susceptors, which are materials that efficiently absorb microwave energy at ambient temperature and are incorporated within the material to be processed, such as in the case of composites whereby one phase does not absorb microwave energy while another does; and, external susceptors, which consist the use of susceptors external to the material, similar to resistances .

[008] At low temperatures the susceptors absorb microwave energy and transmit the heat to the ceramic material which consequently also heats and at the higher temperatures will efficiently interact with the microwaves and rapidly heat up. External susceptors also have the purpose of minimising the energy gradients within the ceramic body by serving as an external heat source and this provides thermal homogeneity to the material being processed even in ultra- fast firing.

[009] In this sense, the susceptors are vital for the homogeneous sintering of ceramic materials that absorb microwave radiation in a highly intense manner because they allow thermal uniformity within the sample during sintering. The fast firing of these types of ceramics in microwaves is jeopardized without the use of susceptors to reduce the major temperature gradients that may form in these materials (gradients going from inside outwards) .

[010] The use of internal susceptors as auxiliary heating systems does nevertheless have some inherent limitations since it is not always possible to use materials (or inclusions) incorporated to the ceramics to be processed as this may result in contamination or alter the densification behaviour or due to heating efficiency limitations. [Oil] The use of conventional heating systems as auxiliary heating agents presents certain difficulties arising from the control of the heating cycle. On the other hand, the use of external susceptors is more widespread with SiC being the material most commonly used.

[012] However, the use of SiC poses certain difficulties, such as : extremely high reflection coefficient and very low reflectiion depth at high temperatures which limits its heating rates at high temperatures; difficulty in preparing the susceptor since the costs and work to sinter a SiC refractory and then cut it finely or have it already made extremely thin are both very expensive; and difficulty in optimising its placement within the sintering arrangement.

[013] Different from state-of-the-art susceptors, use of the system comprising calcium aluminates, or a matrix of calcium aluminates and additions of SiC and/or Zrθ₂ and/or AIN such as that proposed in the present invention affords great ease of preparation of the susceptors, regardless of shape or thickness, due to their cementing characteristics, which exceed the preparatory limitations of SiC susceptors. Furthermore, they possess heating rates at high temperatures superior to SiC susceptors due to their reduced reflection coefficients .

[014] These characteristics allow the susceptors object of the present invention to be applied not only to the heating of ceramic materials, but also as auxiliary heating agents at high temperatures by rendering thermal gradients less abrupt even in the case of ultra-fast firing, which is not done efficiently by SiC at high temperatures .

[015] Patents U.S. 4.147.911 and 4.219.361 describe the use of internal susceptors for the sintering of ceramic materials. They were limited to the incorporation of materials having high dielectric losses (that greatly absorb microwaves) to ceramics with low dielectric losses (transparent to microwaves) at ambient temperature. As example, this includes the addition of 0.1 to 10%, by weight, of aluminium, silicon carbide, silicon, magnesium, siliceous-iron alloys, beta alumina and chrome oxide in refractory oxide matrixes such as alumina, mullite, zirconia and magnesia.

[016] Other examples provided are the addition of oxides such as alumina, silica and magnesia having high dielectric losses in low loss ceramic matrixes, such as Fe₃U₄, MnO₄, NiO, Ni2U3, in ratios of 1 to 90%, by weight. However, there is no known commercial use for these studies due to their technical limitations and reduced applicable flexibility.

[017] As previously mentioned, the absolute majority of research intending the use of external susceptors as auxiliary heating agents are limited to SiC. The use of other materials as external susceptor agents is the subject of research dating to the nineties, with the use of some materials having being approved, but with the vast majority having limitations either due to the temperature of use such as CuO and Mnθ₂, or cost, such as LaCrC>₃ or even in the heating capability (limitations in the heating rates at certain temperature ranges) and in the manner of preparing the actual susceptor product itself.

[018] The susceptors of the present invention, constituted of calcium aluminates, or a matrix of calcium aluminates and additions of SiC and/or ZrO₂ and/or AIN, do not possess the limitations of the new materials under study that are intended to be used as external susceptors because to their high fusion temperatures of over 1600° C, low production costs and the possibility of being prepared in various forms and sizes.

[019] Research is being directed to the search for materials to be used as external susceptors that are at once easy to handle, flexible enough to sinter several types of ceramics, present structural resistance at high temperatures, resistance to degradation following various heating cycles and thermal shock as well as high rates of heating without, however, competing with the sample for the microwave energy at high temperatures. Or, in other words, that possess low capacitance and that their absorbance of microwaves does not markedly increase at high temperature.

[020] Due to their dielectric and physical characteristics, the susceptors developed herein readily fulfil present market requirements, with the difference that they offer low production costs while at the same time providing a flexibility of application that allows them to be used not only for sintering ceramics, but also in a series of processes that use microwave energy, such as sintering metals and metal-ceramic composites, synthesis of ceramics and the vitrification and destruction of residues, amongst others .

SUMMARY OF THE INVENTION

[021] In a broad manner, the present invention describes compositions for external susceptores that comprise the combination of post-precursors with CaO rates varying between 40 and 5% and AI₂O₃ rates varying between 60 and 95% in mass, water to form a homogenous dispersion with cementing characteristics adequate for the process of casting in molds, curing and alternatively firing at temperatures between 1200° C and 1600° C during 15 minutes to 4 hours and the susceptors obtained from these for sintering ceramics by microwaves at the frequencies of 2.45 GHz and 912 MHz.

[022] The susceptors present a high heating capability, and their compositions comprise calcium aluminates or calcium aluminates mixed with other constituents, namely, SiC and/or Zrθ2 and/or AIN.

[023] Therefore, the present invention provides compositions of calcium aluminates or mixtures of calcium aluminates and SiC and/or ZrC>₂ and/or AIN for the manufacture of external susceptors for sintering ceramics at the frequencies of 2.45 GHz and 912 MHz.

[024] The present invention also provides external susceptors for sintering ceramics at the frequencies of 2.45 GHz and 912 MHz.

[025] The present invention further provides external susceptors for sintering ceramics at the frequencies of 2.45 GHz and 912 MHz from compositions of calcium aluminates or mixtures of calcium aluminates and SiC and/or ZrO₂ and/or AIN having cementing characteristics.

[026] The present invention also provides external susceptors for sintering ceramics at the frequencies of 2.45 GHz and 912 MHz that act as auxiliary heating agents at high temperatures, reducing the thermal gradients even with ultra-fast firing.

BRIEF DESCRIPTION OF THE DRAWINGS

[027] FIGURE 1 attached comprises the illustrative graphs of the derivative heating curves for susceptor P, when exposed to microwaves at a frequency of 2.45 GHz.

[028] FIGURE 2 attached comprises the X-ray diffraction graphs for susceptor P. [029] FIGURE 3 attached presents the derivative heating curves for susceptor C80.

[030] FIGURE 4 attached presents the X-ray diffraction graphs for susceptor C80.

[031] FIGURE 5 attached presents the derivative heating curves for susceptors Z4-80 and Z4-60 at 1400° C, as well as the derivative heating curves for susceptors CZl-60, CZ2-60 and CZ3-60.

[032] FIGURE 6 attached presents the derivative heating curves for susceptors C80, Z4-80 and P after degradation.

[033] FIGURE 7A and 7B attached present the results of the pyro-deformation trials performed on the test bodies of the susceptors of the present invention. FIGURE 7A illustrates susceptors C80, P. Pl, Z4-80, while FIGURE 7B illustrates susceptors CZ3-60, CZ4-60, CA20 and CA60 after the pyro- deformation trials. It can be noted from these illustrations that the susceptors P, Pl and Z4-80 present high pyro-deformation resistance.

[034] FIGURE 8A and 8B attached illustrate susceptors according to the present invention, prepared in various forms and several sizes.

DETAILED DESCRIPTION OF THE INVENTION [035] The compositions for obtaining the susceptors in accordance with the present invention do not only comprise mixtures of calcium aluminates; besides the mixtures of calcium aluminates, the compositions alternatively include other constituents such as SiC and/or ZrC>₂ and/or AIN.

[036] The compositions formulated with calcium aluminates present cementing characteristics and, thus, following the addition of water to the post-precursors to form a homogenous dispersion having cementing characteristics, the susceptors are prepared by casting the dispersion in plastic molds .

[037] Therefore, an aspect of the invention comprises compositions of pure calcium aluminates or in mixtures with SiC and/or ZrO₂ and/or AIN.

[038] Another aspect of the invention comprises the external susceptors obtained from the said compositions.

[039] The susceptors present high heating rates when exposed to microwave energy, which allows them to be used as auxiliary heating agents in the sintering of ceramics by microwaves. The susceptors allow the rapid sintering of ceramics that are normally transparent to microwaves at ambient temperature.

[040] The use of the susceptors object of the present invention allow obtaining ceramics with densities exceeding 98% of the theoretical density in firing cycles of under 30 minutes. The susceptors (such as shown in Figure 8A) were used in the sintering of alumina (in firing cycles of approximately 30 minutes) , of zirconia (in cycles of under 20 minutes) , of porcelains (in cycles of under 30 minutes) and of NiZn ferrites (in cycles of under 40 minutes) . In the sintering processes, pellets of the above materials were placed inside the susceptors (Figures 8A and 8B) and this arrangement was enclosed in thermal insulation matting and placed inside the microwave equipment whereupon it was submitted to heat by microwave radiation.

[041] The raw materials used in the course of developing the technology that led to the present invention include: a) for the susceptors comprised solely of a mixture of calcium aluminates; aluminous cements of high alumina, alumina, calcium hydroxide and calcium carbonate; b) for the susceptors comprised of a mixture of calcium aluminates and one or more constituents; aluminous cements of high alumina, alumina, calcium hydroxide and calcium carbonate, silicon carbide, aluminium nitride and zirconia nitride and/or any sources of Zrθ₂.

[042] The methodology applied for obtaining external susceptors for sintering ceramics by microwaves is based on the mixture of raw materials so as to obtain compositions comprised solely of calcium aluminates with cementing properties, or, of a mixture of calcium aluminates with cementing properties and one or more constituents, with these being SiC and/or ZrO₂ and/or AIN.

[043] The mixture of calcium aluminates present final compositions with CaO rates varying between 40 and 5% and

AI₂O₃ rates varying between 60 and 95%. In the compositions containing one or more other phases besides the mixture of calcium aluminates, the rate of these phases varies between

5 and 45% of the total. Water is added to the compositions obtained and the susceptors are prepared by casting in plastic molds.

[044] After curing by standing without any major ambient control other than protection from drafts comparable to conventional cement and without the use of any additional curing agent, the susceptors are dried and fired at various temperatures, ranging from 1200° C to 1600° C from 15 minutes to 4 hours for firing. The drying occurred at ambient temperature for 24 hours and at 110° C for 24 hours .

[045] The susceptors of the present invention are used as external susceptors in a raw state immediately after the curing of the cementing material and drying of the susceptor.

[046] Alternatively, the external susceptors according to the present invention are used after firing processes in conventional or microwave ovens, at temperatures ranging from 1200° C to 1600° C from 15 minutes to 4 hours, with the densities after firing varying between 60 and 95% of the theoretical density of the material.

[047] The invention shall be illustrated by the following Examples, which, however, are not to be considered as limiting.

[048] The methodology described below relates to that for obtaining the susceptors termed P and Pa, respectively. Table 1 lists the raw materials used.

TABLE 1

[049] To prepare the susceptor, water is added to the aluminous cements, under agitation. This agitation may be manual or mechanical. For the purpose of optimisation, it should be done mechanically and in accordance with the standard practices for cement technology. The granulometric range of the cement shall be selected so as to afford good compaction and fineness, generally below ABNT 200 mesh. When the cements attain the consistency of paste they are poured into plastic molds. Susceptor P presented 20% of CaO and Pa presented 30% of CaO. The susceptors are fired at 1400° C and 1600° C.

[050] For analysis of heating characteristics, the susceptors are submitted to heating at the frequencies of 2.45 GHz and 912 MHz and the temperature is monitored with a thermocouple.

[051] For analysis of degradation, the susceptors are submitted to between 5 and 20 heating cycles at high temperatures between 1500 and 1600° C after which they are again exposed to microwaves in order to assess their heating behaviour. The susceptors are submitted to a pyro- deformation trial at high temperatures between 1500° C and 1600° C.

[052] The methodology described bolow relates to that for obtaining the susceptors termed Pl and P2, respectively. Table 2 lists the raw materials used.

TABLE 2

[053] The preparation of susceptor Pl occurs through the mixture of aluminous cement and calcium hydroxide until obtaining a composition of 25% CaO and 75% AI₂O₃. Water is added to the formulation until it attains the consistency of paste following which it is poured into a plastic mold. Susceptor Pl is obtained after curing and is then fired at 1400° C.

[054] The susceptor Pl is prepared through the mixture of alumina and calcium hydroxide until obtaining a composition of 22% CaO and 78% AI₂O₃. The post-precursors are mixed and homogenised in a ball mill, but are not limited to this.

[055] The formulation is submitted to a fast firing process at high temperature and then rapidly cooled. The material is pulverised and again submitted to a fast firing process. This procedure is repeated until obtaining the intended initial precursor phases. For such, the fast firing temperatures vary from 1000 to 1400° C with firing cycles varying between 30 and 90 minutes (with rapid cooling at around 30 to 45 minutes) . The product is pulverised and passed through an ABNT 200 mesh sieve and an ABNT 325 mesh sieve, thus working with materials 100% finer than 74 μm and materials 100% finer than 45 μm.

[056] Water is then added to the system and the mass is poured into a plastic mold. Susceptor P2 is obtained after curing and is then fired at 1400° C. [057] For analysis of heating characteristics, the susceptors are submitted to heating at the frequency of 2.45 GHz and the temperature is monitored with a thermocouple.

[058] For analysis of degradation and pyro-deformation, the susceptors are submitted to several heating cycles at high temperatures after which they are again exposed to microwaves in order to assess their heating behavior. The susceptors are submitted to a pyro-deformation trial at high temperatures.

[059] The methodology described below relates to that for obtaining the susceptors termed C80, CZ4-60, Z4-80, CZl-60, CZ2-60, CZ3-60 and susceptors CA20 and CA60. Table 3 lists the raw materials used.

TABLE 3

[060] Preparation of Susceptor: The susceptor C80 is prepared from a mixture of aluminous cement and silicon carbide in a ratio of 80:20, by mass. The susceptor Z4-80 is prepared from a mixture of aluminous cement and zirconia in a ratio of 80:20, by mass. The susceptor CZ4-60 is prepared from a mixture of aluminous cement, silicon cardobe and zirconia in a ratio of 60:20:20, by mass. The susceptor CZl-60 is prepared from a mixture of aluminous cement, silicon carbide and monoclinic zirconia in a ratio of 60:20:20. The susceptor CZ2-60 is prepared from a mixture of aluminous cement, silicon carbide and zirconite

(alternative source of zirconia) in a ratio of 60:20:20.

The susceptor CZ3-60 60 is prepared from a mixture of aluminous cement, silicon carbide and zirconia refractory (alternative source of zirconia) . The susceptor CA20 is prepared from a mixture of aluminous cement, silicon carbide and alumina in a ratio of 20:20:60.

[061] After mechanical or manual mixing, water is added to the formulations until they attain the consistency of paste following which they are poured into a plastic mold. The susceptors are obtained after curing.

[062] The susceptor C80 is fired at 1300° C, 1400° C and 1500° C, the susceptor Z4-80 is fired at 1400° C and 1600° C and susceptors CZ4-60, CZl-60, CZ2-60, CZ3-60, CA20 and CA60 at 1400° C. [063] Heating Analysis: The susceptors are submitted to heating at the frequencies of 2.45 GHz and 912 MHz and the temperature is monitored with a thermocouple.

[064] Degradation and Pyro-deformation Analysis: the susceptors are submitted to several heating cycles (between 5 and 20 cycles) at high temperatures between 1500 and 1600° C after which they are again exposed to microwaves in order to assess their heating behavior. The susceptors are submitted to a pyro-deformation trial at high temperatures between 1500° C and 1600° C for 3 hours.

[065] The invention shall be further described by reference to the Figures attached.

[066] Figure 1 presents the derivates of the heating curves of susceptor P, when exposed to microwaves at the frequency of 2.45 GHz. Figure 1 shows that the heating rate of susceptor P increases in proportion to the increase of firing temperatures. This behaviour is linked to the phases that are developed in the system, which is confirmed by the analysis of x-ray diffraction. This heating behaviour is also seen when exposed to the frequency of 912 MHz.

[067] The decrease in the heating rate above 600° C is associated to the insulation system used for the heating trials. The decrease in the heating rate is not thus a characteristic of the susceptor but rather an influence of the heat losses inherent to the system. This is confirmed by simulation. Therefore, the derivative curves for the heating should only be analysed together for comparative purposes and should not be considered as providing "absolute" values since they are obtained at a precise power radiation with a specific insulation system and in a microwave oven having a specific chamber dimension and homogenisation system. This means that under other conditions such as, for example, another oven or another power radiation, the values of the heating rates shall be different because the heating behaviour of the susceptor is dependent on the power, insulation and oven used.

[068] Figure 2 presents the X-ray diffraction curves of susceptor P fired at 1400° C and 1600° C and reveals micro- structural alterations with the changes in the firing temperatures .

[069] Figure 3 presents the derivative heating curves for susceptor C80.

[070] Figure 3 shows that the heating rate of susceptor C80 are influenced by the firing temperatures which is related to its micro-structural evolution. This analysis is confirmed by complementary results from X-ray diffraction.

[071] Figure 4 presents the X-ray diffraction curves of susceptor C80 fired at temperatures of 1300, 1400 and 1500° C. Figure 4 reveals the micro-structural alterations to C80 linked the firing temperatures used when preparing the susceptor .

[072] Figure 5 presents the derivative heating curves for susceptors Z4-80, CZ4-60, CZ1-60, CZ2-60 e CZ3-60. Figure 5 corroborates that the raw material used as source of zirconia has an influence on the heating rates of the susceptors and that the combination of silicon carbide with sources of zirconia also influenced the heating rates. The use of zirconite (CZ2-60) as an alternative source does not lead to better (higher) heating rates, while the use of monoclinic zirconia (CZ1-60) affords the best heating rates up until 1000° C. However, susceptor Z4-80 demonstrates the best heating rates over 1000° C, which proves to be extremely desirable for fast firing cycles of ceramics at high temperatures (above 1250° C, for example) .

[073] Figure 6 presents the derivative heating curves for susceptors C80, Z4-80 and P after degradation. It can be noted that the heating rates of susceptor P at low temperatures are better than those of susceptors Z4-80 and C80 but, however, the heating rates of susceptor Z4-80 above 600° C are superior to those of the other two susceptors .

[074] It is also possible to perceive from Figure β that after degradation the heating rates of susceptor C80 presented a decrease which is associated to the phases formed during the firing cycles at high temperatures . On the other hand, susceptors Z4-80 and P showed an increase in heating capacity after the heating cycles (degradation) also associated to the phases formed during the high temperature heating cycles .

[075] Figure 7A presents some results from the pyro- deformation trials. These trials are performed in "straight" bodies resting on supports and submitted to several long heating cycles (3 hours) attaining 1500° C and, in some cases, 1600° C (P, Pl, Z4-P) . The susceptors that remained "straight" - Figure 7A - were considered resistant for applications at high temperatures, while those that sagged - Figure 7B - were not considered adequate for applications at high temperatures (above 1400° C) . The pyro- deformation trials indicated that susceptors P, Pl and Z4- 80 presented high structural resistance at high temperatures and may be used in applications requiring temperatures of up to 1600° C, while susceptors C80, CZ4-60, CZ3-60, CA20 and CA60 presented limited structural resistance at high temperatures and are only indicated for uses in applications requiring temperatures not exceeding 1200° C.

[076] Overall, the pyro-deformation trials demonstrated that the susceptors without SiC presented structural resistance at high temperatures (evaluated by their pyro- deformation) superior to those susceptors containing SiC. This is noted both in susceptors containing SiC in combination with other phases (Zrθ₂ and/or AlN) as well as susceptors prepared entirely from SiC.

[077] The use of optimised cement compositions, an adequate combination of raw materials and the correct firing temperature (prior firing) provided external susceptors affording high heating performance and resistance to degradation after several heating cycles .

[078] Complementary results showed that the heating performance of susceptors P, Z4-80 and all the others containing additions of zirconia or AlN is extremely high at temperatures above 1200° C.

[079] Furthermore, these susceptors possessed the efficiency and flexibility to be used for sintering various ceramic materials, rendering the rapid and uniform sintering of the following materials possible: alumina (in heating cycles under 30 minutes) , mullite (cycles under 20 minutes), zirconia (cycles under 25 minutes), ferrites (heating rates of up to 100° C per minute, with no micro- structural heterogeneities) , ZnO-CuO varistors (cycles under 15 minutes) and porcelaneous materials in general (cycles under 40 minutes) . All materials present high densities, equal or superior to those obtained by conventional sintering processes conducted in parallel. Table 4 shows some of these results. TABLE 4

[080] With regard the flexibility and ease of preparation of the susceptors, Figures 8A and 8B exhibits susceptors prepared in various shapes and several sizes. It can therefore be seen that it is possible to prepare susceptors either with very thin or very thick wall thicknesses and that it is possible to produce these in the most diverse shapes regardless of complexity, merely through adequate control of the system' s viscosity and preparation of the appropriate mold. It can also be seen that it is possible to prepare susceptors of - practically - any size, which, combined with the possibility of production in complex shapes, allows sintering large parts in a homogenous manner without confronting the difficulties normally encountered when using SiC plates (the most frequently used susceptors are made of silicon carbide) .

[081] Thus, it is possible to conclude that the susceptors developed herein present an excellent performance as aids to sintering ceramic materials by microwave being superior to the performance seen in susceptors constituted solely of silicon carbide, allowing a fast and uniform sintering of a series of ceramics possessing distinct dielectric characteristics, without the need of contrivances or lengthy studies related to enhancing the uniformity of the electromagnetic field distribution inside the chamber of the oven and/or improving the insulation system of the sintering arrangement.

[082] Therefore, in view of the broad sintering flexibility and high heating capacity of the susceptors developed herein and their significantly reduced cost compared to the other susceptors under development based on different SiC materials, it is possible to use the susceptors of the present invention in a series of thermal treatments using microwave energy, such as calcination, annealing, etc. and not merely the sintering process. [083] The present invention fulfils the demands of technology for processing ceramics by microwaves and essentially consists the unprecedented application of materials based on calcium aluminates . The susceptors developed herein may be prepared in a broad variety of forms, in the most diverse thicknesses, in a quick and cheap manner, contrary to the procedures used for making the SiC susceptors that are currently the most widely used. The susceptors developed by the Applicant present important characteristics, such as: high heating rates both at low and high temperatures (above 1400° C) , high structural integrity at high temperatures and great resistance to thermal shock. These susceptors also demonstrate low reflection coefficients and greater in depth penetration at high temperatures when compared to SiC, which indicates they expose the sample better and do not markedly compete for the microwave energy at high temperatures.

Claims

1. A composition for manufacturing external susceptores for sintering ceramics by microwaves at the frequencies of 2.45 GHz and 912 MHz, comprising the combination of post- precursors with CaO rates varying between 40 and 5% and Al₂O₃ rates varying between 60 and 95% by mass, water to form a homogenous dispersion with cementing characteristics adequate for the process of casting in molds, curing and alternatively firing at temperatures between 1200° C and 1600° C during 15 minutes to 4 hours.

2. The composition according to claim 1, wherein, alternatively, the post-precursors are mixed and homogenised mechanically and the resulting composition is submitted to fast firing during 30 to 90 minutes at temperatures between 1000° C and 1400° C and then rapidly cooled during 30 to 45 minutes, pulverised, passed through an ABNT 200 mesh sieve followed by an ABNT 325 mesh sieve and again submitted to a fast firing cycle with this procedure being repeated until obtaining the intended initial precursor phases whereupon water is added to the system and the mass is poured into a plastic mold from which the susceptor is recovered and fired at 1200 to 1600° C.

3. The composition according to claim 1, wherein it further comprises between 5% a 45% by mass of SiC and/or AlN and/or ZrO₂ in the final material.

4. The composition according to claim 1, wherein the ZrO₂ is zirconia, monoclinic zirconia ZrO₂, zirconite ZrO₂SiO₂ or refractory zirconia ZrO₂Al₂Oa.

5. The composition according to claim 1 and 3, wherein the rate of CaO in the post-precursors is of 20% by mass and that the rate of Al₂θ₃ is of 80% by mass.

6. The composition according to claim 1 and 3, wherein the rate of CaO in the post-precursors is of 30% by mass and that the rate of AI₂O₃ is of 70% by mass.

7. An external susceptor capable of absorbing microwave energy for sintering ceramics at the frequencies of 2.45

GHz and 912 MHz, being manufactured from the composition of post-ceramics as defined in claim 1.

8. The external susceptor according to claim 7, wherein it is used as an external susceptor in a raw state, immediately after curing of the cementing material and drying of the susceptor.

9. The external susceptor according to claim 7, wherein it is used as an external susceptor after a firing process in conventional kilns or microwave ovens, at temperatures between 1200° C and 1600° C, during 15 minutes to 4h, with densities after firing varying between 60 and 95% of the theoretical density of the material.

10. The external susceptor according to claim 7, wherein it maintains structural integrity at temperatures of up to 1200° C.

11. The external susceptor according to claim 7, wherein it maintains structural integrity at temperatures over 1400° C.

12. The external susceptor according to claim 7, wherein it maintains structural integrity at temperatures over 1600° C.

13. The external susceptor according to claim I₁ wherein it may be prepared in circular, rectangular or any geometry.