• Journal of the Korean Crystal Growth and Crystal Technology
• /
• v.15 no.2
• /
• pp.75-78
• /
• 2005

Silicon boride ($SiB_6$) is very promising for use as thermoelectric materials at high temperature because of its high melting point and relatively large Seebeck coefficient. In the present work, spark plasma sintering (SPS) was applied for preparing dense $SiB_6$ ceramics, and their thermoelectric properties were investigated, together with their microstructural evaluation. The SPS process was found to be effective in densifying a $SiB_6$ ceramic, typically 99 % of the theoretical density at low temperature of $1500^{\circ}C$. In comparison with $SiB_6$ specimen prepared by hot-pressing, the SPS-processed specimen exhibited the significantly improved Seebeck coefficient, resulting in the higher power factor.

• The Korean Journal of Ceramics
• /
• v.6 no.1
• /
• pp.27-31
• /
• 2000

A series experiments were performed to evaluate mechanical behavior of non-oxide boride type ceramics formed on the AISI 1040 plain carbon steel. Boronizing was performed in a slurry salt bath consisting of borax, boric acid, and ferro-silicon at $950^{\circ}C$ for 2-6h. The AISI 1040 steel used as substrate material was containing 0.4%C, 0.13%Si, 0.65%Mn, 0.02%P, 0.014%S. The presence of non-oxide boride type ceramics $Fe_2B $ and FeB formed on the surface of steel was confirmed by metallographic technique and X-ray diffraction (XRD) analysis. The hardness of borides measured via Vickers indenter with a load of 2N reached a microhardness of up to 1800 DPN. The hardness of unborided steel was 185 DPN. The fracture toughness of borides measured by means of Vickers indenter with a load of 10N was about 2.30 MPa.$m^{1/2}$. The thickness of boride layers ranged from 72$\mu\textrm{m}$ to 145$\mu\textrm{m}$. Boride layers have a columnar morphology.

• Proceedings of the Materials Research Society of Korea Conference
• /
• 2011.05a
• /
• pp.15-15
• /
• 2011

As you know, boron compounds, borax ($Na_2B_4O_5(OH)_4{\cdot}8H_2O$) etc. were known thousands of years ago. As for natural boron, it has two naturally occurring and stable isotopes, boron 11 ($^{11}B$) and boron 10 ($^{10}B$). The neutron absorption $^{10}B$ is included about 19~20% with 80~81% $^{11}B$. Boron is similar to carbon in its capability to form stable covalently bonded molecular networks. The mass difference results in a wide range of ${\beta}$ values between the $^{11}B$ and $^{10}B$. The $^{10}B$ isotope, stable with 5 neutrons is excellent at capturing thermal neutrons. For example, it is possible to decrease a thermal neutron required for the nuclear reaction of uranium 235 ($^{235}U$). If $^{10}B$ absorbs a neutron ($^1n$), it will change to $^7Li+^1{\alpha}$ (${\alpha}$ ray, like $^4He$) with prompt ${\gamma}$ ray from $^{11}B$ $^{11}B$ (equation 1). $$^{10}B+^1n\;{\rightarrow}\;^{11}B\;{\rightarrow}\; prompt \;{\gamma}\;ray (478 keV), \;^7Li+4{\alpha}\;(4He)\;\;\;\;{\cdots}\; (1)$$ If about 1% boron is added to stainless steel, it is known that a neutron shielding effect will be 3 times the boron free steel. Enriched boron or $^{10}B$ is used in both radiation shielding and in boron neutron capture therapy. Then, $^{10}B$ is used for reactivity control and in emergency shutdown systems in nuclear reactors. Furthermore, boron carbide, $B_4C$, is used as the charge of a nuclear fission reaction control rod material and neutron cover material for nuclear reactors. The $B_4C$ powder of natural B composition is used as a charge of a control material of a boiling water reactor (BWR) which occupies commercial power reactors in nuclear power generation. The $B_4C$ sintered body which adjusted $^{10}B$ concentration is used as a charge of a control material of the fast breeder reactor (FBR) currently developed aiming at establishment of a nuclear fuel cycle. In this study for new boron compound, silicon boride ceramics for capturing thermal neutrons, preparation and characterization of both silicon tetraboride ($SiB_4$) and silicon hexaboride ($SiB_6$) and ceramics produced by sintering were investigated in order to determine the suitability of this material for nuclear power generation. The relative density increased with increasing sintering temperature. With a sintering temperature of 1,923 K, a sintered body having a relative density of more than 99% was obtained. The Vickers hardness increased with increasing sintering temperature. The best result was a Vickers hardness of 28 GPa for the $SiB_6$ sintered at 1,923K for 1 h. The high temperature Vickers hardness of the $SiB_6$ sintered body changed from 28 to 12 GPa in the temperature range of room temperature to 1,273 K. The thermal conductivity of the SiB6 sintered body changed from 9.1 to 2.4 W/mK in the range of room temperature to 1,273 K.

• The Transactions of The Korean Institute of Electrical Engineers
• /
• v.56 no.9
• /
• pp.1602-1608
• /
• 2007

The composites were fabricated, respectively, using 61[vol.%] SiC-39[vol.%] $TiB_2$ and using 61[vol.%] SiC-39[vol.%] $ZrB_2$ powders with the liquid forming additives of 12[wt%] $Al_2O_3+Y_2O_3$ by hot pressing annealing at $1650[^{\circ}C]$ for 4 hours. Reactions between SiC and transition metal $TiB_2$, $ZrB_2$ were not observed in this microstructure. The result of phase analysis of composites by XRD revealed SiC(6H, 3C), $TiB_2$, $ZrB_2$ and $YAG(Al_5Y_3O_{12})$ crystal phase on the Liquid-Phase-Sintered(LPS) $SiC-TiB_2$, and $SiC-ZrB_2$ composite. $\beta\rightarrow\alpha-SiC$ phase transformation was occurred on the $SiC-TiB_2$ and $SiC-ZrB_2$ composite. The relative density, the flexural strength and Young's modulus showed the highest value of 98.57[%], 249.42[MPa] and 91.64[GPa] in $SiC-ZrB_2$ composite at room temperature respectively. The electrical resistivity showed the lowest value of $7.96{\times}10^{-4}[\Omega{\cdot}cm]$ for $SiC-ZrB_2$ composite at $25[^{\circ}C]$. The electrical resistivity of the $SiC-TiB_2$ and $SiC-ZrB_2$ composite was all positive temperature coefficient resistance (PTCR) in the temperature ranges from $25[^{\circ}C]$ to $700[^{\circ}C]$. The resistance temperature coefficient of composite showed the lowest value of $1.319\times10^{-3}/[^{\circ}C]$ for $SiC-ZrB_2$ composite in the temperature ranges from $100[^{\circ}C]$ to $300[^{\circ}C]$ Compositional design and optimization of processing parameters are key factors for controlling and improving the properties of SiC-based electroconductive ceramic composites.

• Journal of the Korean Crystal Growth and Crystal Technology
• /
• v.11 no.1
• /
• pp.20-26
• /
• 2001

Titanium boride ($\textrn{TiB}_{x}$) films were deposited on (100) silicon substrates at the substrate temperature of $500^{\circ}C$ by means of the co-evaporation of titanium and boron evaporants during deposition. The co-evaporation method makes it possible to deposit the non-stoichiometric films with different boron-to-titanium ratio($0{\le}B/Ti \le 2.5$). The resistivity increases linearly as the boron-to-titanium ratio in the as-deposited films is increased. The surface roughness of $\textrn{TiB}_{x}$ films is changed as a function of the boron-to-titanium ratio. The XRD spectrum for pure titanium film shows a highly (002) preferred orientation. For B/Ti=0.59 ratio only a single TiB phase that shows a (111) preferred orientation is observed. However, the $\textrn{TiB}_{x}$ phase with the hexagonal structure of the $AlB_2$(C32) type appears as the boron concentration increase, and only a single $\textrn{TiB}_{x}$ phase is observed for $B/Ti \ge 2.0$ ratio. The $\textrn{TiB}_{x}$/Si samples reveal a tensile stress (3~$20{\times}^9$dyn/$\textrm{cm}^2$) in the overall composition of the films, although the magnitude of the residual stresses is depended on the nominal B/Ti ratio.

• Korean Journal of Materials Research
• /
• v.20 no.4
• /
• pp.217-222
• /
• 2010

Zirconium diboride (ZrB2) and mixed diboride of (Zr0.7Ta0.3)B2 containing 30 vol.% silicon carbide (SiC) composites were prepared by hot-pressing at $1800^{\circ}C$. XRD analysis identified the high crystalline metal diboride-SiC composites at $1800^{\circ}C$. The TaB2 addition to ZrB2-SiC showed a slight peak shift to a higher angle of 2-theta of ZrB2, which confirmed the presence of a homogeneous solid solution. Elastic modulus, hardness and fracture toughness were slightly increased by addition of TaB2. A volatility diagram was calculated to understand the oxidation behavior. Oxidation behavior was investigated at $1500^{\circ}C$ under ambient and low oxygen partial pressure (pO2~10-8 Pa). In an ambient environment, the TaB2 addition to the ZrB2-SiC improved the oxidation resistance over entire range of evaluated temperatures by formation of a less porous oxide layer beneath the surface SiO2. Exposure of metal boride-SiC at low pO2 resulted in active oxidation of SiC due to the high vapor pressure of SiO (g), and, as a result, it produced a porous surface layer. The depth variations of the oxidized layer were measured by SEM. In the ZrB2-SiC composite, the thickness of the reaction layer linearly increased as a function of time and showed active oxidation kinetics. The TaB2 addition to the ZrB2-SiC composite showed improved oxidation resistance with slight deviation from the linearity in depth variation.