Advanced ceramic

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Bull. Mater. Sci., Vol. 16,NO. 6, December 1993, pp . 533-541. Printed in India.

Advanced ceramics: Com bustion synthesis and properties

K C PATILDep artme nt of Ino rganic and Physical Ch emistry, Indian Institute of Science, Bangalore56001 2, India

Abstract. Fine-p article ceram ic powders such as chromites, manganites, ferrites, cobaltites,aluminas (a-Al, 0 3 ,C r3 + A1 20 3, zirconia-toughened alumina, mullite and cordierite), ceria,titania, zirconia (t, m, c and PSZ), ielectric oxides (MTiO,, PZT and PLZT) as well ashigh T, cuprates have been prepa red by the combustion of redox compound s or mixtures.The c omb ustion -derive d oxide mate rials are of submicron size with a large surface area andare sinteractive.

Keywords. Combustion synthesis; metal oxides; ceramics.

1. Introduction

Traditionally, the wo rd ceram ics is associated with clay-based products such as bricks,tiles, pottery, table ware, san itary wa re an d glass. Naturally occurring minerals likesand, quartz, bauxite, feldspar etc are used in the manufacture of these materials.Advanced ceram ics differ from conv entional ceramics in their high-mechanical strength,fracture toughness, wear resistance, refractory, dielectric, magnetic and opticalproperties. Advanced ceramics or fine ceramics are high value-added inorganicmaterials produced from high purity synthetic powders to control microstructureand properties. Wet-chemical routes (Segal 1989) e.g. coprecipitation, sol-gel,spray-dry , freeze-dry etc a re usually em ployed to prepare advanced ceramic powders havingsubmicron size, narrow size distribution and absence of particle agglomerates.However, these methods are quite involved, require long processing time, costlychemicals and special eq uipm ent. Therefore, there is a need for an alternative to thewet-chemical methods.

2. Combustion synthesis

M an y researchers hav e attempted to use the heat generatedby exothermic chemicalreactions in the synthesis of high-temperature ceramic materials. Aluminothermiteprocess,

pioneered by Goldschmidt (1898) was used to prepare metals and alloys (Carlson1973). Walton and Pou los (1959) synthesized a number of refractory cermets: oxide-bofid2, oxide carb ide a nd oxide-silicide. M ore recently, Merzhanov used the exothermicreaction between the elements (metal-fuel. an d nonmetal-oxidizer) to produce a hostof materials: borides, carbides, carbonitrides, cermets, chalcogenides, hydrides,nitrides, oxides and silkides (Merzhanov 1993).This process, popularly known asself-propagating high-temperature synthesis(SHS), is also called furnaceless or firesynthesis.

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Table 1. Properties of oxide materials prepared by thk combustionof redox compounds.

Redox compound ProductSurface Av. aggl.

area (m2/g) size(,urn)

M O , M 2 0 3 ( M= Mg, Mn to ~ n ) ~M O , M 2 0 3 ( M= Mg, Mn to Zn)+L n 2 0 3 ( L n= rare earths), C e0 2MO, (M =T i an d Zr and their ss)M O , M 2 0 3 ( M= M g, M n to ~ n ) 7M O, M 2 0 3 ( M = Mg, Mn to Zn)+MFe ,04 (M = Mg, Mn, Co to Zn)M x Z n ,- ,Fe20, (M = Mg , Mn and Ni)MCo,O,(M = Mg, Mn, Co to Zn)M Mn, 0 4 ( M = C o a nd Ni)

M Mn, O,(M = Mg and Zn)PbMO, (M = Ti and Zr and their ss)

Note hc = N 2 H 3 C O 0 , s = solid solution, 'Fe20, rorms y-phase.

Although the preparation of fine particle oxides by the combustion of redoxcompounds is quite simple and attractive, it,has certain limitations. First, it takesseveral days to prepare the metal hydrazine carboxylate precursors. Secondly, theyield of th e oxide is only z 0%. Finally, not all metals form hydrazine carboxylatecomplexes and therefore it was not possible to prepare other oxides like aluminasand chromites which are formed a t fairly high temperatures(> 1000C). This hasbeen achieved by using flaming reactions employ ing metal nitrates and energetic fuelslike urea and hydrazides.

2.2 Preparation of oxides by the combustion of redox mixtures

Redox mixtu res like KN O,+ C + S (gun powder) once ignited undergo self-propagating,gas-producing exothermic decomposition. Similar exothermic reactions of redoxmixtures are used in rocket propellants e.g. NH4C10,-A1-CTPB. These redox mixturescontaining metallic ingredients(Mg, Al) ar e known to give metal oxides as undesiredproducts of combustion. This phenomenon has now been exploited to actuallysynthesize me tal oxides by the comb ustion o f stoichiometric mixtures of metal nitrates(oxidizer) and urealhydrazine-b ased fuels.

The stoichiom etric composition of the m etal nitrate (oxidizer) and fuel redox mixtureswas calculated based on the total oxidizing and reducing valency of the oxidizer andthe fuel which serve asa num erica l coefficient for stoichiom etric balance such tha tthe equivalence ratio, a, is unity, i.e. O/ F = 1 and the energy released is maximum

, an d H areJain et al 1981). In propellant chemistry, the speciesM 2 + , M 3 + ,M4* Cconsidered to be reducing with co rresponding valencies+ 2, + 3, + 4, +4 and + I.Elemental oxygen is considered to be an oxidizing species with valency- . Thevaiency of nit rogen is considered to. be zero. A ccording to this oxidizing and re ducingvalencies of the fuels such as urea ( C H 4 N 2 0 ,U), carbohydrazide (C H ,N 40 , CH),tetra formyl tris-azine (C4 H ,,N, 0 , TFTA), oxalyl dihydrazide(C, H 6 N 4 0 2 ,ODH ) ,maleic hydrazide (C, H 4 N 2O,, MH), and malonic dihydrazide (C, H 8 N 4 0 2 ,M D H )are + 6, + 8, + 28, + 10, + 16, and + 16 respectively.


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2.2a Preparation of oxides: Stoichiometric am oun t of metal nitrate (oxidizer) andfuel when ignited-50-500C undergo self-propagating, gas-producing comb usti onreaction (smouldering or flaming) (temp. 900- 1500C) to yield voluminous me taloxide in less than 5 min. The high in situ temperature ( 2 50O0C)attained during thecombustion synthesis of a-Al,O, using Al(NO ,),-urea mixtu re ha sbeen attributedto the gas phase reaction of ccnnbustible decomposition products of urea(CO, HNCO,NH,) an d th e aluminium nitrate (nitrogen oxides).A number of advanced ceramicoxide materials (table 2) have been prepared by the combustion of metal nitrate-urea/hydrazide fuels. These include: (i) refractory oxides such as a-alumina (Kingsleyand Patil 1988),ruby (Kingsleyet a1 1988, 1990a), metal aluminates (Kingsleyet a21990b), othoaluminates and aluminium garnet (Kingsleyet al 1990c), ceria (Sekaret a1 1990), zirconia (Arul Dhas a nd Patil 1992), pyrochlore zirconates (Arul D ha s

and Patil 1993b), metal chromites (Mano haraneta1

1990, 1992a; Go pic ha nd ranandPatil 1992a); (ii) low-thermal expansion coefficient materials likemullite (Gopichandranand Patil 1990), cordierite (Gopic handran and Pat il 1993); (iii) toug hen ed cer am icssuch as partially stabilized zirconia(PSZ) (Arul Dhas and Patil 1992), zirconia-alumina composites (Kingsleyand Patil 1990d; Arul Dh as and Pa til 1 9 9 3 ~ ) ;iv)magnetic materials like 7-Fe 20 , (Suresh and Patil 1993), spinel ferrites, o th of er ri te sand iron garnet (Sureshet al 1991); v) dielectric oxides such as MTiO ,, M Z rO ,,PZT,PLZT (Sekar et al 1992a,b); vi) high T, cuprates La, -,Sr,CuO, (Manoharanef a11992b; Gopi chandra n and Patil 1992b) and (vii) miscellaneous oxid es l ike sp in elmanganites (Arul Dhas and Patil 1993a), perovskite m anganites a nd ni ck el at es(Manoharan and Patil 1993).

The particulate properties suchas surface area and particle sizeof oxide materialsprepared by th e combustion of redox mixtures are summ arized in table2. T h e surface

Table 2. Proper ties of oxides prepa redby the combustion of redox mixtures.

OxidesSurface areat(m2/g)

Av . aggl. size( c L ~ )

a-A1, O3M Al, O,(M = Mg, Co , Ni, Cu and Zn)r - rOJAl, O ,3Al, O3-2SiO, (mullite)Mg, Al,Si, O,,(cordierite)Y3Al501,MCr2C,(M = Mg, Fe, Co, Ni, Cu and Zn)LnCrO,(Ln = La, Pr, Nd, Sm, Gd , Dy and Y)LnFeO,(Ln = La, Pr, Nd, Sm,Gd, Dy and Y)MFe,O,(M = Mg, Fe, Co, Ni, Cu and Zn)La,(Sr)CuO,Y3FeS012

Y- ez03ZrO,, PSZLn,Zr,O,(Ln = La to Dy)MTiO, (Ca, Sr, Ba andPb)CeO,MMn,O,(M = Mg, Co, Ni, Cu and Zn)

8 U)1-85 (U, CH )3-65 (U, CH )12-45 (U)45-135 (U)3-7 (U)13-70 (U, TFTA)6-35 (TFTA)4-90 (ODH,TFTA)20-100 (ODH,TFTA)3-5 (TFTA, MH)10-90 (ODH)45(MDH)3-13 (CH)4-20 (CH, U)4-30 (TFTA, O DH )14-90 (U, TFTA, CH, ODH)30-60 (MH)

tFuels used for the combustion are indicated in the ~a rent hes es.


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Table 3. Comparison of properties of oxides prepared by the combustion of redoxcompounds (A) and redox mixtures(B).

OxideSurface area (m2/g) Particle size (,urn)

A B A B

CeO,Y - F e z 0 3

MFe, 04 (M= Mg, Mn to Zn)Ni, -,Zn,Fe,04MCo,O,(M = Mg , M n to Zn)MMn,O, (M = Mg, Co to Zn)MTiO,(M = Ca, Sr , Ba an d Pb)

PZ TPLZT

area values of th e metal oxide powde rs preparedby the combustion of redox mixtureare in the ra nge of 2-135 m2/g . The averag e agglom erate particle sizes of these powdersare in the range of0.6- 15 pm.

2.3 Comparison of properties of oxides prepared by the combustion of redoxcompounds and mixtures

Only few oxides could be prepared by both the processes.These include, cobaltites,ferrites a n d dielectrics (PbZr,,,Ti,.,,03, PZ T etc.) which are formed at lowtemperature (< 1000C).A com para tive acco unt of properties of oxides prepared byboth combustion processesis given in table 3. It c an be seen tha t the surface area ofthe oxide pow ders prepared by the c omb ustion of redox compound s are higher thanthose obta ined by the com bustion of redox m ixtures. This is because redox compoundsundergo flameless (smouldering) combustion whereas redox mixture burns withaflame. Conse quen tly, the averag e agglo mera te pa rticle size (light scattering sedimenta-tion method) of the oxide powders prepared by the combustion of redox mixturesis lower than tha t of oxide powders o btain ed by the com bustion of redox compounds.The marked difference in surface area and particle size in both the processes canbeexplained by the amount of gaseous products that are evolved during combustion.

For example, the combustion synthesis of y-Fe,03 by both the methods can berepresented as:a ir

2 ( N 2 H , ) F e ( N , H 3 C O O ) , . H 2 O---+ e203 , ) + 6C02,g18N2(g) 16H20, ,

(30 mol of gases/Fe203) (1)


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The com bustion of redox comp ound (equation (1)) liberates30 moles of gaseousproducts whereas redox mixture gives only20 moles (equ ation (2)). It is interestingto note th at com bustion of redox compo und yields low tempe rature phase(y-Fe203),whereas combustion of redox mixture gives usually a high temperature phase (a-A12Q3,a-Fe20, etc). However by changing the fuel one could alter the energetics of theredox mixture and prepare various metastable phases.

3. Sintering and microstructure 1

Sintering behaviour of the combustion derived powders was studiedby crushing theas-synthesized powders in an agate mor tar an d pelletizing under 20-70 M P a uniaxialcold pressing. Typically ZrO,, Al,O, an d Al, 0,- ZrO , composite when sintered at

1300-1650C for4 h

achieved~ 9 9 %

heoretical density. The scanning electron

Figure la-b.


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Figure 1. SEM micrographs of the surfaces of sintered pellets: (a) ZrO, (1250C,4 ), (b)A120 , (1650C,4 ), (c) Al,O,-ZrO, composite (1650C,4 h) and (d) PZT (12WC, 1 h).

microscope(SEM)micrograph of sinteredY, O , stabilized ZrO,, AI, O,, A1203-Zr02composite an d PZT are show n in figure 1. The microstructureof ZrO, with 4 mol%Y 2 0 3 f igure la) s intered at1250C reveals fine-grained microstructure and the grainsare almost equiaxed with an average grain sizeof 1 to 3pm. he microstructure ofAI2O3 figure lb ) sintered a t 1650 C shows nearly pore-free nature and occasionallythe presence of exaggerated grains is noticed. Exaggerated grain growth could beavoided by adding a small amount of impurities such as MgO, Y203,TiO, etc.Microstructure of A120, with10 wt% ZrO , composite (figure lc) sintered at 1650Cshows alumina grains (black contrast) with linear grain boundaries and pore-freenature of the body. Z irconia gra ins (white contrast) are almost equiaxed with 1-2pm


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Table 4. A comparison between solution combustion hnd Merzhanov'sSHS.-- - - - - - - - -SHS

Gasless solid state (i.e. solid-t solid) processHeterogeneousFlame temperature > 2000CSelf-prop agating only if the adiabatic tem perature(Tad) is > 1800CPowders could be sinteredto high density bysimultaneous applicationof pressureCommercialized

Solution combustion

Gas-pro ducing solution (i.e. solution +solid) processHomogeneous or heterogeneousFlame temperature 2 1000CSelf-propagating even when the flame temperature> 900CCompacted and sintered to near theoretical densityat low temperaturesYet to be commercially exploited

size and occupy the grain boundaries of large alumina gra ins. Microstructureof PZT

(figure Id ) also shows uniform grain growth with a n average grain sizes of 2w .

4. Conclusions

The combustion process described here has several advantages overthe o t h e rmethods in te rms of simplicity, cost effectiveness, energy saving, purity andhomogeneity.The main differences between Merzhanov'sSHS and the combustion processdescribedhere (solution comb ustion ) are given in table 4.

References

Arul Dhas N and Patil K C 1992 ln t . J. SHS 1 576Arul Dhas N and Patil K C 1993aJ. Solid State Cltem. 102 440 ,Arul Dhas N and Patil K C 1993bJ. Mater. Chem. (in press)Arul Dhas N and Patii K C 1993cCeram. Int. (in press)Carlson 0 N 1973 in Progress in extractive metallurgy (ed.)F Haboshi (New York: Gordonan d Breach)

Vol. 1 p. 189Goldschmidt H 1898 Liebigs Ann. Chem.301 18Gopichandran R and Patil K C 1990Mater. Lett. 10 291Gopichandran R and Patil K C 1992a Mater. Lett. 12 437Gopichandran R and Patil K C 1992b Mater. Res. Bull. 27 147Gopichandran R and Patil K C 1993Br. Ceram. Trans. (in press)Jain S R, Adiga K C and Pai Verneker V 1981 Combust. Flame 40 71Kingsley J J, ManikamN and Patil K C 1990a Bull. Mater. Sci. 13 179Kingsley J J, Suresh K and Patil K C 1990b J. Mater. Sci. 25 1305Kingsley J J, SureshK and Patil K C 1990cJ. Solid State Chem.88 43 5Kingsley J J and Patil K C 1988 Mater. Lett. 6 427Kingsley J J and Patil K C 1990d Ceram. Trans. 12 217Mahesh G V, Ravindranathan P and Patil K C 1986 Proc. Indian Acad. Sci. (Chem. Sci.) 97 117Manoharan S S, Kumar N R S and Patil K C 1990 Mater. Res. Bull. 25 731Manoharan S S and Patil K C 1989 in Advances in ferrites (eds)C M Srivastava and M J Pat ni(New Delhi:

Oxford and IBH) vol. 1,p. 43Manoharan S S and Patil IGC 1992a J. Am. Ceram. Soc. 75 1012Manoharan S S and Patil K C 1993 J. Solid State Chem.102 267Manoharan S S, Prasad K, Subramanium SV and Patil K C 1992bPhysica C190 225Merzhanov AG 1993 in Chemistry of advanced materials(ed.)C N R Rao (Oxford: BlackwellScientific) p. I 9Patil K C 1986Proc. Indian Acad. Sci. (Chem. Sci.)96 459Patil K C 1991 Contributions to the chemistry of hydrazine deriuatives and finite oxi de m at er ia ls ,D. SC.

Thesis, Indian Institute of Science. Banealore


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Ravindranathan P and Patil K C 1986 J . Mate r. Sci. Lett. 5 22 1Ravindranathan P, Mahesh G V and Patil K C 1987aJ . Solid State Chern. 66 20Ravindranathan P and Patil K C 1987b Cera m. Bull. 66 688Ravindranathan P an d Patil K C 1987c J. Mater. Sci. 22 3261Segal D 1989 Chemical synthesis of advanced ceramic materials (Cambridge: University Press)Sekar M A, Manoharan S S and Patil K C 1990 J. Mater. Sci. Lett. 9 1205Sekar M A and Patil K C 1992a J. Mater. Chem. 2 749Sekar M A, Dhanaraj G, Bhat H L and Patil K C 1992b J. Mater. Sci. Elect. Mat. 3 237Sekar M A and Patil K C 1993 Mater. Res. Bull. 28 485

- Suresh K, Mahesh G V and Patil K C 1989a J. Therm. Anal. 35 1137Suresh K, Mahesh G V and Patil K C 1989b in Advances in ferrites (eds)C M Srivastava and M J Patni

(New Delhi: Oxford and IBH) ol. 1, p. 103Suresh K, Kurnar N R S and Patil K C 1991 Ado. Mater. 30 148Suresh K and Patil K C 1992 J . Solid State Chem. 99 12Suresh K and Patil K C 1993 J. Mater. Sci. Lett. 12 572

Walton J D and Poulos N E 1959 J. Am . Ceram. Soc. 42 40

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