Synthesis of lanthanum aluminate by thermal decomposition of hydrated nitrates

Synthesis Thermal

M.D.

Shaji

of Lanthanum Decomposition Nitrates

Kumar,“T.M.

Srinivasan,a

1 INTRODUCTION 1.anthanun-i aluminate belongs to the pcrcrvskite ~>pc osidc materials and has attracted much altcntion for various applications in high frequency capacitors. ’ as gas sensors.’ and is considercd as an electrically insulating buffer layer in the form of lhin films on suMrate for the deposit c>f fcrroelcctric materials. I,’ Sin&z crysral lanthanum aluminate ( LaAIOI) is ;I promtsinb! substrate material for depositron of thin films of‘ rhe high temperature superconductors ~CXIUMZof ins good lattice match and dielectric properties.‘.” For practical applications, high T, \upcrconductIng tnin tilms on silicon or sapphire subztratcs wit11 ;I LaAI03 buffer layer would be more desirable than thin tilms on single crystal \ubstratc.The direct formation of LaA103 from aluminium oxide (AI,O;) and lanthanum oxide (La;OI) occurs typically in the range of 1500 170WC. with melting occurring at about I MO’ (_.’ This conventional ceramic method h! the direct combination of corrc$ponding ouidcs by calcining at high tcmperaturcx and ball-milling arc not adequate for man> and suffers from manq actvsnccd applications

Aluminate by of Hydrated

C. Subramanian”

& P. Ramasamyb

inherent shortcomings. Problem< ha\c .triscn with poor sintering behaviour and n~,i7h~)tno~eneit4, ujhich ha1.e ;I detrimental cfiect con the clcctrical and mcchanicnl propertics. Sc~crltl \\cr-chemical methods“ ” have been used to obtain bcttcl homogcncity and control 01‘ stoichiomctrk t-11w cra1 other researchers. Among ~hcse. ~~oprecipitation and sol gel arc the most wrdcl> Investigated for ceramic pouder preparation\. T‘hc po~dci obtained from the coprccipitarion tc’chnique is hi@l> ;tgglomeratcd in nature and t‘urther hallmilling is necessary to obtain tint po\+der\ with small particle size. The SIICCC~~of sol gel process; 111rho synthesis of ceramic powders is due. in particllllar. 10 the possibility of working at IOU ternpcraturc and attaining good homogenelt\ in :hc solution phase. Ne\,ertheless. for complex \:onlpoGtions the sol-~gel process is dclicatc to uw hccausc. in certain cases. it requires very specific control 01‘ pH concentration or sequence of addition. The all-alkoxide sol gel route requires either s!nthcsizing bi-- or tri-metallic alkoxidc:,. ;i proces\ that may be complicated and costI>. o? using alkoxide mixtures with inherent dit?jc.ultic\ caused by the mark4 differences in rcactivit>, of those

M. D. Slwji Kumur et al.

420

compounds. Since for commercial large scale production the synthesizing method has to be relatively simple and cheap, a cost-effective technique using readily available starting materials has to be developed. The main purposes of the present work were to synthesise fine particle LaA103 powder by the thermal decomposition of hydrated metal nitrates and to study the dielectric and the sintering behaviour of these powders.

2 EXPERIMENTAL 2.1 Powder preparation

Figure 1 shows the flow diagram of the experimental processing route for LaA103 powder. The starting materials A1(N0&.9Hz0 and La(NO&. 6HZ0 were all analytical reagent grade supplied by BDH, Bombay, India. Stoichiometric amounts of aluminium nitrate and lanthanum nitrate were allowed to dissolve in its crystallization water by gradually heating it up to 120 “C, until a clear solution is obtained. This solution was quenched at room temperature to avoid phase separation. The resulting solid mass was broken into coarse lumps and dehydrated for 24 h at a temperature of 8& 90°C and was transferred into a Pyrex beaker and heated at a temperature of 400 “C. The coarse mass decomposes to form the corresponding mixed oxides. The products were hand ground and calcined for 4 h at different temperatures (600, 650, 800, 900 and 1000 “C). Aluminium nitrate

Lanthanum nitrate

I

Melting

2.2 Characterization

methods

Simultaneous thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were carried out, using a Seiko TGA/DTA model 242 thermal analyser, for the quenched product before decomposition from room temperature to 1000 “C at a heating rate of 10°C min I, in nitrogen atmosphere. X-ray diffraction (XRD) patterns were recorded using a Rigaku X-ray diffractometer with Ni filtered CuK, radiation (1.5418 A) to determine the phases present in the calcined powders and to estimate crystallite size. The crystallite size was estimated from the X-ray line broadening of the (110) diffraction peak using the Scherrer formula:” D=-

0.9h fiCOS@

where D is the crystallite size in nm, h is the radiation wavelength, 8 is the diffraction peak angle and 10 tb& \JIOpp&+ZE8 LIVE XL&h s(T ~krh@-L&YK ~VTEWSlTb

hE

E&SIK ppC&&VtV’/

$oppuhcx-

+Ci~ppE\JytlW

+_!lp

tVCXpQlEVK%h

XCrO j.lC&E U0tV-y

jI = (h$,,$ - h’)“‘.:

TbkE

X~E~F

9appE.v

hr,h,, is the

line width at half-peak intensity related to LaA103 and h is the line width of the diffraction peak (004) of the metallic silicon which is used as the standard. The surface morphology of the synthesized powder was studied using a Hitachi S2400 model scanning electron microscope (SEM). The specific surface area of the powder was measured by the conventional BET technique with nitrogen adsorption. The particle size measurements were done using a Malvern Master sizer particle size analyser employing the laser diffraction technique. For sintering studies, the powders were uniaxially cold pressed at a compaction pressure of 200 MPa using 3% polyvinyl alcohol (PVA) solution as a binder. These pellets were sintered at various

T

Quenching

Exe t

c

Fig. 1. Flow

diagram

showing the LaAlO, powder.

processing

route

400 600 Temperature PC I-

for

Fig. 2. TGA-DTA

800

curve for the quenched

solid mass.

~cmpcraturcs

for sintcring studies. The green dencompacts were calculated from the dimensions of the samples and their weight and the sintcred dcnsit\, was determined by Archimedes principle. The sintercd pellcra were well polished and both the tliccs vkc’rz ccatcd with conducting silver paste. Dielectric constant and loss factor of LaAIO1 were mc*asured. using ;L HP 4 193A complex impedance .lnJ>scr. III the t’rcquenq range from 30 Hz to Ii MHz ;ti rc)c)m rcmperature. S~IIC\ of

the

3 RESULTS

AND

DISCUSSION

.3. 1 Thermai analysis TGiI and DTA curves for the \ol~d mass beforc decomposition from room temperature to 1000 (‘. It has been observed that there arc two major weight losscs. The first weight loss in rhc TGA correlating with the small endothermic peak in the DTA is because of the dehydration of \\.atcr. The second maior weight loss at about MO C‘. w,hich corresponds to the large endothcrmic DTA peak, i \ caused by the nitrate dccompo\ition. The DTA analysis also revealed ;I small cuc)thcrmic peak ;lt 650 ‘C. which was exactly the tcmperaturc of crystalline LaAIO? formation as t:igiIrc‘

7 hho\\b

the

indicated by X-ray analysis. Thr: TGA analysis did not show any weight change in (hi!, remperature ranye.

3.2 X-ray diffraction

analysis

The XRD patterns of the reaction product prcsented in Fig. 3 show how progrcssi\,ely the LaAIOT phase was formed with an increase in calcination temperature from 600’ C to IO00 c‘. The as-prepared powder was ?-ray amorphous. At 6.50 ‘C the powder became f‘eebl!, cr!.stnlline and further heating increased the intcnsit! of the X-ray peaks without any phase change. Powders calcined at 1000 ‘C showed the f‘u11\ cr>st:tlline structure of LaAIOJ. The lattic’c constants calculated were in good agreement with those reported in the literature.‘” 3.3 Surface area measurement Specific surface area (SHI..,) ol’thc I_aAIO1 powders calcined at different temperatures \vcrc tn the range 8 23 m7 g ‘, Table 1 shon,s the \.ariation of surface arca with calcination tcmperalure. It was observed that the surface area decreases as the catcination temperature increascx. 5incc t hc cr!,stallite size increases with temperarurc. 3.4 Particle size distribution The crystallite sizes calculated from XRD line broadening arc below I.50 nm. The mc;tn particlc size ( D)11_.,) obtained from surface area was c;I Iculated based on the equation.

where (I is the theoretical densi of the powder (5X60 k gm J ). ‘I Particle sizes calculated from the surface area are in the range of 30 170 nm. Variations of the particle size with cal#cinatmg temperature are listed in Table 1. The particle sizes calculated from surface area arc comparable with the crystallite sizes calculated from XRD lint Table

1. Variation

of with

surface area temperature

~~

Cm*!3 ') ~~ ~

(nm)

23 16 13 IO 8

45 64 79 102 128

Annealing CC

600 650 800 900 1000

'C)

&ET _

and

particle

D BET

size

M. D. Slwji Kurmu et al.

422

broadening. The particle size distribution of asprepared LaA103 (Fig. 4) shows that the average agglomerate particle size is less than 3 urn. 3.5 Microstructure

and densification

Figure 5 shows the SEM image of the as-prepared powder which reflects the agglomerate nature of the powder and shows irregular morphology. The green density was calculated to be about 30% of the theoretical density. The sintering density of the pellets heated at 1500°C for 15 h was determined to be 5390 kg mP3, which was about 92% of the theoretical density. The low sintered density may be caused by the agglomerative nature of the powders. Figure 6 shows the variation of sintering density with temperature. It was observed that the sintering density increases due to the breakdown of agglomerates by grinding. The powders ground for 10 h could be sintered to 96% of theoretical density.

(e’) and loss factor (tan&) with frequency (1og.h are shown in Figs 7 and 8. Both the dielectric constant and loss tangent decrease with increasing frequency and were measured to be 22 and 8x 10 4 at 10 kHz. respectively. These results were comparable with the reported values for the LaA103 single crystals.‘5 This relatively low dielectric constant and low loss tangent, makes lanthanum aluminate 100

60-l 55 1

i

/

I

1200

1300

1400

Temperature

3.6 Dielectric studies Fig. 6. Variation

Dielectric properties of LaAIOj were studied in the frequency range 30 Hz to 13 MHz at room temperature. The variations of the dielectric constant

J

1500

(“0

of the sintercd bulk density powder with temperature.

for

LaAIO?

23.0

22.5

I

Particle Fig. 4. Particle

IO

size (pm 1

size distribution of the LaAQ cined at 1000°C for 4 h.

powder

cal-

21.0

’

,

1

7

2.48

4.00

5.48

7.00

Log f Fig. 7. Variation

of dielectric

constant (P’) with frequency.

1 -)0.0001 2

3

4

5

6

7

Log f Fig. 5. SEM

image of the as-prepared

LaAIOs

powder.

Fig. 8. Variation

of loss tangent (tan&) with frequency

fl

4

CONCLUSION 7

s

‘)

ACKNOWI_EDGEMENT ()llC

,,I

(‘ouncil the

;I\+

rh:

authors

01‘ Scicntitic at-d 01

;I

REFERENCES

Senior

IO

(M.I>.S) ;md

is

Industrial

Rcscarch

grateful

to

Rcscarch

Fellowship.

the l’or I I

I7 I:

II

Ii

\uhstrate conductor 2679 HR.4NIII.E. perovsktte ./. ,\ltr/r,,‘. SANDt.. NFSFL.

I’or epttaxial thin lilnix.

growth .~/I/J/.

P/II~\

01’ Ittt_‘h tempt’ratttresup~ri’.,,ii.. 5.1 ( IYXX) 2677

C‘. I>. & FRAT ELLO. \’ .1. f’t-eparatton .. GRIGORIU. C‘. CY BI’NF SC’U. M. C’.. L.aAIOI thttt liltns deposited on \~licon atrd sapphtre ;15 huffcr layer\ I’oI- YHazCiiJ) I .I. tjdii7 St,;. /.~ii.. I.3 (IYW) I”7 1215 HOhDER. I. A. & VINO<;R/\I:tOh \. 2 1 . I’hasc ec~utlibrtum in the lanthanum t~xidc ~~lutn~na system. /Id/. .-lr d. SC i. I ‘SSR f Ey//.\/r tr’ coprectpitatton /_I.. -I/id. .\‘d SSSR. .\~~CJ/X!. l.fC//(,K. 2 (1%,6) IhOh 101 I KRYLOL’. \‘. S.. HELOVA. I. 1. hlAC;l NO\‘. Ii. L ._ KO%LO\‘. V. II. KAL.lh~‘ll~\KO. ,I. I’. & KROTKO. N. F. Prepartion 01’ rawcarlI alutn~nates front aqueott\ solution\. /:I, 1Xd ZlrtlX. .S.\‘.SR. 9 (10771 I3Xh I3YO Vll>\r’ASA<;.4R. K.. (;0~‘:2L:2Kl~IS~i~i.l~h. .I. B RAO. C’. h. R.. Synrhes~\ of cotnl~le\ metal ouidc\ ttstng h\droxidc. cyantde, and nitrdte vjltd ~oltt1ton prccurwr\ .I Scditl S/o/,, (‘lrrwt.. 58 (IY)tc5) ?Y :KLI’C;. II P. & 4lLEXANDER. I. I \-/&/~I D///rw ICOH/‘t~whw\. Wiley. New York. lctS4. pp hlh 703 (iFI LER. S. & H.4L.A. V. H.. (‘r\~lallo~rapllt~ \tuCl~cs 01 pero\skltc-like compounds: II. Raw C..II-111 ;tlumttt;ltc\ .~IC./C.’ (‘/.l~.\rtr//.. 9 I lY56, IO10 lO7i 1’011&I. d/ffvt~cl~w //iv. C‘ard no. .: I-22 .lotn t C‘cm~nttttee on I’o\+dcr Dilll-action Stanrlard\ .1( PIIS. Slat-thmorc. Pennsylvania. PIllLIPS. .I M.. SIE<;AL. M I’. \‘,ll\ I)O\‘ER. Ii. H.. TIEFI:L.. ‘I-. II.. MARSti,\Ll.. .I t1.. BR.45 IILl:. (‘. I>.. BtRKSTRESSF,R. <;.. SlRI:SS. /\. J,. t AlIE).. Ii. b.. SENC’CIP’I-Au. S. ( ASSAhHO. A B .IEf%SSl:N. 11. P.. (‘otttpartxcln OI‘ H,I,Y(‘~I~O~ thtn liltn\ grown on \ariott\ pc~o\\l\ttc \uhstt-ate\ h! cot’\ aporalton .I .\ltrtc,v. KC,.\ : ( I W)1) 2650 2657