Preparation of nanostructured TiO2ceramics by spark plasma sintering

1
Preparation of nanostructured TiO2ceramics
byspark plasma sintering
???? ??? ? ? ?
2
Contents

1. Spark plasma sintering
2. Abstract
3. Introduction
4. Experimental
5. Result and discussion
6. conclusions

3

• ? ?
• ?? ???? ??(Spark Plasma Sintering) ??(??) ???
  ??? ??? ?
• ???? ??? ???? ??? ???? ???? ???.
•   
• ?? ???? ??? ??? ?????(Hot Press)? ???,
• ????? ? 1/31/5??? ???? ??? ??? ???
• ??? ??
• ?????? ??
• ????? ?? ??? ???? ??
• ??? ??? ???? ??
• High Speed ?? ?? ? ?? ??? ?? ??
• . ? ??? ???? ??? ???? ?????? ??? ? ?? ??? ???
• ?? ???? ?? ??? ???? ?? ??.

1. Spark Plasma sintering
4
2. ??     ?? ???? ?? ????? ???(???) ????
??? ????? ???? ???? ????, ??????? ??? ?????
???? ?? ????(?????? ????? ?????? ????? ????
??)? ?? ???? ??? ? ???? ??? ????? ??? ???.
???? 2000? ??? ?????? ??? ??? ??? ??? 200500?
?? ?? ?????? ?? ? ????? ???? ????? 520? ???
???? ??? ????.
5
(No Transcript)
6
(No Transcript)
7
On-Off ?? ?? ????? ?? ???
P
Pulse Current
Powder
Thermo-couple
On-Off DC Pulse Current
Die
Punch
Electrode
P
8
(No Transcript)
9
2. Abstract
The effect of spark plasma sintering (SPS) on
the densification of TiO2 ceramics was
investigated using a nanocrystalline TiO2 powder.
A fully-dense TiO2 specimen with an average grain
size of 200 nm was obtained by SPS at 700 C
for 1 h. In contrast, a theoretical density
specimen could only be obtained using
conventional sintering above 900 C for 1 h with
an average grain size of 12 ?.
10

• 3. Introduction
• There is an increasing interest in
  nanocrystalline TiO2 both in powder and ceramics
  forms.
• The highly active surface of a nanocrystalline
  TiO2 powder plays a key role in its catalytic
  1, photocatalytic1, and gas-sensing
  properties 2. Moreover, a nanocrystalline
  powder can provide fast densification kinetics
  with a lower sintering temperature 3.
• preparing a fully-dense fine-grained TiO2
  specimen is difficult
• ? the grains grow rapidly at the later
  stages of sintering 6,7. The sintered
• microstructure is largely determined
  by the powder characteristics and the
• initial microstructure

11
Other approaches are the employment of a new
sintering process including phase-transformation
assisted sintering 9, hot pressing 1012, and
millimeter-wave radiation 7. The key approach
is to lower the sintering temperature without
deteriorating the densification. Spark plasma
sintering (SPS) is a newly developed sintering
process that makes use of a microscopic electric
discharge between the particles under pressure
13. This has been acknowledged to reduce the
densification temperature to a great extent with
a minimum grain growth. In this report,
fully-dense, TiO2 ceramics with an average grain
size of 200 nm was prepared by SPS using a
commercially available nanocrystalline powder at
700 C, and the results were compared with that
of a conventionally sintered specimen.
12

• 4. Experimental
• The starting material a high purity
  nanocrystalline TiO2 powder (P25, 70
  
• anatase and
  30 rutile, Degussa Co., Frankfurt,
• Germany)
  with an average particle size of 20 nm.
• A powder compact was prepared by the uniaxial
  pressing of 2.2 g powder at 5 MPa, which was then
  placed into a 10 mm graphite die.
• An electric current 1000 A was applied under
  a pressure of 62 MPa.
• The heating rate 150 C/min.
• The sintering temperature range 600 to 800C
  for up to 1 h
• Conventional sintering was also conducted for a
  comparison. The powder compact was isostatically
  pressed at 100 MPa and then sintered at 6001000
  C for 1 h.

13
5. Results and discussion

• Relative density

• conventional sintering
• fully dinsified 900 C for 1 h.
• Spark plasma sintering
• 99 at 700 C. for 1h.
• high density 600700 C for 5min.

Comparison SPS lowers the densification
temperature by approximately 200 C.
Fig. 1 shows the relative densities of the
specimens prepared by SPS and conventional
sintering as a function of the sintering
temperature.
14

• XRD pattern

• only the rutile phase
• no phase change with further heat treatment.

• main phase anatase with a minor amount
• of rutle phase
• similar to the starting powder.

Fig. 2. XRD patterns of (a) TiO2 specimen
prepared by spark plasma sintering at 600 C
for 5 min and (b) TiO2 powder calcined at 600 C
for 5 min.
This suggests that heat treatment at 600 C
for 5 min is insufficient for inducing a
phase -transformation without any applied
pressure.
15

• Fractured surface

Fig. 3. Fractured surfaces of the TiO2 specimens
prepared (a) by spark plasma sintering at 700 C
for 1 h and (b) by conventional sintering at
900 C for 1 h.
The grain size was noticeably different
depending on the sintering method used.
16

• Average grain size

• 600, 650, and 700 C
• SPS for 1h 160, 170, and 200 nm
• SPS for 5min similar grain size

• 900 C, 1000 C
• Conventional sintering larger than 1?
• ? significant growth occur

Fig. 4. Average grain size of the TiO2 specimens
prepared by spark plasma sintering and
conventional sintering.
17
6. Conclusions TiO2 ceramics with an average
grain size of 200 nm could be densified to 99 of
the theoretical density by SPS at 700 C for 1 h
under a pressure of 62 MPa. However, a
fully-dense specimen could only be obtained above
900 C for 1 h by conventional sintering where
the average grain sizes ranged 12 um. The SPS
process was effective in obtaining
fully-densified TiO2 ceramics with a minimum
grain growth at a low sintering temperature.
18
References 1 B. Levy, J. Electroceram. 1
(1997) 239. 2 M. Ferroni, M.C. Carotta, V.
Guidi, G. Martinelli, F. Ronconi, M. Sacerdoti,
E. Traversa, Sens. Actuators B 77 (2001) 163. 3
J.-G. Li, T. Ikegami, J.-H. Lee, T. Mori, Acta
Mater. 49 (2001) 419. 4 H. Hahn, S. Averback,
J. Am. Ceram. Soc. 74 (1991) 2918. 5 S.
Demetry, X. Shi, Solid State Ionics 118 (1999)
271. 6 I.-W Chen, X.-H. Wang, Nature 404 (2000)
168. 7 Y. Bykov, A. Eremeev, S. Egorov, V.
Iranov, Y. Kotov, V. Khrustov, A. Sorokin,
Nanostruct. Mater. 12 (1999) 115. 8 D. Lee, S.
Yang, M. Choi, Appl. Phys. Lett. 79 (2001)
2459. 9 K.-N.P. Kumar, K. Keizer, A.J.
Burggraaf, T. Okubo, H. Nagamoto, S. Morooka,
Nature 358 (1992) 48. 10 S.-C. Liao, K.-D. Pae,
W.E. Mayo, Mater. Sci. Eng. A 204 (1995)
152. 11 S.-C. Liao, Y.-J. Chen, W.E. Mayo, B.H.
Kear, Nanostruct. Mater. 11 (1999) 553. 12
S.-C. Liao, K.-D. Pae, W.E. Mayo, Nanostruct.
Mater. 5 (1995) 319. 13 M. Omori, Mater. Sci.
Eng. A 287 (2000) 183. 14 J.-H. Han, D.-Y. Kim,
Acta Metall. Mater. 43 (1995) 3185. 15 L. Gao,
J.S. Hong, H. Miyamoto, S.D.D.L. Torre, J. Eur.
Ceram. Soc. 20 (2000) 2149. Y.I. Lee et al. /
Materials Research Bulletin 38 (2003) 925930 929
19
In conventional sintering, the specimen was
fully densified after sintering at temperatures
above 900 C for 1 h. In contrast, when the
samples were prepared by SPS, the relative
density was 90 of the theoretical value at 600
C for 1 h and reached 99 at 700 C.
Moreover, even 5 min of treatment using SPS at
600700 C resulted in a significantly high
density(7595). A comparison of the two
densification curves obtained from conventional
sintering and SPS for 1 h indicates that
employing SPS lowers the densification
temperature by approximately 200 C. The
enhanced densification by SPS has been observed
in other systems such as Al2O3 15,
SiC-Al2O316, PMN-PT 17, and was attributed to
self-heat generation by the microscopic discharge
between the particles, activation of the particle
surfaces, and the high speed mass and heat
transfer during the sintering process 18.
20
The apparent density of the sintered specimen
was measured using the Archemedes method in
water. The phases of the powder and sintered
specimens were measured by X-ray diffraction
(XRD). The microstructure of the samples was
examined by field emission scanning electron
microscope (FE SEM 6330F, JEOL, Japan). The
average grain size was determined from the
fractured or polished sections 14.