Preparation of nanostructured TiO 2 ceramics by spark plasma sintering

1. Preparation of nanostructured TiO2ceramics byspark plasma sintering 플라즈마 연구실 김 형 용

2. Contents • Spark plasma sintering • Abstract • Introduction • Experimental • Result and discussion • conclusions

3. 개 요 • 방전 플라즈마 소결(Spark Plasma Sintering) : 분체(粉體) 재료에 압력과 저전압 및 • 대전류를 걸어서 고품질의 제품을 단시간에 소결하는 것이다. • 방전 플라즈마 소결은 종래의 열간압축법(Hot Press)에 비해서, • 전력소비가 약 1/3~1/5정도로 적어지는 에너지 절감형 소결법 • 취급이 간편 • 러닝코스트가 저렴 • 소결기술에 대한 숙련이 필요하지 않음 • 재료의 선정에 다양성이 있음 • High Speed 소결 등의 수 많은 장점을 갖고 있다 • . 이 공법은 신재료에 있어서 프로세스 이노베이션을 실현할 수 있는 것으로 폭넓은 • 선진 신재료의 제조 분야에 응용되어 가고 있다. 1. Spark Plasma sintering

4. 2. 원리 방전 플라즈마 소결 프로세스는 압분체(壓粉體) 입자간의 틈새에 저전압으로 펄스상의 대전류를 투입하고, 불꽃방전현상에 의하여 순간적으로 발생하는 방전 플라즈마(고온플라즈마: 순간적으로 수천~일만℃의 고온도장이 입자간에 생성)의 높은 에너지를 열확산 및 전계확산 등으로 효과적으로 응용한 것이다. 저온에서 2000℃ 이상의 초고온영역에 있어서 종래의 방법에 비하여 200~500℃ 정도 낮은 온도영역에서 승온 및 유지시간을 포함해서 개략적으로 5~20분 정도의 단시간에 소결을 완료한다.

5. SPS장치 구성도 P Oil Pressure System Upper Electrode Powder Thermocouple Pyrometer Upper Punch Controller On-Off DC Pulse Generator Die Lower Punch Temperature Pressure Current - Voltage Vacuum Longitudinal displacement Vacuum chamber Lower Electrode P

6. Spark Plasma Sintering vs. Hot Pressing DC pulse Heater P P ⊙ ⊙   ⊙ ⊙   ⊙ ⊙    ⊙ ⊙ ⊙ ⊙ P P Joule heating by applying on-off DC pulse Heating by heater located outside of mold

7. On-Off 제어 직류 펄스전류의 통전 모식도 P Pulse Current Powder Thermo-couple On-Off DC PulseCurrent Die Punch Electrode P

8. 분말입자간 통전 모식도 Pulse current Particle Die Joule heat Discharge

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 1~2 ㎛.

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], photocatalytic[1], 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-transformationassisted sintering [9], hot pressing [10–12], 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 800°C 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 600–1000 ° 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 600–700 °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 1–2 um. The SPS process was effective in obtaining fully-densified TiO2 ceramics with a minimum grain growth at a low sintering temperature.

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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 600–700 °C resulted in a significantly high density(75–95%). 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-Al2O3[16], 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].