Solar sintering of cordierite-based ceramics at low temperatures
Introduction
In recent series of works using solar furnace (SF) at PSA (Plataforma Solar de Almería) in southern Spain (Amaral et al., 2000; Cruz Fernandes et al., 1998, Cruz Fernandes et al., 1999, Cruz Fernandes et al., 2002, Guerra Rosa et al., 1999, Martínez et al., 1999; Rodríguez et al., 2001; Shohoji et al., 1999, Shohoji et al., 2000), we demonstrated that concentrated solar radiation can be used as a sustainable ecological heat source for production of carbides (in Ar gas environment) and carbonitrides (in N2 gas environment) of transition metals from compacted powder mixtures consisting of the metal and carbon (either graphite or amorphous carbon). By the standard 30 min reaction period in Ar gas environment with the maximum heating temperature 1600 °C under solar irradiation, Si and d-group transition metals excluding Hf were all fully converted to carbide with high degree of crystallinity. Some combinations of d-transition metals and carbon yielded results not readily understandable with reference to available equilibrium phase relationship data suggesting that some photochemical effect besides a thermal effect might be involved in the reaction taking place under certain circumstances of solar heating (Rodríguez et al., 2001).
According to our solar-sintering experiments for oxide ceramics (Cruz Fernandes et al., 2000) and for non-oxide ceramics (Guerra Rosa et al., 2002), the mechanical properties of the ceramic sintered body prepared under solar irradiation were almost comparable to those of the counterpart prepared through conventional industrial sintering processes. Thus, we felt it desirable to further pursue experimental verifications for applicability of SF-sintering for a variety of types of ceramic materials because the quick, cheap, ecological and sustainable nature of solar-sintering would raise productivity of ceramic components with acceptable quality for certain limited applications at reasonable price with minimized environmental load.
In the present study, two commercial ceramic powder mixtures possessing compositions leading to the formation of cordierite (2MgO · 2Al2O3 · 5SiO2) phase by heating were sintered at about 950 °C (measured temperature using a type B thermocouple; see Section 3.2.2 for details of the temperature measurement and setting in the present work) in a SF at PSA and the mechanical properties of the sintered disks were evaluated. Phases emerged during the sintering were identified by XRD (X-ray diffraction) analysis. For the sake of comparison, the same two powder mixtures were also sintered at 950 °C in a conventional electric furnace (EF) and subjected to the similar characterization.
There were two main reasons for undertaking the present solar-sintering experiments at such relatively low temperature as 950 °C:
- (1)
consolidating cordierite at relatively low temperature (<1000 °C) in order to satisfy the industrial demand from the semiconductor sector to fabricate cordierite at temperature lower than the melting points of Cu, Ag or Au;
- (2)
avoiding undesired melting of the starting materials during heating prior to formation of cordierite ceramics due to inherent difficulty of knowing true temperature in the SF at PSA as explained later in Section 3.2.2.1.
Section snippets
Consolidated cordierite-based ceramics
Consolidated cordierite ceramic components appear to have great industrial significance in either dense form (Sumi et al., 1998, Sumi et al., 1999) or porous form (Izuhara et al., 2000). Dense sintered bodies of cordierite are used as catalyst carrier for automotive exhaust in form of honeycomb monoliths (Sumi et al., 1998) and also as heat sinks and semiconductor packaging substrates (Sumi et al., 1999) owing to their low dielectric constant (∼5 at 1 MHz) and thermal expansion coefficient ((1–2)
Materials
As mentioned above, the starting materials were two types of commercial powder mixtures referred to as BL7 and RP7. These two powder mixtures were supplied by Rauschert Portuguesa Lda. When sintered in EF, these would lead to the formation of cordierite by heating to 1300 °C.
The raw powders were uniaxially pressed at 45 MPa to fabricate test disks (30 mm in diameter × 3.5 mm in thickness). After that, some of the test disks were sintered at about 950 °C for 20 min (dwell time) in air using a SF at PSA
Mechanical behavior
The mechanical properties determined for the solar-sintered specimens are summarized in Table 2(Panel A). It is noticed that the Young’s modulus E of the solar-sintered BL7 specimens was by about two times greater than that of the solar-sintered RP7 whereas the shear modulus G of the BL7 disk specimen was twice as large as that of the RP7. Anyway, the mean values of MOR of these two materials were found to be comparable to each other.
As mentioned earlier in the text, sintering experiments for
Concluding remarks
By the SF-sintering of the BL7 and RP7 powder mixtures up to the measured temperature 800 °C using the B type thermocouple held in alumina sheath (that is, the true sample temperature no higher than 950 °C taking into account about 150 K margin of deviation between the two thermocouple configurations; Fig. 2), the cordierite phase failed to form. On the other hand, formation of trace indialite (polymorphic form of cordierite) was detected by XRD in the EF-heated RP7 disk specimen to the comparable
Acknowledgments
The authors thank the valuable technical assistance given by Ms. S. Dias, Mrs. T. Magalhães (XRD), and Mrs. P. Coelho (SEM) at INETI. Mr. D. Dias of Rauschert Portuguesa Lda. is acknowledged for provision of the raw materials used in this study. The authors are also grateful to Dr. J. Rodríguez and Dr. D. Martínez for their assistance in undertaking the solar-sintering experiments at PSA.
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