8094 J Mater Sci(2006)41:8093-8100 dditive for densification is used. Various reports show the deposit were monitored continuously. The densi- that EPD has been successful for preparing thick and ties of the fresh(wet) deposits were calculated from thin films and various free-standing materials [11-13, the buoyancy in the suspension, and the solids content for example, bulk SiC-based ceramics with boron, in the deposits was determined by 4s were fir measuring the arbon, Al2O or AIN as sintering additives [14-16. weight change during drying. The deposits were fired at Another example of using EPD is the infiltration of temperatures from 1300 to 1500C in an Ar atmo- [17, 18]. For the material to be used in the first wall of a TEM and ED ered samples were inspected by SEM, lumina or carbon fibres woven with a ceramic matrix sphere. The sint fusion reactor suitable low-neutron -activation sinter ing additives must be used. Such additives would allow low-temperature densification without any detrimental Results and discussion effects on the fibres This excludes the conventional additives for SiC, such as B, C, and additives from the Figure 1 shows the zeta-potential for the used powders system Al2 O3(AIN)-Y2O3 that require high sintering in an ethanol suspension as a function of pH. It is clear temperatures, typically above 1800C. In our study that the point of zero charge(PzC)for the um-Sic two grades of the Sic powder was used and powder appears at about pH 6.0. At lower values the ed as the additive: this particles carry a relatively high positive net-charge, enables densification below the temperature limit of while the zP in the alkaline region are negative. The 1500 .C at close-to-zero shrinkage. The conditions for addition of aluminium phosphate(AP) shifted the the EPD of a SiC-based matrix material on SiC-fibres PZc to a lower value, suggesting that it is adsorbed on were investigated the surface of the Sic particles. The measured ZP values for the nano-Sic powder were much lower than those for the um-SiC across the whole range of PH Experimental The PZC appears at a lower pH value, suggesting that the fine Sic particles have an oxidised surface layer Two grades of Sic powder were used in this investi- Figures 2a and b show the morphology of the um-SIC gation:"H-SiC", a submicron B-SiC BF-12(H Starck, and nano-SiC powders, while Figs 2c and d show Goslar, Germany) with an average particle size of HRTEM images of the oxygen-containing amorphous 0.5 um; and"nano-SiC", a powder with an average surface layers on the particles of both powders. In particle size of 50 nm(Hefei KiIn Nanom Technol. Figs 2e and f EDXS spectra of the amorphous layer Dev. Co. Ltd, China). Aluminium phosphate, "AP", and bulk Sic particle of um-SiC are shown. It is (TKI doo, Slovenia)was used as an additive for the demonstrated, that the thickness of the layers is similar in a ball-mill for at leae e pe s of the powder and the for both powders(1-2 nm). The XPS analysis revealed 2 h. The characteri istics of the powder, while in the nano-SiC powder, beside oxyca particles in the suspensions were quantified by mea- bides also SiOz was found suring the zeta-potential(ZP) using a ZetaPals zeta- meter(Brookhaven, USA). The operational ph was adjusted using HCl, citric acid or NH,OH, and measured with a pH meter(Metron Ltd, Switzerland) For simplicity, the measured values are designated as 00四 Hin this investigation. Surface chemistry of the powders was analysed by X-ray troscopy(XPS) using TFA spectrometer(Pysical N Electronics, USA) The EPD experiments were performed at a constant voltage of 60 V. The electrodes were either square stainless steel plates, 2 x 2 cm2, placed vertically at distance of 2 cm or a bundle of the sic fibres 0123456789101112 Nippon Carbon Co, Ltd). The solids conte uspensions for the EPD varied from Fig. 1 Zeta-potential of um-SiC (without and with addition sition the electrical current and the weight change of function of pl phate)and nano-SiC powders in eth (4.3 vol %)to 70 wt%(37.3 vol %) During the depo- aluminium pho 2 Springeradditive for densification is used. Various reports show that EPD has been successful for preparing thick and thin films and various free-standing materials [11–13], for example, bulk SiC-based ceramics with boron, carbon, Al2O3 or AlN as sintering additives [14–16]. Another example of using EPD is the infiltration of alumina or carbon fibres woven with a ceramic matrix [17, 18]. For the material to be used in the first wall of a fusion reactor, suitable low-neutron-activation sinter￾ing additives must be used. Such additives would allow low-temperature densification without any detrimental effects on the fibres. This excludes the conventional additives for SiC, such as B, C, and additives from the system Al2O3(AlN)-Y2O3 that require high sintering temperatures, typically above 1800 C. In our study two grades of the SiC powder was used and a phosphate glass was employed as the additive; this enables densification below the temperature limit of 1500 C at close-to-zero shrinkage. The conditions for the EPD of a SiC-based matrix material on SiC-fibres were investigated. Experimental Two grades of SiC powder were used in this investi￾gation: ‘‘l-SiC’’, a submicron b-SiC BF-12 (H. Starck, Goslar, Germany) with an average particle size of 0.5 lm; and ‘‘nano-SiC’’, a powder with an average particle size of 50 nm (Hefei Kiln Nanom.Technol. Dev. Co. Ltd, China). Aluminium phosphate, ‘‘AP’’, (TKI doo, Slovenia) was used as an additive for the densification. The suspensions of the powder and the additive in ethanol were prepared by homogenisation in a ball-mill for at least 2 h. The characteristics of the particles in the suspensions were quantified by mea￾suring the zeta-potential (ZP) using a ZetaPals zeta￾meter (Brookhaven, USA). The operational pH was adjusted using HCl, citric acid or NH4OH, and measured with a pH meter (Metron Ltd, Switzerland). For simplicity, the measured values are designated as ‘‘pH’’ in this investigation. Surface chemistry of the powders was analysed by X-ray photoelectron spec￾troscopy (XPS) using TFA spectrometer (Pysical Electronics, USA). The EPD experiments were performed at a constant voltage of 60 V. The electrodes were either square stainless steel plates, 2 · 2 cm2 , placed vertically at a distance of 2 cm, or a bundle of the SiC fibres (Nicalon, Nippon Carbon Co., Ltd). The solids content in the suspensions for the EPD varied from 15 wt.% (4.3 vol.%) to 70 wt.% (37.3 vol.%). During the depo￾sition the electrical current and the weight change of the deposit were monitored continuously. The densi￾ties of the fresh (wet) deposits were calculated from the buoyancy in the suspension, and the solids content in the deposits was determined by measuring the weight change during drying. The deposits were fired at temperatures from 1300 to 1500 C in an Ar atmo￾sphere. The sintered samples were inspected by SEM, TEM and EDS. Results and discussion Figure 1 shows the zeta-potential for the used powders in an ethanol suspension as a function of pH. It is clear that the point of zero charge (PZC) for the lm-SiC powder appears at about pH 6.0. At lower values the particles carry a relatively high positive net-charge, while the ZP in the alkaline region are negative. The addition of aluminium phosphate (AP) shifted the PZC to a lower value, suggesting that it is adsorbed on the surface of the SiC particles. The measured ZP values for the nano-SiC powder were much lower than those for the lm-SiC across the whole range of pH. The PZC appears at a lower pH value, suggesting that the fine SiC particles have an oxidised surface layer. Figures 2a and b show the morphology of the lm-SiC and nano-SiC powders, while Figs. 2c and d show HRTEM images of the oxygen-containing amorphous surface layers on the particles of both powders. In Figs. 2e and f EDXS spectra of the amorphous layer and bulk SiC particle of lm-SiC are shown. It is demonstrated, that the thickness of the layers is similar for both powders (1–2 nm). The XPS analysis revealed presence of oxycarbides at the surface of the lm-SiC powder, while in the nano-SiC powder, beside oxycar￾bides also SiO2 was found. -60 -40 -20 0 20 40 60 pH ZP (mV) µm-SiC µm-SiC + AP nano-SiC 0 1 2 3 4 5 6 7 8 9 10 11 12 Fig. 1 Zeta-potential of lm-SiC (without and with addition of aluminium phosphate) and nano-SiC powders in ethanol as a function of pH 8094 J Mater Sci (2006) 41:8093–8100 123