Through the Direct Nitriding of Silicon Compacts
| @Synthesis and characteristics of porous silicon nitride containing
fine pores was attempted by the nitridation of Si compacts. Small
pore size, 70 nm in average, was achieved by heat
- treating Si compacts at 1350 for 8 hours in 0.1 MPa nitrogen
atmosphere. After nitridation, the samples were essentially composed
of Ώ - Si3N4.
And no trace of Si metal was observed. Porosity of the sample was
27% and the thermal expansion coefficient of the sample was 3.1 ~10-6
/from room temperature to 1000 . Three point bending strength was
200 MPa at room temperature and 1000 . Addition of acrylic spheres
as a pore forming agent was effective to increase the porosity but
not detrimental to maintaining the size of pores small. *Research Center |
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| @ | 1. Introduction |
| @ | 2. Experimental Procedure |
| @ | 3. Results and Discussion |
| @ | 4. Conclusions |
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| 1. Introduction |
| @Porous ceramic materials have been attracting a great interest
for various separating applications in severe environmental conditionsi1
j in which other materials such as metals or organic materials
can not be used. Porous ceramic thin films with fine pores, for instance,
are candidate materials to separate specific gas, liquid and solid
phases under the high temperature or high corrosive environments.
S. Kondoh et al reported a mesoporous silica film with oriented through
channels perpendicular to the surfacei2j. In
order to use this film as a filter,@however, the thin films have to
be carried on porous substrates, so porous ceramics materials composed
of pores with controlled size and the size distribution are needed
to support the films on the surface without formation of defects in
them. @Porous silicon nitride is one of candidates for the support of ceramic thin films or membranes because it has excellent stability under high temperature condition and severe chemical environmenti3j. Formation of silica layer, which is formed on the surface of silicon nitride by heating in air, is also considered favorable to improve the adhesion between the substrate and the film containing silica as a component. In this study, fabrication of porous silicon nitride with fine pores through the nitridation of Si compacts was investigated. |
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| 2. Experimental Procedure |
| @Commercially available Si raw powder with the size of under 75
Κ was used as a starting material. Acrylic sphere with the average
size of 20 Κ were also used in some cases to form pores in silicon
nitride matrix. The Si powder was ball - milled
for 20 hours by using alumina grinding media in ethanol. After ball
- milling, the powder was dried and crushed in mortar to break
agglomerates. In the case of adding acrylic spheres as a pore forming
agent, both milled Si powder and the spheres were mixed in dry blender.
The amount of added acrylic spheres was 30 mass%to the total mass
of Si and acrylic spheres. @The Si powder and the mixed powder of Si and acrylic spheres were then mold pressed in a square die (80 mm ~80 mm)at the applied pressure 19.6 MPa. Then they are cold isostatically pressed at 98 MPa to form green compacts. The compacts were settled in a silicon nitride container and heat - treated in the electric furnace. Heat - treatment was performed under 0.1 MPa nitrogen atmosphere at three different conditions:1100 for 12 hours, 1350 for 8 hours and 1600 for 4 hours. Before the sintering test of powder compacts, thermogravimetry and differential thermal analysis (TG - DTA)was employed to examine the nitridation behavior of Si powder and to design the heat - treatment conditions. The density of the sintered samples was measured by the Archimedes method in water. The crystalline phases of sintered specimens were identified by X - ray diffractometry (XRD)and the microstructure of these samples was examined by scanning electron microscope (SEM). Mercury porosimetry was employed to investigate the size distribution and volumes of pore channels in sintered bodies. @Thermal expansion coefficient was measured for specimens cut from the sintered bodies. Fracture strength of these samples was measured by three point bending mode with span length of 30 mm for the specimens with the size 3 ~4 ~40 mm that were also cut from the sintered bodies. The surfaces of specimens were finished with a diamond grinding wheel. Cross head speed was adjusted to be 0.5 mm/min. |
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| 3. Results and Discussion |
| @Figure 1
shows the result of weight change of Si powder with heating from room
temperature to 1400 at the ramping rate of 10 /min in nitrogen flow.
As shown, the value of TG began to increase around 1100 , suggesting
the onset of nitridation of Si compact. The increase in weight occurred
rapidly above 1200 and reached plateau at around 1400 .It was conducted
from the results that the nitridation of Si compacts was promoted
above 1200 under a normal nitrogen pressure. @Table 1 shows the nitridation ratio of Si compacts heat - treated at three different conditions. Nitridation ratio was calculated from the equation, |
| Nitridation ratio (%)=1.504 ~(WSB - WGB)/WGB ~100 |
| where WSB and WGB are
the weight of sintered and green bodies respectively. As shown, nitridation
ratio of the specimen heat - treated at 1100
was only 7.5%even after nitriding for 12 hours, whereas the ratio
reached up to 90%for the samples sintered at 1350 and 1600 . @The XRD results indicated that the samples heat - treated at 1350 and 1600 were composed of Ώ - Si3N4, and no peaks of Si were observed, it is suggested that the nitridation almost completed in these samples although the nitridation ratios did not reach 100%. It is due to the vaporization of Si and other impurities such as SiO is considered responsible for the difference in nitridation ratios. @Figure 2 shows the pore distribution curves of three samples. The size of the most frequent pores is clearly smallest for the sample heat - treated at 1350 . The mean pore size was small, `70 nm. @Figure 3 shows the SEM micrographs of the fracture surface of the samples heat - treated at 1350 and 1600 . It is clear that the size of the grains heat treated at 1350 is smaller than that of at 1600 . Formation of silicon nitride by the nitridation of Si metal grains causes not only the increase in weight but also the expansion of 27%in volumei4 j . Small pore size in the present silicon nitride is probably caused by the volume expansion of grains during nitridation. The increase in the size of pores for the sample heat - treated at 1600 can be ascribed to the coalescence of pores with the growth of silicon nitride grains. @Table 2 shows the properties evaluated for the samples heat - treated at 1350 for 8 hours. Porosity of the sample was 27%. Thermal expansion coefficient of the sample was 3.1 ~10|6 /@between room temperature and 1000 and almost the same with that of silicon nitride ceramics reported previously. Three point bending strength was 200 MPa at both room temperature and 1000 @Table 3 shows the porosity and average mean pore size of Si compacts containing acrylic spheres heat - treated at 1350 for 8 h and 1600 for 4h. Remarkable increase in porosity was observed by adding acrylic spheres to the system. However, the increase in the mean pore size was small. The average pore sizes were 0.21 Κ for the sample sintered at 1350 and 1.11 Κ at 1600 respectively. This is probably caused by the formation of silicon nitride whiskers which grew at the surface of additive originated pores as presented in Fig.4. |
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| 4. Conclusions | ||||||||||
| @Porous silicon nitride ceramics with fine pores could be synthesized
through the nitridation of Si powder compacts. Small pore size, 70
nm in average, was achieved by heat - treating
Si compacts at 1350 for 8h in 0.1 MPa nitrogen atmosphere. Addition
of acrylic spheres was effective to increase the porosity but not
detrimental to maintaining the size of pores small. It was considered
that the present porous silicon nitride ceramics was suitable for
the substrate to support carry the ceramic thin films or membranes. |
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| |Acknowledgement | @This work has been supported by NEDO, as a part of Synergy Ceramics Project promoted by AIST, MITI, Japan. The authors are members of the Joint Research Consortium of Synergy Ceramics. |References | |
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