It is also noted that the electrical resistivity increases with incrcabiiii; Si concentration... Therm al conductivity is large.[r]
Trang 1V N l ) Journal o f S c i c n c c , M athem a tics - Physics 25 (2009) 193*197
D o T h i K im A n h ' *, M a k i o K u ris u ^
^Facidiy o fF h w ii s, ỉỉíin o i Utỉivcrsily ofSciejw e VNU, 334 Nị^u\-en Trai, Thanh Xiuuì, Ha Noi Japan A dvanced ỉiìsíitute o f Science and Technology', School of M aterials Science.
Nomi, hhikawa 923 - 1292 , Japan
Received 2 O ctober 2009
A b s t r a c t T h e crystal structure and thcrm ocleclric propcities o f La(Fei.^Si,)i3 co m p o u n d s were investigated b y m eans o f X-ray p o w d e r dilTraction and clcctrical resistivity, th e rm o p o w e r and ihernial c o nduc tivity iucasarcm ents T h e single N a Z n |i-l> p c cubic structure phase is stabilized for the co m p o u n d s v\iih X = 0 1 2 , 0 1 4 a n d 0 1 5 I'hcse niagnclic phase transitions are also seen in the electrical resisiivily, th e rm o p o w c r and thermal conductivity m easurem ents All com pounds have the small values o f t h c r m o p o w c r and lattice conductivity H ow e ver, therm al conductivity is large
1 Introdu ction
rh e míìpn<Mir propcrtu's »>I I aT ii r r = l-'e nnH C o) ronip<)iinik o f the Na7nn-iyp<* cnhic stnictiire
have been intensively sludicd T hese co m p o u n d s have the largest am o u n t o f transition mclal in the cryslallmc formula unit am ong the rarc-carth transition intcrmctallics [1,2] T he cubic N aZ n n -ty p c structure IS easily s ta h ili/c d in the binary La-Co com pound I'or the I.a-Fc co m p o u n d , this structure can be formed only in pscudo-;>inary La(Fci,^M ,)i3 (M = Al, Si) com pouĩids [3] r h c m agnetic state in La(l*Cj.,Al,)u c o m p o u n d s IS íciTomagnctic for 0.14 < t < 0.38, and antifcrrom agnetic for 0.08 < X < 0.14 |4 | l.ad'Ci ,Siv)i3 com|)t)unds are fcưom agiietic in the region 0.14 < V < 0.38 However, their
C urie temperature 7’c dccrca s w ith increasing Fe concentration, w hereas the saturation magnetic
m om ent increases [1| I'or ilicsc La(F ei.,S i,)i3 com pounds, it w as reported that in the high Fe concentration region, an itiiiciant-electron m etam agnetic (1ÍỈM) transition, i.e a fieM-induced first- ordcr param agnctic-fcrrom aunctic transition, accom panied by a large negative lattice expansion, appeared just above the C u n c temperature It is interesting to m ention that the pseudo-binary La(Fe| ,M^)i3 com pounds w ith M - Si and A1 exhibit a giant m agnctoslriction effect, w h ich is prom ising for applications [5]
Magnetic properties have j c c n extensively investigated for La(Fei-iSir)i3 c o m p o u n d s (x = 0.12, 0.14 and 0.15) In these coniỊ.vmnds, an itinerant elcclron n.olamagnetic (lE M ) transition near Tc has
'C o rre s p o n d in g author- Tcl.: 84-9:'-:‘ 43849
i:-mail: k im a n h 7 2 ^ g n ì a i l c ơ i n
193
Trang 2194 D T K Anh A/ K urisii \ 'NIJ J o u n u il o fS a c n c c \ỉ,iĩhị.Ịiìiitii S - PhvMí S 25 ị2(Uì9) Ị9 3 - I 9 7
b c c n d e m o n s t r a t e d | 6 | ' H i c Ỉ H M I r a n s i l i o n 1S c l o s c l y r c l a l c d t o t h e l a r e c p o s i t i v e c u r \ a t u r c í ) t ' t h e
density o f State (D O S ) ut xìiC 1-cmii level m the com pounds |7 | , ĩhcrcỉorc u c can cxpccl that the
La(ỉ*Ci,^Si()i3 compounds Ịtosscss a laruc thcrniopowcr (Sccbcck cocnicicnt) A small phonon thermal
conductivity is also cxpcctod S ince the com pounds have ihc N aZ iin structure in which 1 12 a to m s arc accom m odatcd in the unil ccli It is also interesting to exam ine the ihcrm oclcctric bchavỉor n e a r the
C u r i e t e m p e r a t u r e i n t h e \ ) u n d s I n t h e p r e s e n t s t u d y , t l i c i h c i m o p o w c r , c l c c l n c a ! r e s i s t i v i t y a n d
thermal conductivity o f La(ỉ S i,)i3 com pounds have been !n\cstig atc d below room temperature
2 E xp erim en tal
The La(Fci ,S i,)i3 c o n ;p c ;n Js (-V = 0.12, 0.14 and 0.15) have been prepared by arc-m elting the appropriate am ounts o f high purity o f La with 99.9” o, I'C willi 99 9 9 % and Si with 99.999% in purified
Ar atmosphere I'h c ingoU' were sealed into cvacuatud lubes and the heal treatm ent for hom ogenization w as e a rn e d I ait at 1 1 0 0 ° c for 1 week
The X - r a y d i f f r a c t i o n ( \ R J ) p a l l c r n s u s e d to d e t e r m i n e i h c i r c r y s ta l s t r u c i u r c p a r a m e t e r s w e r e
c o l l e c t e d b y R i g a k u R m t - 2 ')0 () w i t h C u K a T h e t h c r n i o p o v v c r , c l c c l n c a l r e s i s t i v i t y a n d t h e r m a l conductivity w ere m easured b u s i n g a Q uantum Design P P M S in the Icmpcralurc range from 5 K to
300 K
3 R esults and d iscu ssion
10000
ci
8000
6000
c/)
c
^ 4000
2000
0
NaZiij^
A _ J l
A k
Tlie La(l'C|_^Si^) 13sam ples
x = 0 1b
x = 0 1 4
_A.
x = 0 - 1 2
I I I I I I I I I ■ I I >1 II nil II II II II II ml nil II '1
J _! _i -1 -1 -1 -1 1
2 0 (dcg.) Fig 1 The X-ray diffraction patterns of La(I C| ,Si,)i3 compounds
Fig 1 show s the X R l patL-ms o f the La(Fei-xSi,)i3 {x = 0.12, 0.14 and 0.15) com pounds X -ray
diffraction confirm s that lh« Solid solution o f La(Fe, ,Si^)i3 c o m p o u n d s crystallizes in the cubic
N a Z n n - type structure w i'h sp a c e group I-m 3 c T he latticc paran.eters o f the co m pounds are listed in Table 1
Trang 3D T K Anh M K iư isu ^ \ su J o u n ia ! o f Science M a th em a tics - P hysics 25 (2009) Ị 9 3 - Ị9 7 195
Table 1 1 he liicrmoclcctric properties of La(Fci.^Si J n compounds and other thermoelectric materials at room temperature
C o m p o u n d i / ( A ) / r ( K ) a {ịiW :K ) p ( ụ í ì cm ) \ ' ( W / K m) ZT
I'hc tem perature dcpendciicc o i'lh c eleclrica! resislivitv (/?) in the La(f'ei.^Si,)i3 (.V = 0.12, 0.14 and 0.15 I sam ples is show n in Fig 2 N ormal metallic behaviour is seen all the com pounds T he electrical resistivity decreases rapidly below the magnetic transition in i.a(F*’e i - t S i c o m p o u n d s due to the freezing o f spin disorder contribution to clectrical resistivity It is also noted that the electrical resistivity increases with incrcabiiii; Si concentration I hc room tem perature electrical resistivity
d ecreases from 159 for A' - 0 15 dow n to 146.4 |j.Q cni for r = 0.12
180
Í 160
>
I 140
(/)
re
o
% 120
o
100
Tem perature (K)
rig 2 Tem perature d e p e n d en ce tti'ilic electrical
resistivity o f L a ( F c i ,S i J i i com pounds
Temperature (K)
F-'ig 3 rcnipcralurc d ep e n d e n c e o f the therm opow e r o f
La{Kci.,Si J |3 com pounds
l ig 3 sliows llic Iciiipcraiurc d ependence o f the therni0|)0Wcr (ư ) in the l.a(['Ci,vSi()i3 ( a * = 0.12,
0.14 and 0.15) com pounds Ali \h : com pounds have ncuuiive thcrmopovver, indicating the n-type
nature o f these materials At room temperature, the ihe rm opow e r o f all tlic co m pounds is ỚT = " 5.5
A growth o f the ỊK‘aV is t'ou:id below Z’f The difference in the value betw een the ferromagnetic
and paramagnetic states is 21 % 18% for ,Y = 0 1 2 and 0.14, respcclivcly
Finally, the thermal co ndik l A itv (v) o f La(Fei,tSi^)ii (A = 0.12, 0.14 and 0.15) co m pounds is shown in i'iii 4 For general, ih.- ilicrmal conductivity o f a material can be described as: v ( 7 ) “ (7) ^ Vpi^ (7), w here and a f j the electronic conductivity and the lattice thermal conductivity,
Trang 4196 /) 7\K Anh M K u risu / V'N V J o u r n a l o f Science M ulhcm aiics - P hysics 25 {2009} ì^ỉ.^-ỉ^)
respectively Tlic lattice tlieriiial conductivity value, Vpii, can be estim ated by subtractiim tỉic electronic contribution k ' q \ from tlie total thermal condiiclivitv a; w here Kf^\ is related with the elcclrical resistivity a c c o rd in g to llic \V ie dem ann-ỈT a nz law Kị.\ = LoT/p, w here Li) is the l.o rcn /
num ber 2.45 X 10'^ W Q K "^ T he value o f AT o f all ihc co m pounds is lai'uc (sec Table 1) The \*p|i contribution to AT is 30 % (inset o f Fig 4) O nlv a small increase is found iii its value at /'(■
Temperature (K)
Fig 4 'r c m p c ra u ir e d e p c n d c n c e o l'ih e thermal conductivity o f La(i-'ej.jSi J i3 c om pounds
I'he therm oelectric p ro p erties o f l.a(l''e|.(Si,)|-Ị co m pounds at room lempc'c.turc arc listed in I'ablc
1, together with Ihc data OÍ o th e r typical thcniioelcclric materials O ur co m p o u n d s have rclativjly
la r g e r t h e r m a l c o n d u c l i v i t y t h a n llic r c f c r e n c c s F u r t h e r m o r e , i h c v a l u e o f i h c r i i i o p o w c r a n d c ic c ric rcsistivily are sm aller than those o f o th e r ihcrmoclcctric materials 'ĩh c iluurc 0 Ỉ' liicrit (Z7), which is defined bv Z7 = ccT!pK\ is found to be very small (see Table 1).
4 C onclusion
The structural and th e rm o c lc c tric properties have been investigated in l.a(ĩ'ei„Si,)i, com pounJs
T he following conclusion can be d ra w n from this study:
- The La(Fci.;(Si,)i-i c o m p o u n d s liavc a cubic NaZrii3- type cr>'stal structure
- The thcrm oclectric p ro p erties o f La(Fe|.jSi^)i3 co m p o u n d s have been investigated below 300 K The values o f th e rm o p o w c r and lattice conductivity are small Therm al conductivity is large The dim ensionless figure o f m e rit (Z 7 ) is very small
A ck n o w led g m en ts, I'h is w o rk w a s supported by the Vietnam N ational U niversity (V N U ) research
program under the grant No Ọ T-09-1 5
Trang 5D T.K Anỉi A/ K lih su / VNU J o u n u d o jS c ic iic e , M iithem atics * P h y sic s 25 (2009) Ỉ 9 3 - Ỉ 9 7 197
Rcl’e r c n c e s
111 ÍM Kr i f i y a k c v i c h , ( ) s / a r c c h n y u k , H I ( j l a d v s h c \ s k y , 0 1 B o d a k , z A n o rịị ( h e m 3 5 8 ( 1 9 6 8 ) 90.
| 2] T T M I’alsl ra, J A \ t \ d o s h , Ci J N i c u w c n h u y s A M V a n d c r K r a a n K H J B u s c h o w , J S íiiịỉn SíciịỊn \fa te r
í l ‘;8.V)2VO
ị 3 | T I ,M P u M r a , (;,J, N i c u w c n h u y s , J.A My d o s l i , K 11J B u s c h o v v , / A p p ỉ r i i y s 5 5 ( 1 9 8 4 ) 2 3 6 7
| 4 j T T M Pal s l r a, G.J, Ni c u \ K c n h u y s , J.A M y d o s h , K H J B u s c h o v v , / % 5 R c v lì 31 ( 1 9 8 5 ) 4 6 2 2
| 5 | A Fuji la, K F u k a m i c h i //:/:/■: 35 {1999) 1 796.
[6] A Fuji ta V A k a m a l s u , K F u k a m i c h i , J A p p L Pltys 8 4 ( 1 9 9 9 ) 4 7 5 6
ị 7 | M , Cy r o t , M L a v a g n a , / A p p ì r h y s 5 0 ( 1 9 7 ' ) ) 2 333.
[S| J p FIcur i al , P r o c -Vi''7'“ 93 ( I V93) ỉ c c t u r c 3.
l ‘; i G JciTrcy S n y d c r T Ca i l ! a t J, p í - ỉ c una l Aíiií / t e ỉ*roc 545 ( 1 9 9 9 ) 339.