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STRUCTURE AND PROPERTIES OF NANOPHASE TiO2
R. Siegel, H. Hahn, S. Ramasamy, L. Zongquan, L. Ting, R. Gronsky
To cite this version:
R. Siegel, H. Hahn, S. Ramasamy, L. Zongquan, L. Ting, et al.. STRUCTURE AND PROPER- TIES OF NANOPHASE TiO2. Journal de Physique Colloques, 1988, 49 (C5), pp.C5-681-C5-686.
�10.1051/jphyscol:1988589�. �jpa-00228086�
STRUCTURE AND PROPERTIES OF NANOPHASE TiO,
R.W. SIEGEL, H. H A H N ( ~ ) , S. R A M A S A M Y ( ~ ) , L. Z O N G Q U A N ( ~ ) , L. T I N G ( ~ ) and R. GRONSKY*
Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, U.S.A.
* ~ a t i o n a l Center for Electron Microscopy, Lawrence Berkeley
Laboratory, University of California, Berkeley, California, U.S.A.
~6sumG
-
-Echantil lons de Ti0 ( r u t i l e ) nanophas6 de grains ul t r a f ins ont 6t6 synthetises par l a metFode,&
condensation dans un gaz, s u i v i e ensuite par compaction.en-situ, e t etudies par microscopie electronique en transmission, par microdurete de Vjckers, e t par spectroscopie d4a_nnihila;ion positronique en fonction de temperature de f r i t t a g e . La denslte des echantillons augmente rapidecment au-dessus de 5 0 0 ~ C ~ a v e c seulement yne legere c_roissance de grains. La durete obtenue par, c e t t e methode, effectuee aux temperatures 400-600'~ plus basses que l a temperature de f r i t t a g e conve?tionnel e t sans avgir besoin des additives de f r i t t a g e , e s t comparable on superieure FI c e l l e des echantillons de gros grains.Abstract
-
Ul t r a f i ne-grai ned, nanophase samples of Ti02 ( r u t i 1 e ) were synthesized by the gas-condensation method and subsequent in-situ compaction, and then studied by transmission electron microscopy, Vickers hardness measurements, and positron annihilation spectroscopy as a function of s i n t e r i ng temperature. The nanophase compacts densified rapidly above 500°C, with only a small increase in grain size. The hardness values obtained by t h i s method are comparable t o or greater than coarser-grained compacts, b u t a t temperatures 400 t o 6 0 0 ~ ~ lower than conventional s i n t e r i n g temperatures and without the need f o r s i n t e r i n g aids.1. INTRODUCTION
The gas-condensation method [I-31 f o r the production of small p a r t i c l e s i n t h e s i z e range of 1 t o 50 nm has recently enabled the synthesis of a new c l a s s of ul trafine-grained materials by the in-situ compaction and s i n t e r i n g of these p a r t i c l e s [41. The resulting nanophase materials, which may contain c r y s t a l l i n e , quasicrystal 1 i n e , or amorphous phases, can be metals
,
ceramics, or composites with rather d i f f e r e n t and improved properties than normal coarse-grained polycrystall ine materials. The work so f a r done on these new materials and t h e i r potential f o r the future have been recently reviewed [5,61. Some advantages of nanophase ceramics should be: ( i ) Their small p a r t i c l e s i z e during synthesis should a l l ow f o r increased s i nterabil i t y a t 1 ower temperatures and smaller residual pore s i z e s owing t o a combination of high driving forces and s h o r t diffusion distances, avoiding the need f o r s i n t e r i n g aids. ( i i ) The exceptional physical and chemical control available i n the gas-condensation method l e t s the p a r t i c l e surfaces be maintained clean a1 1 owing subsequent high grain-boundary purity and thus negligible i n t e r f a c i a l phase formation. ( i i i ) The large fraction(Qresent address: Materials Research Laboratory, University of I l l i n o i s , Urbana, I l l i n o i s USA.
(%emanent address: Department of Nuclear Physics, University of Madras, Madras,
$:g:;ent address: t n s t i t u t e of Sol id State Physics, Academia Sinica, Hefei PRC.
h e m a n e n t address: I n s t i t u t e of Low Energy Nuclear Physics, Bei j i ng Normal University, Bei jing PRC.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988589
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of atoms residing i n i n t e r f a c e s , almost one-half in the case of a 5 nm grain s i z e , may allow f o r new atomic arrangements and t h u s novel and improved ceramic properties. Such properties may include, f o r exampl e , mechanical properties which might be improved through higher grain-boundary purity and the absence of b r i t t l e phases therein or small grain s i z e s allowing f o r more e f f i c i e n t deformation mechanisms and more e f f e c t i v e crack dissipation.
In order t o explore the f e a s i b i l i t y of creating nanophase ceramics with such improved properties, ul t r a f i ne-grained nanophase Ti02 ( r u t i l e ) samples were synthesized i n the present work by the gas-condensation method and then studied by a variety of techniques as a function of s i n t e r i n g temperature. The desi rabil i t j r of small and uniform p a r t i c l e s i z e f o r obtaining qua1 i t y ceramics has been well documented f o r ceramics in general C71 and f o r Ti02 s p e c i f i c a l l y [8,91. Nanophase ceramics, w i t h t h e i r high grai n-boundary purity and very small and rather uniform p a r t i c l e s i z e , are expected t o s i n t e r a t lower temperatures than normally avail able ceramics with l a r g e r p a r t i c l e s i z e s , and t o exhibit improved properties C61. Consequently, f o r comparison with the nanophase samples
,
coarser-grai ned samples were a l s o synthesized from commercial powders and similar measurements were carried out.2. EXPERIMENTAL PROCEDURE
The ultra-high vacuum chamber used f o r the preparation of nanophase Ti02 by the gas-condensation method has been described el sewhere [6]. Titanium (99.7% pure) was evaporated from a resistance-heated tungsten boat a t temperatures between 1550'~ and 1650'~ into a 0.3-0.7 kPa helium atmosphere over a period of 15 t o 30 min. The small Ti p a r t i c l e s formed by condensation in the He-gas were deposited on the cold-finger of the production chamber, and subsequently oxidized by the introduction of about 2 kPa of oxygen i n t o the chamber. The p a r t i c l e s were then compacted a t 1.4 GPa a t room temperature, resulting in a Ti02 nanophase compact of 9 mm diameter by about 0.2 mn thick with a mean grain diameter of 12 nm.
For comparison with the nanophase Ti02, samples were also synthesized from commercial Ti02 powder, which was ball-milled using NiO b a l l s t o an average grain s i z e of 1.3
m,
with a maximum grain s i z e of 2.5m.
Three samples were made from t h i s commercial powder. The f i r s t was compacted a t 1.4 GPa a t room temperature without any s i n t e r i n g aid, as a d i r e c t comparison with the nanophase sample, whi'le the second was compacted a t the same pressure, but using a 5%aqueous solution of polyvinyl alcohol (pva) as a s i n t e r i n g aid. The t h i r d sample was compacted a t 0.1 GPa w i t h the same pva solution, t h i s method being e s s e n t i a l l y t h e conventional one f o r the preparation of such a ceramic. The d e n s i t i e s of the green p e l l e t s ranged from 55 t o 70% of theoretical density.
Grain-size d i s t r i b u t i o n s were determined in the nanophase samples by transmission electron microscopy (TEM)
,
and i n the coarser-grai ned samples by scanning electron microscopy (SEMI. Vickers microhardness was measured a t room temperature, using a load of 15 g and an indentation time of 25 s , on the as-compacted samples and a f t e r si nteri n.g successively f o r one-ha1 f hour a t temperatures up t o 1 4 0 0 ~ ~ . Coniplementary positron annihilation spectroscopy (PAS) 1 ifetime and Doppler-broadening measurements were a1 so made in order t o monitor sample porosity as a function of s i n t e r i n g up t o 900°C. In addition, high-resol ution electron microscopy (HREM) observations of grain and grain-boundary structures were also carried out on selected nanophase samples using the I\tomic Resolution Microscope a t the ~ a t f o n a l Center f o r Electron Microscopy, LBL. An operating voltage of 1 MeV was chosen t o provide b e t t e r penetration of some of the thicker p a r t i c l e s while retaining resolution a t t h e 0.16 nm l e v e l . No attempt was made t o o r i e n t the individual p a r t i c l e s under the electron beam; instead, the samples were scanned f o r l a r g e t h i n areas and imaged in through-focus s e r i e s bracketing the minimum contrast[lo]
condition.250- 1 1 1 1 1 1
200-
I
-
150-N
100
-
5 0 -
0 0 5 10 15 20 25 30 35
-
GRAIN SIZE (nm)3h=
P
I I I II
ISINTERING TEMPERATURE ("C)
F i g u r e 1. Average g r a i n s i z e o f nanophase Ti02 ( r u t i l e f as a f u n c t i o n o f s i n t e r i n g temperature determined by TEM. The s i n t e r i n g anneals were one-half hour i n d u r a t i o n a t each successive temperature. The g r a i n - s i z e d i s t r i b u t i o n f o r t h e as-compacted sample i s shown i n t h e i n s e t .
3. RESULTS AND DISCUSSION
The r e s u l t s o f the g r a i n - s i z e determinations using TEM on the nanophase Ti02 as a f u n c t i o n o f s i n t e r i n g temperature a r e presented i n F i g u r e 1. It can be seen t h a t t h e g r a i n - s i z e d i s t r i b u t i o n f o r t h e as-compacted sample i s r a t h e r narrow and t y p i c a l o f t h e p a r t i c l e - s i z e d i s t r i b u t i o n produced i n t h e gas-condensation method
[21. The d i s t r i b u t i o n appears t o remain unchanged by t h e T i o x i d a t i o n and subsequent compaction processes. It can a l s o be r e a d i l y seen t h a t t h e average g r a i n s i z e increases very l i t t l e up t o about 550°C, and o n l y r a t h e r s l o w l y w i t h s i n t e r i n g temperature up t o about 800°C, a t which temperature g r a i n growth becomes f a i r l y rapid. A s i m i l a r g r a i n - s i z e s t a b i l i t y a g a i n s t temperature has been found f o r n a n o c r y s t a l l i n e i r o n w i t h an i n i t i a l average g r a i n s i z e o f 6 nm
C111.
F i g u r e 2 shows a h i g h - r e s o l u t i o n e l e c t r o n micrograph from a r a t h e r t y p i c a l r e g i o n o f the nanophase Ti02 sample s i n t e r e d f o r one-half hour a t 500'~. The g r a i n s here are seen t o be e s s e n t i a l l y equiaxed w i t h r e l a t i v e l y p l a n a r boundaries. The g r a i n boundaries i n the as-compacted sample, on t h e o t h e r hand, appeared t o be s i g n i f i c a n t l y l e s s p l a n a r than t h i s , b u t d e t a i l e d atomic s t r u c t u r a l s t u d i e s o f these nanophase boundaries have n o t y e t been completed. The l a t t i c e f r i n g e s seen i n F i g u r e 2 are those r e p r e s e n t a t i v e o f t h e atomic planes o f r u t i l e , as confirmed by b o t h e l e c t r o n and X-ray d i f f r a c t i o n p a t t e r n s on a l l o f these samples.
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Figure 2. High-resol ution transmission e l e c t r o n micrograph of nanophase Ti02 ( r u t i l e ) a f t e r s i n t e r i n g f o r one-half hour a t 5 0 0 ~ ~ . The sample was prepared f o r TEM observation by f r a c t u r i n g t h e s i n t e r e d compact, which gave r i s e t o t h e open areas seen i n t h e micrograph.
The Vickers microhardness measured a t room temperature is shown i n Figure 3 as a function of s i n t e r i n g temperature f o r t h r e e d i f f e r e n t Ti02 ( r u t i l e) samples, t h e nanophase si~mple with 12 nm i n i t i a l average grain s i z e compacted a t 1.4 GPa, and two samples with 1.3 p i n i t i a l average g r a i n s i z e compacted a t 1.4 GPa and 0.1 GPa. The l a t t e r sample, prepared e s s e n t i a l l y i n accord with standard ceramic processing methods, was t h e only one of t h e s e t h r e e which was s i n t e r e d with t h e a i d of pva. The r e s u l t s f o r a f o u r t h sample prepared i n a s i m i l a r manner with pva, but compacted a t 1.4 GPa, a r e not shown i n Figure 3; they a r e very s i m i l a r t o those f o r t h e 1.3 urn, 0.1 GPa sample, but s h i f t e d t o lower temperatures by about 150°c. I t can be r e a d i l y seen from t h e s e microhardness measurements t h a t t h e nanophase Ti0 s i n t e r s a t considerably lower (between 400 and 6 0 0 ~ ~ ) temperatures than $he commercial 1.3 fl powder with t h e a i d of pva, y i e l d i n g comparable o r g r e a t e r microhardness values
.
For r e f e r e n c e , t h e Vickers microhardness of a s i n g l e c r y s t a l of Ti02 measured under i d e n t i c a l conditions is 1036+
66 .kcqf/mm2. Preliminary f r a c t u r e toughness s t u d i e s on t h e s e samples, made by measurina t h e crack lengths emanating from microindentations a t higher l o a d s , appear t o confirm t h e s i m i l a r o r b e t t e r mechanical p r o p e r t i e s of t h e nanophase Ti02 i n c o n ~ ~ a r i s o n with t h e coarser-grained material and s i n g l e - c r y s t a l Ti02 ( r u t i l e ) a s we1 1 . Without t h e a i d of pva, t h e commerci a1 powders a r e seen t o s i n t e r r a t h e r poorly and e x h i b i t i n f e r i o r mechanical p r o p e r t i e s , as expected.Although i t seems apparent from t h e microhardness measurements t h a t d e n s i f i c a t i o n of t h e nannphase sample was taking p l a c e upon s i n t e r i n g above 500°c, PAS measurements were a l s o c a r r i e d out i n o r d e r t o monitor t h i s d e n s i f i c a t i o n more d i r e c t l y . The r e s u l t s , which w i l l be published elsewhere by t h e p r e s e n t authors i n a more complete account of t h i s study of nanophase Ti02, show i n t h e i r simple two-state behavior t h a t both t h e 12 nm, 1.4 GPa nanophase sample and t h e 1.3 um, 1.4 GPa commerci a1 -powder sample ( s e e Figure 3 ) began densifyi ng rapid1 y above 500°c, but t h a t t h e nanophase sample did so more r a p i d l y with i n c r e a s i n g
d e n s i t y a t 9 0 0 ~ ~ . As m i g h t have been expected, t h e PAS l i f e t i m e measurements, which are a l s o s e n s i t i v e t o v a r y i n g pore s i z e s when they a r e small, c l e a r l y i n d i c a t e s m a l l e r pore s i z e s i n t h e nanophase sample r e l a t i v e t o the coarser- g r a i n e d sample a t a l l t h e s i n t e r i ng temperatures i n v e s t i g a t e d by PAS.
The r e s u l t s o f these f i r s t i n v e s t i g a t i o n s on nanophase Ti02 i n d i c a t e t h a t these compacts, a1 though already r a t h e r we1 1 bonded on compaction a t room temperature, d e n s i f y r a p i d l y above 500°c, w i t h o n l y a small increase i n g r a i n size. The hardness values obtained by t h i s method are comparable t o o r g r e a t e r than those o f s i n g l e - c r y s t a l T i 02 o r c o a r s e r - g r a i ned compacts, b u t a t temperatures some 400 t o 6000C l o w e r than conventional s i n t e r i n g temperatures and w i t h o u t t h e need f o r s i n t e r i n g aids. Much work s t i l l needs t o be done regarding t h e c h a r a c t e r i z a t i o n o f nanophase ceramics and the e l u c i d a t i o n o f t h e i r f u l l p o t e n t i a l . However, t h e r e s u l t s o f t h i s f i r s t study appear t o h o l d considerable promise f o r the f u t u r e o f nanophase ceramics.
F i g u r e 3. Vickers microhardness i n kgf/mm2 o f Ti02 ( r u t i l e ) measured a t room temperature as a f u n c t i o n of one-half hour s i n t e r i n g a t successively increased temperatures. Results f o r a nanophase sample (squares) w i t h an i n i t i a l average g r a i n s i z e o f 12 nm compacted a t 1.4 GPa are compared w i t h those f o r coarser- grained compacts w i t h 1.3 pm i n i t i a l average g r a i n s i z e s i n t e r e d a t 0.1 GPa w i t h (diamonds) and a t 1.4 GPa w i t h o u t ( c i r c l e s ) t h e a i d o f p o l y v i n y l alcohol from commerci a1 powder.
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ACKNOWLEDGEMENT
This work was supported by t h e U . S. Department of Energy, BES-Materials Sciences, under Contracts W-31-109-Eng-38 a t ANL and DE-AC03- 76SF00098 a t LBL..
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