Ž .Journal of Nuclear Materials 253 1998 203–212

Synthesis of Li TiO ceramic breeder powders by the combustion2 3

process

C.H. Jung a,) , J.Y. Park a, S.J. Oh a, H.K. Park a, Y.S. Kim a, D.K. Kim b, J.H. Kim b

a Hi-Tech Ceramics Team, Korea Atomic Energy Research Institute, Taejon, South Koreab Department of Materials Science and Engineering, Korea AdÕanced Institute of Science and Technology, Taejon, South Korea

Abstract

The synthesis of the ultra-fine Li TiO powder by the combustion reaction of lithium nitrate, titanium nitrate and specific2 3

fuels was investigated. Ultrafine Li TiO powders could be synthesized using glycine or a mixture of urea and citric acid. A2 3

pure Li TiO phase was obtained by the simple process without further calcination reaction. The specific surface area of the2 3

as-synthesized powder was 10 to 14 m2rg and the primary particle size was about 30 nm. The Li TiO body sintered at2 3

8008C for 3 h had dense agglomerates which were formed by the inter-agglomerate sintering process. Each of theagglomerates consisted of very fine grains with a size of 0.3 to 0.5 mm. q 1998 Elsevier Science B.V.

1. Introduction

The choice of a suitable breeding material for D–Tfueled fusion reactors is very important in view of theeffective power conversion, tritium release behaviors and

w xsafety considerations 1 . Lithium-based ceramics such asLi O, g-LiAlO , Li SiO , Li ZrO and Li TiO have2 2 4 4 2 3 2 3

been considered as candidates for tritium breeding materi-w xals in D–T fusion reactors 2,3 .

Li TiO is known to be a low-activation ceramic mate-2 3

rial for use in tritium breeding. To date, however, verylittle work has been done on the fabrication of the Li TiO2 3

ceramic powder, the sintering behavior, the tritium releasew x w xbehavior and the properties database 4 . Finn et al. 5

performed preliminary research which indicated thatLi TiO possessed attractive properties. Li TiO has the2 3 2 3

same lithium density as that for Li ZiO and its melting2 3

point is similar. The thermal conductivity is better thanthat for lithium aluminate or lithium zirconate. Long-termwaste problems should be low because titanium is the lowactivation element. In addition, Li TiO appears to possess2 3

excellent tritium release behaviors at temperatures beloww x3008C 4 .

) Corresponding author. Fax: q82-42 862 5496; e-mail:chung1@nanum.karei.re.kr.

In this study, a novel method for preparing fine-crystal-line Li TiO powders, that is based on the combustion2 3

reaction, is reported. Several conventional powder synthe-sis techniques including solid state synthesis, co-precipita-

w xtion and sol–gel process are available 6 . However, thesereactions are complicated and require multi-step reactionroutes with long term reactions. Therefore, a simple pro-cess is needed. One of the simplest powder preparation

w xprocesses is combustion synthesis 7,8 . This process is anexploitation of an exothermic and usually very rapid chem-

w xical reaction to form final reaction products 9 . A keyfeature of this process is that the heat required to maintainthe chemical reaction is supplied from the reaction itselfŽ .internal source and not from an external source.

Many advantages of this technique have been demon-Ž .strated in the past as follows. 1 The method needs little

Ž .heating to start a rapid reaction. 2 The method requireslittle or no further processing such as calcination andmilling. There is a considerable savings in time, energyand equipment compared with the conventional techniques.Ž .3 The process can be applied to prepare novel-oxidescontaining dopant elements, low-temperature compoundsand cermets which are difficult to synthesize by the con-

Ž .ventional methods. 4 The process uses the heat generatedby the chemical reaction between fuel and metal nitrates to

w xconvert the metal ions to the target ceramics materials 10 .

0022-3115r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.Ž .PII S0022-3115 97 00313-9

( )C.H. Jung et al.rJournal of Nuclear Materials 253 1998 203–212204

Ž .5 The combustion synthesized powders with wet chemi-cal techniques have good sinterability because most ofthem are produced in ultrafine form. This wet chemicalroute allows the degree of mixing of the reactants to bebetter than by directly blending the solid reactants.

In this study, the combustion behavior of various fuelsand fuel composition on the phase formation were investi-gated to synthesize the ultra-fine Li TiO powder by the2 3

combustion reaction. Additionally, the sintering behaviorof the combustion synthesized powder will also be dis-cussed.

2. Experimental procedure

The combustion process is a self-sustaining combustionsynthesis technique that produces fine homogeneousmetal-oxide powders. Aqueous precursor solutions contain-ing metal nitrates and fuel such as urea, citric acid andglycine were heated until the water was evaporated. Afterevaporation of the water, the precursor mixture auto-ignitedand underwent combustion spontaneously to form themetal-oxides.

Titanium nitrates were used as a source material ofŽ .titanium because the water soluble Ti IV ion exists only

in a strong acid media. Titanium nitrate is not commer-cially available and was prepared according to the pathshown in Fig. 1. It was prepared by dissolving hydratedtitania in nitric acid. The hydrated titania was obtained byadding TiCl aqueous solution to ammonia solution. Hy-4

drated titania was carefully washed with distilled wateruntil the chloride ions were fully removed. After these

Fig. 1. Flow sheet of the combustion process for synthesis of theLi TiO powder.2 3

Ž Ž . .metal salts LiNO and TiO NO and fuel were dis-3 3 2

solved in water, this precursor solution was heated withstirring. As soon as the water was evaporated, the precur-sor mixture spontaneously ignited by burning of the metalnitrate and fuel and fine Li TiO powder was formed with2 3

the release of gas like N , CO and H O. The composition2 2 2

of fuel and metal nitrates was calculated according to thew xmethod of Jain et al. 11 . A ‘stoichiometric’ composition

denotes a fuel-to-nitrate ratio in which the fuel can reactcompletely with all of the metal nitrates in the samemixture, such that no residual fuel or nitrate remains in theproduct materials.

The combustion synthesis for Li TiO was done ac-2 3

cording to the flow sheet shown in Fig. 1. The combustionreaction was completed within 20 s. The crystalline phasesof as-synthesized powders were determined by X-ray

Ž .diffractometry with radiation of Cu Ka Rigaku at thescan speed of 18rmin. Scanning electron microscopyŽ . ŽSEM, JEOL and transmission electron microscopy TEM,

.JEOL 2000FX-2, 200 keV , were used to observe themorphology and size of the synthesized powders. Simulta-neous thermogravimetric and differential thermal analysisŽ . Ž .TGrDTA and differential scanning calorimetry DSCwere performed to analyze the thermal behaviors of theprecursor solution and the synthesized powders. The spe-cific surface area of the synthesized Li TiO powders was2 3

measured with BET method.To compare the characteristics of combustion synthe-

sized powders with those of solid state reacted ones, otherpowders were also prepared by the mechanical mixing ofLi CO and TiO and calcined at 400 to 10008C for 3 h.2 3 2

3. Results and discussion

3.1. Phase formation with fuel type and composition

The type and the composition of fuels in the combus-tion synthesis can strongly influence the reactivity of thecombustion reaction and the phases formed in the synthe-sized powders. If only nitrates were dissolved in a precur-sor solution without any fuels, then hard lumps remainedwithout any combustion reaction during heating. But whenfuels were mixed along with nitrates, the precursor solu-tion could be reacted with or without a flame present. Thereactivity of the combustion reaction is dependent upon theligand groups of the fuel molecule and the compositionalratio of fuel to nitrates.

Therefore, the reactivity and the product phases fromŽ .metal Li, Ti nitrates with various fuels were investigated

before the synthesis of Li TiO powder by the combustion2 3

reaction was undertaken. These results are summarized inTable 1. LiNO is reacted with citric acid or glycine.3

However, the produced phase is not Li O but Li CO ,2 2 3

which seems to be to due to the CO adsorption character-2

( )C.H. Jung et al.rJournal of Nuclear Materials 253 1998 203–212 205

Table 1Ž .Combustion behavior of LiNO and TiO NO with various fuels3 3 2

Ž .Reactant composition stoichiometric Reactivity Phase

LiNO urea no reaction Li CO qLiNO crystalline3 2 3 3

citric acid very explosive Li CO crystalline2 3

glycine very explosive Li CO qLiNO crystalline2 3 3

Ž .TiO NO urea no reaction anatase3 2

citric acid weak reaction anataseglycine weak reaction anatase

istics of Li-compounds from glycine or citric acid duringw xthe combustion reaction and atmosphere 12 . Although

glycine reacts on lithium nitrate with flame, not all phasesare Li CO . This seems to result from the excess amine2 3

group which is not complexed with lithium ions.Ž .TiO NO is not reacted with urea but weakly reacted3 2

with glycine or citric acid. However, all the phases areanatase which is the low temperature phase of titaniumoxide, i.e. the heat from the combustion reaction seems tobe too small to produce the rutile phase. The reactivity of

Ž .the mixture of LiNO –TiO NO was different from that3 3 2

of each of the nitrates. The fuels consisted of both thecarboxylic acid group and amine group like glycine or amixture of urea and citric acid were strongly reacted with

Ž .mixed metal Li, Ti nitrates. The reactivity of this systemwas strongly dependent on the composition of fuels. Thestrong reaction occurred only at the stoichiometric ratio offuel and nitrates. The reactivity and the produced phasesare summarized in Table 2. Among various compositionsof fuels, the stoichiometric composition of glycine or amixture of urea and citric acid produced the Li TiO phase2 3

with a strong reaction. The powders prepared by thecombustion process had the specific surface area of 10 to20 m2rg which was dependant on the type and composi-tion of the fuel.

The self-sustaining nature of the combustion can beexplained as follows. The precursor mixture contains, astrong oxidizer, metal nitrate, and also contains glycine or

citric acid, a readily combustible fuel. On heating, theprecursor mixture, after evaporating water, nitrate decom-

Ž .poses giving oxides of nitrogen NO . The gaseous NOx xŽ .reacts with the fuel glycine or citric acid generating heat

Ž .flame and more gases. The homogeneous gas-phaseexothermic oxidation–reduction increases the temperatureof the intact viscous mixture immediately adjoining to thecombustion zone, causing it to react. The reaction processproceeds rapidly and sustainedly until all of the intact zoneis consumed. The rapid temperature increase with theorange flame and the volume of gas generated during thereaction depend on the fuel type, composition and content;in general, a higher fuel content or stoichiometric composi-tion gives rise to higher temperature and to more evolvedgas.

XRD patterns of the synthesized powder prepared byŽvarious fuels are shown in Fig. 2. When pure urea mark A

.in Fig. 2 or mixed fuel of urea and citric acid withŽ .composition of urearcitric acid equal to 12 mark B was

Ž .used as a fuel, LiNO and TiO anatase phases were3 2

found in the synthesized powders. This observation seemsto be related to heat produced from the combustion reac-

Ž .tion. When pure glycine mark F and mixed fuels of ureaŽ .and citric acid with the same ratio mark C or excess of

Ž .citric acid mark D were used as fuels, Li TiO could be2 3

easily obtained. However, an unreacted residue phaseseemed to exist along with Li TiO in the excess citric2 3

acid fuel. Similar results were observed with mixed fuel

Table 2Effects of fuel type and composition on the synthesis of Li TiO powder2 3

2Ž . Ž . Ž .Composition of the fuel mol Reactivity Phase XRD Remark BET m rgŽ .for 1 mol of Li TiO2 3

urea citric acid glycine

Ž .10r3 – – no reaction LiNO , TiO anatase A –3 2Ž .8r3 2r9 – no reaction LiNO , TiO anatase B –3 2

5r3 5r9 – strong reaction Li TiO C 17.012 3Ž .2r3 8r9 – moderate reaction Li TiO impurity D 19.232 3Ž .– 10r9 – weak reaction Li TiO impurity E 28.492 3

– – 20r9 very strong reaction Li TiO F 13.502 3

( )C.H. Jung et al.rJournal of Nuclear Materials 253 1998 203–212206

Fig. 2. XRD patterns of as-synthesized powders with various fuel types and compositions.

( )C.H. Jung et al.rJournal of Nuclear Materials 253 1998 203–212 207

Ž . Ž . Ž . Ž .Fig. 3. SEM photographs of as-synthesized powders with fuel type: A urea, B urea)citric acid, C ureascitric acid, D urea-citricŽ . Ž .acid, E citric acid and F glycine.

( )C.H. Jung et al.rJournal of Nuclear Materials 253 1998 203–212208

with excess citric acid in the case of pure citric acid as afuel. The reactivity of pure citric acid was lower thanmixed fuel with excess citric acid, i.e. to synthesizeLi TiO powder by combustion reaction, a fuel must2 3

possess both the carboxylic acid group and amine group.The microstructures of the synthesized powders using

various fuels were observed. A typical micrograph of thesynthesized powders is shown in Fig. 3. The powders

Ž .prepared from pure urea A in Fig. 3 and a mixed fuelŽ .with excess urea composition B in Fig. 3 had a cluster of

Ž .aggregates, which were LiNO , TiO anatase and unre-3 2

acted fuels. In both cases, the reaction is very weak. Bycontrast, the synthesized Li TiO powders using glycine2 3Ž .F in Fig. 3 formed the foamy agglomerated particles.

A lot of pores were found on the surface of thesynthesized powders, which seems to result from theevolved gas during the combustion reaction. The SADŽ .selected area diffraction pattern and the microstructure ofas-synthesized powder observed by TEM are shown in Fig.4. While the morphology of secondary particles was aporous skeleton, the primary particles were ultra-fine andequiaxed crystalline with a particle size of about 20 nm.The SAD pattern shows clear rings, which means that thesynthesized powder is very fine crystalline.

Considering the above results, the Li TiO powder2 3

could be easily synthesized by the combustion reactionwith glycine or a mixed fuel of urea and citric acid whichis consisted of both of amine group and carboxylic group.

3.2. Phase formation with ratio of fuel to nitrate

It is known that one of the major parameters thatdetermines the phase formation and the morphology of thesynthesized powder is the ratio of fuel to metal nitrate in

w xthe combustion reaction 9,10 . When a precursor solutionhas the stoichiometric ratio of fuels to metal nitrates, thereactivity and the phase formation are the most desirable.In this study, effects of the ratio of fuels to metal nitrateson the characteristics of the synthesized powder wereinvestigated in a precursor solution consisting of glycine asa fuel and a mixed nitrate of lithium and titanium.

Fig. 5 shows XRD patterns for Li TiO produced at2 3

three different ratios of glycine to nitrates. Three differentcompositions are fuel-lean which denotes the precursorsolution contains glycine with one-half of the stoichiomet-ric ratio, fuel-rich which denotes the precursor solutioncontains two times of glycine and fuel-stoichiometric. The

Fig. 4. Bright-field TEM image and SAD pattern of as-synthesized Li TiO powders.2 3

( )C.H. Jung et al.rJournal of Nuclear Materials 253 1998 203–212 209

ŽFig. 5. XRD patterns for Li TiO synthesized by the combustion reaction with three different ratios of glycine to nitrate open circle:2 3.anatase .

ŽFig. 6. DSC curve of precursor mixture with glycine and Li and.Ti nitrates.

Fig. 7. DTArTG analysis curves of as-synthesized Li TiO pow-2 3

der by the combustion reaction.

( )C.H. Jung et al.rJournal of Nuclear Materials 253 1998 203–212210

synthesized powders prepared using the fuel-stoichiometricand fuel-rich consisted of fully crystalline Li TiO with an2 3

average primary crystalline size of about 30 nm which isdetermined by XRD line broadening analysis. However,unreacted residues co-existed in the synthesized powderprepared by the fuel-rich precursor solution. Most of thecrystalline phases produced by the precursor solution withthe fuel-lean process are anatase and small amounts ofunreacted residue also co-existed. This observation meansthat the ratio of glycine to mixed nitrates of lithium andtitanium is important in obtaining the fine crystallineLi TiO powders by the combustion reaction.2 3

Ž .Variations of heat or calorie produced from the reac-tions during the heating of a precursor solution were

analyzed by DSC. The small lump sample was prepared bypre-drying of the precursor solution with a fuel-stoichio-metric composition before the combustion reaction. Themeasured DSC curve was shown in Fig. 6. The exothermicpeak occurs at 2088C which is associated with the redoxreactions of burning the chemical groups involved inglycine. Up to 4008C, the decomposition was completedand the total weight loss was about 95%. The TG and DTanalysis were also performed to investigate the existenceof an unreacted residue in the as-synthesized powder pre-pared by glycine with fuel-stoichiometric composition.Fig. 7 shows that weight losses were less than 5% and nothermal peaks were observed up to 10008C, which indi-cated that pure Li TiO powder without a further calcina-2 3

Ž . Ž . Ž .Fig. 8. XRD patterns of the Li TiO powder prepared by solid state reaction; a after oven-drying, b after calcining at 5508C for 3 h, c2 3Ž .after calcining at 7008C for 3 h and d after calcining at 10008C for 3 h.

( )C.H. Jung et al.rJournal of Nuclear Materials 253 1998 203–212 211

Fig. 9. The surface microstructure of Li TiO body sintered at 8008C for 3 h using the combustion synthesized powder.2 3

tion process could be easily synthesized by this simplecombustion process.

3.3. Comparison of solid-state process with combustionprocess

A powder synthesis process by the solid state reactionis the most well known method for synthesis of the oxideceramics including Li TiO . Li TiO powder was pro-2 3 2 3

Ž .duced by the solid state reaction of TiO anatase and2

Li CO in order to compare the characteristics of the2 3

synthesized powders. A 1:1 stoichiometric mixture of tita-nia and lithium carbonate was prepared by mixing togetherwith a zirconia ball in ethanol. Then, the mixed powderwas dried at 1008C for 24 h and calcined in the range 400to 10008C for 3 h.

Fig. 8 shows the XRD patterns of as-dried powderswith various calcination temperatures. The as-dried powderdid not contain the Li TiO phase but did contain TiO2 3 2Ž .anatase and small amounts of Li CO . A Li TiO phase2 3 2 3

Ž .appeared at 5508C, but TiO anatase and small amounts2

of Li CO were also observed along with Li TiO phase2 3 2 3

at that temperature. When the calcination temperaturereached 7008C, the observed phase was only Li TiO .2 3

These phenomena can be confirmed by the TGrDT analy-sis. The DTA curve shows a broad endothermic peak inthe range of 400 to 7008C, which is in agreement with thecalcining results above. Although Li TiO is produced at2 3

7008C in the solid state reaction, this reaction required along reaction time.

The shrinkage curves of the combustion synthesizedpowder were shifted toward a lower temperature thanthose of the solid state reacted one. Fig. 9 shows thesurface microstructure of a sintered Li TiO body using2 3

the combustion synthesized powder which was sintered at8008C for 3 h. This Li TiO sintered body has dense2 3

agglomerates which are formed by the inter-agglomeratesintering process. Each of the agglomerates consist of veryfine grains with a size of 0.3 to 0.5 mm. Large porespresent at the intra-agglomerates in the sintered bodydecrease the sintered density of the Li TiO powder com-2 3

pact. Therefore, an additional de-agglomeration processwould be necessary to increase the packing density andthus the sintering density.

4. Conclusion

Ultrafine Li TiO powders could be easily synthesized2 3

by a combustion reaction in a short time using glycine or amixture of urea and citric acid with a stoichiometriccomposition. A pure Li TiO phase was obtained by the2 3

simple process without further calcination reaction. Thesynthesized powder was very fine and equiaxed. The spe-cific surface area of the as-synthesized powder is 10 to 14m2rg and the primary particle size is about 30 nm.

A Li TiO body sintered at 8008C for 3 h has dense2 3

agglomerates which were formed by an inter-agglomeratesintering process. Each of agglomerates consist of veryfine grains with a size of 0.3 to 0.5 mm.

( )C.H. Jung et al.rJournal of Nuclear Materials 253 1998 203–212212

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