Study of the mechanism of andalusite-Al°-N2 reaction using the combination of DTA-TG-DTG techniques

Estudo do mecanismo da reação andalusita-Al°-N2 usando a combinação das técnicas ATD-ATG-TGD

Study of the mechanism of andalusite-Al°-N₂ reaction using the combination of DTA-TG-DTG techniques

(Estudo do mecanismo da reação andalusita-Al°-N₂ usando a combinação das técnicas ATD-ATG-TGD)

A. D. Mazzoni, E. F. Aglietti

CETMIC (Centro de Recursos Minerales y Cerámica),

Camino Centenario y 506. C.C. 49 (1897) M.B.Gonnet. Buenos Aires. Argentina

tel.: [phone], fax: [phone], e-mail: [e-mail]

Abstract

Alumina based ceramics have important engineering applications due to their good physicochemical properties but their performance may be enhanced by addition of a second phase. Aluminosilicates as andalusite submitted to aluminium reduction and the simultaneous nitriding (N₂ atmosphere) leads to the obtention of alumina and phases of the Si-Al-O-N system. These phases are of great interest for structural ceramics. Mixes prepared with andalusite and Al° (different content) were treated with N₂ up to 1550 °C., using a heating rate of 5 °/min. Thermal effects (DTA) and weight changes (TG, DTG) of nitriding reactions during heating were evaluated. The reaction mechanism was determined using DTA-TG-DTG techniques. Results indicate that Si° and AlN are important intermediate phases during reactions. The final products were a-Al₂O₃ with sialons, mainly b’ phase and polytype sialons. The final formed phases depend on the Al° content and can be predicted using the Si-Al-O-N phase diagram.

Keywords: andalusite, nitriding mechanism, Si-Al-O-N.

Resumo

Cerâmicas à base de alumina apresentam importantes aplicações em engenharia por causa de suas boas propriedades físico-químicas; entretanto, seu desempenho pode ser melhorado pela adição de uma segunda fase. Aluminosilicatos tais como andalusita submetida a redução do alumínio e simultânea nitretação ( atmosfera de N₂) leva a obtenção de alumina e fases do sistema Si-Al-O-N. Essas fases são de graande interesse em cerâmica estrutural. Misturas preparadas com andalusita e Al° (teor diferente) foram tratadas com N₂ até 1550° C, com taxa de aquecimento de 5° /min. Os efeitos térmicos (ATD) e variações de massa (TG, DTG) das reações de nitretação durante aquecimento foram determinados. O mecanismo da reação foi determinado usando as técnicas ATD-ATG-DTG. Os resultados mostram que Si° e AlN são fases intermediárias importantes durante as reações. Os produtos finais foram a-Al₂O₃ com sialons, principalmente a fase b’ e sialons politipos. As fases finais formadas dependem do teor de Al° e podem ser previstas usando o diagrama de fases do Si-Al-O-N.

Palavras-chave: andalusita, mecanismo de nitretação, Si-Al-O-N.

INTRODUCTION

The advanced ceramics have important technological applications due to their good mechanical and chemical properties specially at high temperatures [1, 2]. Alumina and sialons are some of these ceramics.

Alumina-sialon composites present dielectric and mechanical properties superior than those of the pure alumina [3]. These composites contain mainly b’-sialon phases. b’-sialons are the most widely known sialons (phases of the Si-Al-O-N system). b’-sialons have a crystalline hexagonal structure derived from b-Si₃N₄ corresponding to the formula with 0<Z£ 4.2 [1]. Other sialons also found in this type of system are the polytype sialons near the AlN composition with the general formula (Si, Al)_m(O,N)_m+1 with m values of 4, 5, 6, 7, 9 or 11 [1, 2].

An interesting method to achieve alumina-sialon composites is the aluminosilicate-Al° reaction in N₂ atmosphere. This type of reaction is in agreement with a general tendency of using natural, abundant and cheap raw material to obtain advanced ceramics (oxyhydrogen as well as nitride).

In this work the "andalusite-Al-N₂" and "mullite-Al° -N₂" reactions during heating of aluminosilicate-Al° mixtures at the sintering temperature are studied. The study is performed taking into account the thermal effects (DTA) and the weight variations (TG, DTG) together with the determination of crystalline phases by X-ray diffraction.

EXPERIMENTAL

The raw materials used were andalusite from South Africa and synthetic mullite FOSECO (2µ mean particle size). The chemical analysis of aluminosilicates is shown in Table I.

The powdered Al° (d₅₀<20µm) had a purity of 99.9%, and the N₂ used had O₂ and H₂O content lower than 5 ppm.

The mixtures were prepared by wet mixing of the calculated amounts of mineral and aluminum (Table II), then they were dried, pressed at 39 MPa and milled for the test.

Experiments were carried out in a NETZSCH STA 409 equipment performing simultaneously thermal (DTA) and gravimetric analyses (TG). The differential thermogravimetry (DTG) was calculated according to the TG.

Samples were placed in alumina sample holders with a-Al₂O₃ as reference and a heating rate of 5 ° C/min. This heating was performed in N₂ atmosphere.

Crystalline phases of the different stages were followed using an X-ray diffraction equipment Philips model X’PERT. Thermodynamic data were obtained from JANAF Tables [4].

RESULTS

The global reaction between the aluminosilicate and Al° in N₂ atmosphere at high temperatures (higher than 1300 ° C) is of the type:

2SiO₂.Al₂O₃ + 2X Al⁰ + X N₂ -----> [Si₂ Al_{2+ 2X} O₇ N_2X] (A)

where [Si₂ Al_{2+ 2X} O₇ N_2X] is the mixture of a-Al₂O₃ and sialons.

Some intermediate stages occur during heating before reaching the sintering temperature (1400-1600 ° C) due to the high reactivity of aluminum. These reactions produce thermal effects and weight variation.

Figs. 1 to 3 summarize results of andalusite-Al-N₂ reactions and Figs. 4 and 5 show results of mullite-Al-N₂ reactions.

Mullite does not present neither thermal effects nor weight variations when it is heated, and andalusite has a weight loss of 0.8% (impurities) and a peak of thermal decomposition at high temperatures which is not detected in the conditions of our test. Thus, all peaks of figures are attributed to reactions of aluminothermy.

The first peak detected is an endothermic peak without weight change corresponding to Al⁰ melting. Aluminium melting occurs at 660 ° C.

Al⁰_(S) -----> Al⁰_(l)DH = 10.71 KJ.mol^-1 (B)

This peak shows the "S" effect due to thermal conductivity changes that occur with the change of the present phases [5].

The following peak is an endothermic band around 830 ° C accompanied by weight gain. The XRD analysis shows the AlN, Si° and alumina formation with Al° disappearance. This corresponds to the equations:

Al⁰_(l) + ½ N₂ -----> AlN_(S)DH_1100K = - 329.3 KJ mol^-1 (C)

3(SiO₂.Al₂O₃)+4 Al° ----->3 Si⁰ + 5 Al₂O₃

DH_1100K= - 637.02KJmol^-1 (D)

3(2SiO₂.3Al₂O₃) + 8 Al° -----> 6Si⁰ +13 Al₂O₃

DH_1100K= -1388.4 Kjmol^-1 (E)

Reactions (C) and (D) in andalusite and (C) and (E) in mullite share the Al° consumption which is not detected at higher temperatures. Reactions (C) to (E) are strongly exothermic.

DTA curves for andalusite around 950 ° C and the ones of mullite at ~1010 ° C present a non-pronounced exothermic peak. The peaks cannot be attributed to a specific reaction since crystalline phases observed before and after this thermal effect were similar. The TG record in these zones shows a small weight gain, but mainly a change is observed in the weight gain slope since Si° begins its nitriding.

At higher temperatures the andalusite begins its thermal decomposition.

3(SiO₂.Al₂O₃) ------> SiO₂ + 2SiO₂. 3 Al₂O₂

DH_1800K = 42.6 KJ mol^-1 Mullite(F)

but andalusite remains without decomposition even at temperatures higher than 1550 ° C.

The final stage of the reactions in these two systems (andalusite-Al-N₂ and mullite-Al-N₂) involves the Si° nitriding with formation of sialons as nitrogenous phases.

The final reaction is of the type:

(3 -Z)Si⁰ + 3(SiO₂.Al₂O₃) + (Z +12)/3 AlN + (2-4/6 Z)N₂-->

--->Si_6-Z Al_ZO_ZN_8-Z + (5- Z/3) Al₂O₃ (G)

Thus, a mixture of a-Al₂O₃, b’-sialon and mullite in excess is obtained when reaching the sintering temperature for mullite-Al-N₂. For andalusite-Al-N₂ system there is a remnant amount of andalusite after reaction. When reaching the sintering temperatures, the N₂ will be completely incorporated to the sample since there is neither Si° nor Al° .

The present phases in that moment are not in equilibrium since they are different to the ones expected according to the Si-Al-O-N diagram (Fig. 6) and to the global composition.

During heating at constant temperature these phases react among them tending to equilibrium. Thus, mullite and andalusite disappear, appearing phases such as polytype sialons [6], [7]. If any weight variation is registered during this period of time, weight loss is observed due to SiO(g) removal by reactions of the type:

A[b’-Sialon₁] + B[2SiO₂.3Al₂O₃] ---> C SiO_(g) + D N₂ + A⁰[ b’-Sialon₂] (H)

A[b’-Sialon₁] +B Al₂O₃ -->C SiO_(g) + DN₂ + A⁰[ b’-Sialon₂]+ E Sialon polytypes (I )

where A, A’, B, C, D, E are stoichiometric coefficients and b’-sialon₂ is aluminum richer than b’-sialon₁. This produces an AlN enrichment in the global composition of the products with an increase of the polytype sialon content. The SiO(g) loss may be minimized if the sintering atmosphere is controlled [5-7].

CONCLUSIONS

1. The aluminothermy reactions of aluminosilicates in N

   ₂ atmosphere are adequate to obtain alumina-sialon composites.

2. Most of intermediate reactions occur during heating at the reaction-sintering temperature.

3. The mineral/aluminum ratio affects the reaction products formed.

4. The main intermediate phases found are AlN, Si° and transition alumina.

5. The DTA-TG-DTG techniques are adequate to study this type of reactions.

6. Nitrogen is incorporated during the heating stage.

7. The loss of SiO(g) must be taken into account.

ACKNOWLEDGEMENTS

We thank Lic. S. Conconi for the thermal analysis experiments and Sra. N. Prieto for the complete typing.

REFERENCES

[1] K. H. Jack, J. Mater. Sci. 11 (1976) 1135-58 .

[2] C. C.Sorrell, J. Aust. Ceram. Soc. 19 (1983) 48-66.

[3] A. D. Mazzoni and E.F.Aglietti, Mater. Chem. and Phys. 48 (1997) 41-47.

[4] JANAF,Thermochemical Tables.Edit.1970 and 1985.

[5] A. D. Mazzoni and E.F.Aglietti, Thermochimica Acta 298 (1997) 123-128 .

[6] A. D.Mazzoni, Mater.Chem. and Phys. 43 (1996) 38-44.

[7] A. D. Mazzoni and E.F. Aglietti, Anales Asoc. Qca Argentina.83,6 (1995) 395-399 .

(Rec. 14/04/98, Ac./ 09/02/99)

Publication Dates

• Publication in this collection
  11 May 2000
• Date of issue
  June 1999

History

• Accepted
  09 Feb 1999
• Received
  14 Apr 1998