Abstracts

Refractory oxides containing aluminium and barium

(Oxidos refratários contendo alumínio e bário)

H. G. Emblem and W. A. Al-Douri

Manchester Materials Science Centre, University of Manchester and UMIST

Grosvenor Street, Manchester M1 7HS, UK.

Abstract

Oxides containing aluminium and barium, optionally with chromium, are refractory with several possible industrial uses. A gel precursor of an oxide having the formula BaO.n(Al_2xCr_2yO₃), where 1<n<6.6, (x+y)=1 and 0<y<0.5 was prepared by mixing a solution of a barium salt with a solution of an aluminium salt or a solution of an aluminium salt and a chromium III salt, then forming a gel which was fired to obtain the desired oxide. Filaments may be drawn as the gel is forming or extruded from a semi-rigid gel. FT-lR, XRD and ²⁷Al NMR studies showed that barium is not incorporated directly into the gel structure. Barium aluminium oxides were obtained only after liquefaction of barium species. A powder mixture suitable for firing to an oxide of the formula BaO.m [Al_2xCr_2y O₃] where 4.6<m<6.6, (x+y)=1 and 0<y<0.5 was prepared from source materials barium hydroxide, barium oxide, barium carbonate and barium mono-aluminate, alumina and/or a hydrated alumina and chromium III oxide, the relative amounts being varied to give the desired values of m and y on firing. A preferred oxide has the formula BaO.6Al₂O₃ (m=6 and y=0). Strong ceramic shapes have been prepared from this. XRD studies of oxide compositions (n or m=6 and y=0) sintered at high temperature showed that barium mono-aluminate is a low-temperature intermediate phase. Chromium III reduces the temperature required to form a barium aluminium oxide. Previously published work is summarised.

Resumo

Oxidos contendo alumínio e bário, opcionalmente com crômio, são refratários com vários possíveis usos industriais. Foi preparado um gel precursor de um óxido de formula BaO.n(Al_2xCr_2yO₃), com 1<n<6,6, (x+y)=1 e 0<y<0,5, pela mistura de um sal de bário com uma solução de um sal de bário com solução de um sal de alumínio ou uma solução de um sal de alumínio e um sal de cromo III, formando então um gel que é queimado para se obter o óxido desejado. Filamentos podem ser estraídos duramente a formação do gel ou extrudados de um gel semi-rígido. Estudos de infravermelho com transformada de Fourier, difração de raios X e ressonância magnética nuclear com ²⁷AL mostraram que o bário não é incorporado diretamente na estrutura do gel. Oxidos de alumínio e bário foram obtidos somente após liquefação de espécies de bário. Foi preparada uma mistura de pós adequada para queima para se obter um óxido de fórmula BaO.m[Al_2x Cr_2y O₃] com 4,6<m<6,6 (x+y)= 1 e 0<y<0,5, com materiais de partida hidróxido de bário, óxido de bário, carbonato de bário e mono-aluminato de bário, alumina e/ou alumina hidratada e óxido de cromo III, com quantidades relativas variando para se obter na queima valores desejados de m e de y. Corpos cerâmicos resistentes da fórmula BaO 6 Al₂O₃ (m=6 e y=0) foram preparados. Estudos de difração de raios X de óxidos de composições (n ou m=6 e y=0) sinterizados em alta temperatura mostraram que mono-aluminato de bário é uma fase intermediária de baixa temperatura. Cromo III reduz a temperatura necessária para formar um óxido de alumínio e bário. É feito um sumário de trabalhos anteriormente publicados.

INTRODUCTION

Oxides containing aluminium and barium are refractory and have several possible industrial uses [1-3], for instance in the glass and metallurgical industries, also as moulds and cores for casting metals. The oxides can be prepared via a sol-gel route (Route (i)) or via a dry powder route (Route (ii)). From phase diagram studies [4, 5] of the binary system BaO-Al₂O₃, composition limits [3, 6] for a possible engineering ceramic material could be BaO.4.6Al₂O₃ to BaO.6.6Al₂O₃, with the well-defined compound BaO.6Al₂O₃ being preferred. Barium hexa-aluminate, like calcium hexa-aluminate and strontium hexa-aluminate has the space group D⁴_6h, a hexagonal crystal system with two molecules per unit cell [7]. Barium mono-aluminate hydrates [8] and may from the basis of cement or grouting compositions.

Chromium III addition can reduce the temperature required to form an aluminium-barium oxide on firing [3]. The system Al₂O₃ - Cr₂O₃ is a simple binary, forming a complete range of solid solutions showing (except for a narrow composition range) expansion of the alumina lattice with increasing chromium content [9]. The lattice expansion can allow barium to react more readily during firing and probably explains why chromium reduces the temperature of formation of an aluminium-barium oxide.

Route (i) - Oxides of the formula (i) BaO.n(Al_2xCr_2yO₃) where 1<n<6.6 and 0<y<0.5 and x+y=1 may be obtained [1] from a gel precursor prepared by mixing a solution of a barium salt with a solution of an aluminium salt or with a solution of an aluminium salt and a chromium III salt, then forming a gel which is fired to obtain the oxide. The preferred aluminium salts are the aluminium chlorohydrates Al₂(OH)_6-nCl_n with n in the range 1-5, e.g. Al₂(OH)₅Cl, available in aqueous solution as ‘Chlorhydrol’. The relative amounts of precursor gel components can be varied to give the desired values of n and y when the gel is fired. Polymerisation and hence gelation may be induced by adding a separate gelling agent, for instance a solution of ammonium acetate, which is basic, or by heating the solution to remove volatiles. A barium, aluminium or chromium III salt already present can produce polymerisation and gelation, barium acetate being one such salt. The gels may be used to bind a refractory grain mix. Under suitable conditions gel filaments, which convert to a ceramic fibre when fired [1] may be obtained from a gel precursor corresponding to an oxide of formula (i), preferably n=6, y=0, i.e. BaO.6Al₂O₃. Barium bromide dihydrate is more soluble in water than is barium chloride dihydrate [10] and is therefore preferable for the preparation of gel precursor solution with oxide stoichiometry BaO.Al₂O₃.

Route (ii) - Oxides of the formula (ii) BaO.m (Al_2xCr_2yO₃) where 4.6<m<6.6 and 0<y<0.5 and x+y=1 were prepared [1-3] from the source materials barium oxide, barium carbonate or barium mono-aluminate and alumina or a hydrous alumina such as boehmite, with, when y is not zero, chromium III oxide, the relative amounts of the components being adjusted to give the desired values of m and y on firing. Sintered compacts of oxide stoichiometry BaO.6Al₂O₃ may be prepared from a mix of barium carbonate and alumina grain.

In the present paper results from previous work are included and summarised. The use of barium acetate to obtain gels from precursor solutions of oxide stoichiometry BaO.6Al₂O₃ or BaO.Al₂O₃ is described. Results of varying the weight ratio barium chloride (or bromide): barium acetate are given and the results obtained [11] when 0.1 M HCl, CH₃, COOH or NH₄OH solutions are added are summarized. The barium source for gels of oxide stoichiometry BaO.6Al₂O₃ can also include an aqueous suspension of barium carbonate. For a gel of oxide stoichiometry BaO.Al₂O₃, the barium source was barium bromide with barium acetate. The use of ammonium acetate solution to form a gel of oxide stoichiometry BaO.Al₂O₃ or BaO.6Al₂O₃ (with or without chromium III addition) is also described. FT-IR, XRD and ²⁷Al NMR studies showed that barium was not incorporated directly into the gel structure obtained from a gel precursor solution having the oxide stoichiometry BaO.6Al₂O₃. Aluminium ion condensation and the formation of gels, with or without added barium, are summarised. The preparation and properties of sintered compacts, of oxide stoichiometry BaO.6Al₂O₃, from a mix of barium carbonate and alumina grain are described (route(ii)). The sintering behaviour of gel filaments and compacts prepared from a dried and powdered gel, each of oxide stoichiometry BaO.6Al₂O₃ (route (i)) is compared with the sintering behaviour of compacts having oxide stoichiometry BaO.6Al₂O₃ prepared according to route (ii).

EXPERIMENTAL

Barium carbonate, soluble barium salts and ammonium acetate were all of laboratory reagent grade. The aluminium salt solution was ‘Chlorhydrol’ solution [Al₂(OH)₅Cl nominal, 22-23 wt% Al as the Al₂O₃ equivalent: Wilfrid Smith Ltd, Edgeware, Middlesex, UK]. Barium chloride dihydrate or barium bromide dihydrate were dissolved in ‘Chlorhydrol’ solution. Barium acetate was dissolved separately in distilled water. The barium acetate: water ratio was constant at 1: 25 (w/w) to make certain that the barium acetate was completely dissolved. This solution was added to the barium chloride (or barium bromide) - ‘Chlorhydrol’ solution, in the quantity required to give the oxide stoichiometry BaO.6Al₂O₃. Chromium III was added as ‘CAC 75’ chromium acetate solution (11.0-11.5 wt% Cr as the Cr₂O₃ equivalent; 12.0-12.5 wt% acetic acid equivalent), (Lancashire Chemicals Ltd, Glossop, Derbyshire, UK) to give the desired values of x and y in formula (i). Barium bromide dihydrate was used to prepare gel precursor solutions having the oxide stoichiometry BaO.Al₂O₃. A barium mono-aluminate gel precursor solution was also prepared from barium chloride and gelled with ammonium acetate solution. Ammonium acetate solution was also used to gel BaO.6Al₂O₃ gel precursor solutions, with or without chromium III addition.

Unless otherwise stated, gel default font for the various solution were determined at ambient temperature (usually about 15 °C). Filament formation was assessed by inserting and then pulling out of the solution a 10 mm diameter glass rod during gelation. The apparatus and procedure used for continuous filament formation are described in [3]. Cylindrical compacts, diameter 10 mm, height 3 mm were prepared from dried and powdered gels (oxide stoichiometry BaO.6Al₂O₃) by compaction at 270 MPa in a steel die. The compacts were sintered in air at given temperatures for given default font. FT-IR and XRD measurements, together with ²⁷Al NMR observations, were used [6] to characterise phases present in the gel before and after sintering.

Barium carbonate was suspended in water, to give weight ratio BaCO₃:H₂O of 0.185:1. Barium chloride dihydrate was dissolved in ‘Chlorhydrol’ solution to give weight ratio BaCl₂:2H₂O: ‘Chlorhydrol’ solution of 0.685:10. On mixing in quantities to give the oxide stoichiometry BaO.6Al₂O₃ a clear gel was formed in about 2 minutes.

Alumina grain (‘low soda’) was either RA107LS (average particle size 0.5 mm; surface area 6.7 m²g^-1, Na₂O 0.05 wt%) or RA207LS (average particle size 0.5 mm; surface area 7.0 m²g^-1, Na₂O 0.08 wt%), both supplied by BA Chemicals Ltd, Gerrards Cross, Buckinghamshire, UK. Sintered components of oxide stoichiometry BaO.6Al₂O₃ were prepared from a mix of alumina grain powder and barium carbonate that had been high-energy milled to break down particle aggregates. Two procedures, A and B, were explored: in procedure A the milled powder mix was compacted uniaxially at 100 MPa, then cold isostatically at 1723 MPa, followed by sintering for 2 h at 1700 °C. In procedure B the milled powder was sintered at 1200 °C, then re-milled, followed by compaction and sintering as for (A). Modulus of rupture determinations were by four-point bend testing at ambient temperature. Phase developments during rapid or slow heating were followed by XRD observations, as for filaments, fibres or compacts prepared from gels of oxide stoichiometry BaO.6Al₂O₃. The identity of crystalline phases was confirmed by XRD patterns with the aid of JCPDS International Centre for Diffraction Data, Inorganic Materials Alphabetical Index, 1984.

A mixture of barium hydroxide dihydrate and alumina grain with the oxide stoichiometry BaO.6Al₂O₃, when sintered and re-milled, gave [6] a powder suitable for compaction and sintering (1750°C for at least 30 min.) to prepare simple shapes, (longer sintering time for larger, more complex shapes).

RESULTS AND DISCUSSION

Route (i) - The influence of the barium chloride: barium acetate weight ratio on the gel time and the time available for drawing filaments from solutions of oxide stoichiometry BaO.6Al₂O₃, at ambient temperature, in shown in Table I. The effect of adding either 0.1 M HCl solution, 0.1 M CH₃COOH solution or 0.1M NH₄OH solution, or diluting the gel precursor solution, or increasing the temperature, was also studied [11]. Table II summarises the results. Gel default font were determined at ambient temperature. All the solutions formed clear, rigid, coherent gels. Barium bromide is an alternative to barium chloride as a source of barium. The effect of the barium bromide: barium acetate weight ratio is shown in Table III, for gels of oxide stoichiometry BaO.6Al₂O₃ and in Table IV for gels of oxide stoichiometry BaO.Al₂O₃. All the gels were air-dried, then milled and sintered to form the oxide BaO.6Al₂O₃ or BaO.Al₂O₃.

Acetates which can form alkaline solutions, ammonium acetate for instance, are known [12] to gel aluminium chlorohydrate solutions, also [1] aluminium chlorohydrate solutions containing barium, with or without added chromium III. Table V gives the preparation and properties of barium mono-and hexa-aluminate precursor gels, having the formula (i) BaO.n(Al_2xCr_2yO₃) using ammonium acetate to form the gel. The precursor gels were air-dried, milled, then sintered to form the oxide. A barium mono-aluminate precursor gel may be the basis of a cement or grouting composition; for this use, the gel should be air-dried, then milled to a fine particle size.

XRD studies of gel particles dried in air showed [6] that barium was present as various barium chloride species. After heating to 650 °C, barium was present as hydrated barium chloride. The FT-IR spectra of gels derived from aluminium chlorohydrate solution (‘Chlorhydrol’) using ammonium acetate solution or a barium chloride-barium acetate solution, weight ratio 7:3 or 2:8, giving the oxide stoichiometry BaO.6Al₂O₃ closely resemble each other [6]. This, together with the XRD observations [3, 6] show that barium is not part of the gel network. This is confirmed [6, 13] by ²⁷Al NMR spectra of Al-O groups, in the solution at the start of gelation, during gelation, and in the gel formed; in each state oxide stoichiometry BaO.6Al₂O₃ existed, with barium acetate as the gel-inducing agent. The NMR spectra are very similar, suggesting that Al-O groups in the solution probably have a structure similar to that of Al-O groups in hydrous alumina gels. The hydrolysis and condensation of aluminium ions has been reviewed by Singhal and Keefer [14], who have also followed these reaction by ²⁷Al NMR observations. The various aluminium ions and the formation of gels from ‘Chlorhydrol’ solutions are described in Table VI.

Route (ii) The preparation and properties of sintered compacts having the preferred oxide stoichiometry BaO.6Al₂O₃ are given in Table VII. The lower modulus of rupture values obtained for sintered compacts prepared by procedure A could be due to CO₂ evolution from barium carbonate producing microcracking during sintering and inhibiting densification. Alumina grain RA107LS and barium carbonate mixed and sintered according to procedure B gave compacts with an intersecting network of small crystals [6]. The observed average particle size and average flaw size of ca. 1 mm explains the good strength of ca 600 MPa and if grain growth is suppressed, it is possible that this strength would be maintained at high temperature [6]. Sintered components prepared from the preferred composition BaO.6Al₂O₃, using RA107LS alumina and following procedure B are stronger than the alumina-chromia [15] and the magnesia-alumina-chromia [16] sintered compacts prepared previously, they are also stronger than the sintered compacts prepared [17] from alumina-chromia and mullite grain mixes.

SINTERING OF FILAMENTS AND POWDER COMPACTS

The behaviour of gel filaments (n=6 in formula (i)) with or without chromium during sintering is given in [3, 13]. For gel filaments which do not contain chromium, no barium-aluminium oxides are formed before the barium species liquefy. Tribarium mono-aluminate is formed first. This could be formed by reaction between the barium-rich surface phase and the aluminium-rich core phase of the filament. As the temperature of sintering is increased, tribarium mono-aluminate converts to barium mono-aluminate and ultimately, with increasing temperature of sintering, barium hexa-aluminate is formed. When the filaments are sintered at high temperatures, (ca 1400 °C), barium hexa-aluminate is formed directly. The presence of chromium reduces to ca 1150 °C the temperature required to obtain a barium hexa-aluminate ceramic fibre [3]. The microstructure of a barium hexa-aluminate ceramic fibre [3, 13] resembles the microstructure found [6] in a sintered compact, strength ca. 600 MPa, prepared according to procedure B, route (ii).

The behaviour of compacts prepared from a powder mix of alumina and barium carbonate, oxide stoichiometry BaO.6Al₂O₃, when sintered is given in Table VIII. Compacts were also prepared from a precursor gel of oxide stoichiometry BaO.6Al₂O₃, after the gel had been air-dried, then milled. The behaviour of these compacts during sintering is also given in Table VIII. In each case, barium mono-aluminate is formed as an intermediate phase during slow sintering to 1600-1700 °C. The formation of barium mono-aluminate during the slow sintering of mixes of barium carbonate and alumina (oxide stoichiometry BaO.6Al₂O₃) is consistent with accounts [18] of the sintering behaviour of mixes of alkaline earth oxides and alumina. Refractory shapes can also be prepared [1, 19] from a refractory grain mix, (for instance an alumina grain mix), bonded with a barium hexa-aluminate precursor gel. After drying to remove volatiles, refractory components with good bonding between grains were obtained by firing to ca 1700 °C.

CONCLUSIONS

- Oxides of the formula (i) BaO.n(Al_2xCr_2yO₃) where 1<n<6.6 and 0<y<0.5 and x+y=1 may be prepared from precursor solutions containing the required amounts of barium, aluminium and chromium.

- Aluminium acetate or barium acetate can be a gel-inducing agent, barium acetate giving better control of gel formation. Barium carbonate can also be a gel-inducing agent.

- Because of the higher solubility in water, barium bromide dihydrate can be preferable to barium chloride dihydrate as a source of barium, particularly for barium mono-aluminate gel precursor solutions.

- Gel filaments which convert to a ceramic fibre on firing can be obtained from a gel corresponding to an oxide of formula (i) in which preferably n=6 and y=0

- Barium is not directly incorporated into the gel structure.

- When chromium III is included in the gel precursor solution (n=6 in formula (i)), the temperature required to form a barium hexa-aluminate ceramic fibre is reduced.

- Oxides of the formula (ii) BaO.m(Al_2xCr_2yO₃), where 4.6<m<6.6 and 0<y<0.5 and x+y=1 may be prepared from powder compositions giving the required values of m and y on firing.

- The preferred oxide of formula (ii) has the stoichiometry BaO.6Al₂O₃ (m=6 and y=0). Sintered compacts having a modulus of rupture of ca 600 MPa can be prepared; a retention of strength at high temperature would indicate use as an engineering ceramic.

- Compacts prepared from micronised gel powders of oxide stoichiometry BaO.6Al₂O₃ (formula (i)) or from an alumina - barium carbonate mix of oxide stoichiometry BaO.6Al₂O₃ form a barium hexa-aluminate species when sintered, with barium mono-aluminate being an intermediate product.

ACKNOWLEDGEMENT

Alcan International Limited is thanked for supporting the work described in this paper

REFERENCES

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(Rec. 03/98, Ac. 04/98)

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