Sinterability of cordierite powders synthesized by different sol-gel methods

Abstract

Dense cordierite-based ceramic is a promising material for technological applications due to its low thermal expansion and dielectric constant, and high chemical and thermal stability. Cordierite ceramics can be prepared by conventional methods, such as solid state reaction technique and crystallization out of glasses. However, it is difficult to sinter cordierite without sintering aids because of the narrow sintering range just before its incongruent melting point. These aids, on the other hand, increase the thermal expansion coefficient and dielectric constant. Some attempts have been made to improve the sinterability of cordierite. By wet chemical routes, cordierite powders with favorable sintering behavior can be synthesized. It is well known that the sol-gel method has the advantage of an excellent control of chemical composition and the possibility of reducing the sintering temperature. In this dissertation, the sinterability of cordierite powders obtained by different sol-gel methods was studied. The cordierite powders were synthesized by colloidal and alkoxide sol-gel methods and starting from silicic acid and magnesium and aluminum salts. Phase transformation and shrinkage during heating of gel obtained starting from silicic acid and the kinetic parameters of cordierite crystallization from the gel, were also investigated in this dissertation. Cordierite powders for the sintering investigation were obtained by the gels calcination at a temperature where α-cordierite is formed or at a temperature where densification without crystallization of the silica-containing component occurred. The influence of synthesis method, temperature of gel calcination, duration of calcined gel grinding and temperature of sintering on microstructure and mechanical properties of sintered materials was analyzed. In order to improve sintering behavior of the cordierite powders, spark-plasma sintering was applied and results were compared to those obtained by conventional sintering. Starting from silicic acid and magnesium and aluminum salts, a homogeneous monophasic cordierite gel was obtained, which was proved by μ-cordierite crystallization at about 900 °C...

Gusta kordijeritna keramika ima veliki značaj u nauci o materijalima i široku primenu u industriji zahvaljujući malom koeficijentu toplotnog širenja kordijerita (2MgO⋅2Al2O3⋅5SiO2), maloj dielektričnoj konstanti, velikoj specifičnoj otpornosti i relativno velikoj vatrostalnosti. Kordijeritna keramika se može dobiti konvencionalnim postupcima, ali je veoma teško sinterovanjem dobiti gust materijal, zbog veoma uskog intervala sinterovanja kordijerita u blizini inkongruentne tačke topljenja. Pokušaji da se problem uskog intervala sinterovanja reši aditivima omogućili su snižavanje temperature nastajanja kordijerita i povećanje gustine gotovog proizvoda, ali su doveli do pogoršanja željenih osobina, odnosno do povećanja koeficijenta toplotnog širenja i dielektrične konstante. Zbog toga su poslednjih godina istraživanja u cilju dobijanja guste kordijeritne keramike usmerena na sintezu prahova kordijerita poboljšane sinterabilnosti. Različitim hemijskim postupcima sinteze mogu se dobiti fini prahovi velike specifične površine, velike unutrašnje energije, a time i velike sinterabilnosti. Sol-gel postupci imaju veliku primenu u sintezi prahova zahvaljujući mogućnosti precizne kontrole hemijskog sastava prahova i sniženja temperature sinterovanja. Predmet ove doktorske disertacije je ispitivanje sinterabilnosti kordijeritnih prahova koji su sintetizovani alkoksidnim i koloidnim sol-gel postupkom, kao i sol-gel postupkom u kome je kao polazni reaktant korišćena silicijumova kiselina, dobijena iz vodenog stakla, kao jeftinog prekursora. Fazne transformacije i skupljanje tokom termičkog tretmana gela dobijenog postupkom sa silicijumovom kiselinom, kao i kinetika tih faznih transformacija su takođe proučene u ovoj doktorskoj disertaciji. Kordijeritni prahovi za ispitivanje sinterabilnosti su dobijeni kalcinacijom suvih gelova na temperaturi pri kojoj dolazi do viskoznog sinterovanja bez kristalizacije komponente koja sadrži SiO2 i na temperaturi na kojoj nastaje α-kordijerit. Pri proučavanju sinterabilnosti kordijeritnih prahova ispitan je uticaj: vrste sinteze gela, temperature kalcinacije gelova, dužine trajanja mlevenja kalcinisanih gelova i temperature sinterovanja na mikrostrukturne i mehaničke karakteristike dobijenih materijala...