• Journal of Sensor Science and Technology
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• v.3 no.2
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• pp.40-49
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• 1994

Potentiometric cell oxygen sensors using $CaF_{2}$ were fabricated for monitering the oxygen partial pressure in the low temperature range ($300^{\circ}C{\sim}$500^{\circ}C). The disk type oxygen sensors consist of a reference electrode: Air($O_{2}:21%$)|Pt, a solid electrolyte $CaF_{2}$, and a sensing metel Pt electrode. And the change in open circuit emf of the disk type cell was about 45mV for the oxygen concentration range, $0.1%{\sim}10%$, at the cell temperature of $400^{\circ}C$. Also, the reference electrode incorporated type sensor showed the change of 40mV for 0.1% to 10% oxygen partial pressure range.

• Korean Journal of Materials Research
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• v.26 no.12
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• pp.681-687
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• 2016

Zirconium(Zr) nuclear fuel cladding tubes are made using a three-time pilgering and annealing process. In order to remove the oxidized layer and impurities on the surface of the tube, a pickling process is required. Zr is dissolved in HF and $HNO_3$ mixed acid during the process and pickling waste acid, including dissolved Zr, is totally discarded after being neutralized. In this study, the waste acid was recycled by adding $BaF_2$, which reacted with the Zr ion involved in the waste acid; $Ba_2ZrF_8$ was subsequently precipitated due to its low solubility in water. It is very difficult to extract zirconium from the as-recovered $Ba_2ZrF_8$ because its melting temperature is $1031^{\circ}C$. Hence, we tried to recover Zr using an electrowinning process with a low temperature molten salt compound that was fabricated by adding $ZrF_4$ to $Ba_2ZrF_8$ to decrease the melting point. Change of the Zr redox potential was observed using cyclic voltammetry; the voltage change of the cell was observed by polarization and chronopotentiometry. The structure of the electrodeposited Zr was analyzed and the electrodeposition characteristics were also evaluated.

• Journal of Oral Medicine and Pain
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• v.21 no.1
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• pp.115-122
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• 1996

This study was performed to compare the anticarious effect of the different fluoride adsorbed(Naf, $NH_4F, Na_2PO_3F, SnF_2, TiF_4$) on synthetic hydroxyapatite and enamel. The amount of fluoride adsorbed in synthetic hydroxyapatite under various concentrations and pH of these fluoride solutions was measured by specific electrode. Enamel samples treated with 5 kinds of 1,000ppm fluoride solutions for 10 minutes were evaluated for fluoride uptake and enamel soubility. The results were as follows. 1. The adsorption of fluoride on synthetic hydroxyapatite increased gradually by the concentration of the fluoride solution, In 1,000ppm fluoride solution, the adsorption of fluoride on synthetic hydroxyapatite treated with NH4F and NaF solutions at pH 4.0 was relatively higher than that of other fluoride solutions. In NH4F and NaF solutions, the adsorption of fluoride on powdered enamel was higher at pH 4.0 solution than at pH 7.0 solution. 2. Fluoride uptake from NH4F solution was relatively high. But that from $Na_2PO_3F$ solution was lower than those from other fluoride solutions. 3. Fluoride solutions were significantly effective on enhancing acid resistance. $NH_4F$ solution was relatively more effective than others on enhancing acid resistance. 4. $SnF_2 and TiF_4$ solutions had the same effect on fluoride adsorption, fluoride uptake, and enamel solubility.

• Journal of Korean society of Dental Hygiene
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• v.1 no.1
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• pp.125-132
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• 2001

This study was carried out to investigate the effect of multi or single application of fluoride plus 0.05% NaF solution on the remineralization of dental caries lesion. The microhardness changes of enamel surface were measured after application of fluoride and precipitation of 0.05% NaF solution on 6 groups of cow's tooth on which the artificial carious lesions were formed first. Test groups were calssified into two step application with NaF, $SnF_2$ and APF under different application time conditions plus 0.05% NaF solution(group I, II, III, IV) and single application with APF plus 0.05% NaF solution (group V) and control(0.05% NaF solution only, group IV). The obtained results were as follows. 1. Regarding microhardness change of enamel surface. microhardness increments in group I(NaF for 1 minute+APF for 3 minutes+0.05% NaF solution for 1 minute), II($SnF_2$ 1 min+APF 3 min+NaF sol.), III($SnF_2$ 2 min+APF 2 min+NaF sol.), IV(NaF 2 min+APF 2 min+NaF sol.) and V(APF 4 min+NaF sol.) were significantly greater than group VI.(P<0.05) 2. Microhardness changes of shorter application time of $SnF_2$ (group II) were significantly greater than group III. (P<0.05) 3. Microhardness changes were variable with kinds of fluoride, application sequence and application time of fluoride. but had no relation with the number of fluoride application.

• Korean Journal of Materials Research
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• v.26 no.9
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• pp.504-511
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• 2016

To prepare $Mn^{4+}$-activated $K_2TiF_6$ phosphor, a precipitation method without using hydrofluoric acid (HF) was designed. In the synthetic reaction, to prevent the decomposition of $K_2MnF_6$, which is used as a source of $Mn^{4+}$ activator, $NH_5F_2$ solution was adopted in place of the HF solution. Single phase $K_2TiF_6$:$Mn^{4+}$ phosphors were successfully synthesized through the designed reaction at room temperature. To acquire high luminance of the phosphor, the reaction conditions such as the type and concentration of the reactants were optimized. Also, the optimum content of $Mn^{4+}$ activator was evaluator based on the emission intensity. Photoluminescence properties such as excitation and emission spectrum, decay curve, and temperature dependence of PL intensity were investigated. In order to examine the applicability of this material to a white LED, the electroluminescence property of a pc-WLED fabricated by combining the $K_2TiF_6$:$Mn^{4+}$ phosphor with a 450 nm blue-LED chip was measured.

• Journal of the Korean Ceramic Society
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• v.41 no.2
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• pp.118-124
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• 2004

The properties of AlN made by carbothermal reaction depend on the starting materials, quantity of liquid, the liquid-vapor phase reaction, the N$_2$ flow rate, and the reaction temperature. AlN whisker was synthesized by the VLS and VS methods. Solid ${\alpha}$-A1$_2$O$_3$(AES-11) was carbothermally reduced with carbon black in a high-purity N$_2$ atmosphere with AlF$_3$ to cause whisker grown and additional aluminum liquid to increase whisker yield. Aluminum nitride was perfectly formed at reaction temperatures of 1600$^{\circ}C$. At reaction temperature higher than 1600$^{\circ}C$ the aluminum nitride was completely formed, while the composition remains unaffected. Needle-shaped whiskers formed best at 1600$^{\circ}C$ while higher temperatures disrupted whisker formation. Adding 0 to 15 wt% aluminum to the synthesis favorably affects the microstructure for formation of needle-shaped AlN whisker. Additions over 15 wt% degraded formation of AlN whisker.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.11 no.12
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• pp.1122-1127
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• 1998

Group II-AF_2$films such as $CaF_2$, $SrF_2$, and $BaF_2$ have been commonly used many practical applications such as silicon on insulatro(SOI), three-dimensional integrated circuits, buffer layers, and gate dielectrics in filed effect transistor. This paper presents electrical and structural properties of fluoride films as a gate dielectric layer. Conventional gate dielectric materials of TFTs like oxide group exhibited problems on high interface trap charge density($D_it$), and interface state incorporation with O-H bond created by mobile hydrogen and oxygen atoms. To overcome such problems in conventional gate insulators, we have investigated $CaF_2$ films on Si substrates. Fluoride films were deposited using a high vacuum evaporation method on the Si and glass substrate. $CaF_2$ films were preferentially grown in (200) plane direction at room temperature. We were able to achieve a minimum lattice mismatch of 0.74% between Si and $CaF_2$ films. Average roughness of $CaF_2$ films was decreased from 54.1 ${\AA}$ to 8.40 ${\AA}$ as temperature increased form RT and $300^{\circ}C$. Well fabricated MIM device showed breakdown electric field of 1.27 MV/cm and low leakage current of $10^{-10}$ A/$cm^2$. Interface trap charge density between $CaF_2$ film and Si substrate was as low as $1.8{\times}10^{11}cm^{-2}eV^{-1}$.

• Journal of the Korean Association of Oral and Maxillofacial Surgeons
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• v.26 no.4
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• pp.363-368
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• 2000

The purposes of the study was to examine changes in thickness and strength of the rat femur after administration of sodium fluoride in the drinking water. 48 female Sprague-Dawley rats were randomly divided into 4 groups, and they were supplied with 0(control), 1, 5 and 50 ppm of fluoride in the drinking water ad libitum for 6 weeks. Rats were killed and both sides of femur were dissected. Bone strength was measured as the stress of failure of femur at the middle point and femoral neck with Instron. Histologic slides were prepared from the femur shaft with routine processing of fixing, demineralizing, embedding and HE staining. Thickness and area of cortical bone and medullary cavity were measured by using Camera Lucida and Image Analyzer. All the collected data were analyzed with one-way ANOVA, Duncan's multiple range test for post-hoc tests and ANCOVA using the SAS 6.12 package at the level of 0.05. Bone strength increased significantly in the animals given 1 and 5 ppm of fluoride in the water, as compared to the control group. There were significant decreases of stress at fracture in 50 ppmF group compared to the 1 and 5 ppmF groups. The similar trends of bone strength at the femoral neck fracture test, but there were no statistical significances. Cortical bone thickness and area of the femur increased in the 1 and 5 ppmF groups compared to the control. However, the thickness of 50 ppmF group also decreased significantly as compared to 1 and 5 ppmF groups. On the other hand, medullary thickness and area increased in all fluoride groups than control group. All of the findings presented support the conclusion that, low fluoride dosage used in water fluoridation could increase the bone strength and might have preventive effect on femur fracture.

• Journal of the Korean Ceramic Society
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• v.34 no.2
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• pp.163-170
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• 1997

The retarding effects of MgSiF6.6H2O on the hydration of portland cement were studied. The setting time, flow value and compressive strength of mortar were measured and the mechanism of retardation was also studied by ion concentration in solution, SEM, BET, and X-ray diffraction. The results are as follows ; 1. Setting time was delayed by the addition of MgSiF6.6H2O. 2. The flow value of mortar decreases depending upon the amount of MgSiF6.6H2O. 3. The compressive strength was almost same or some increase on 28 days hydration. 4. The main retardation mechanism of MgSiF6 on the hydration of portland cement may be explained by the following hypothesis. MgSiF6 depressing the Ca++ and K+ ion concentration of cement paste solution be-cause of the recrystalization of K2SiF6 and CaF2 phase. The new products of K2SiF6 and CaF2 deposit on the surface of unhydrated cement powder and harzard the mass transfer through these layer. The low con-centration of Ca++, K+ ion in solution was decreasing the hydration rate of portland cement.

• Membrane Journal
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• v.27 no.5
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• pp.389-398
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• 2017

The most important characteristic of the polymer electrolyte membranes (PEMs) for fuel cells, the proton conducting ability is mainly influenced by the distribution and morphology of the water channels inside the PEMs. Non-perfluorinated hydrocarbon PEMs are known to have weaker water channels than perfluorinated PEM, Nafion, and thus relatively low proton conducting ability. In this study, we used a mesoscale simulation technique to observe the water channel formation and phase separation behavior of hydrocarbon PEM, sulfonated polyimides, under the humidification condition. It was observed that the water molecules were distributed evenly through the entire hydrophilic region, and clear water clusters were formed only in the sulfonated polyimide having high sulfonation degree. In addition, it was observed that sulfonated polyimides have a difficulty in forming water channel under the low hydrated condition. These results clearly support the theories of the formation of water channels in non-perfluorinated hydrocarbon PEMs, and also well explain the tendency of proton conducting abilities of sulfonated polyimides. Thus, it is confirmed that mesoscale simulation techniques can be very effective in analyzing phase separation behavior and water channel formation in PEMs for fuel cells and elucidating the ion conducting abilities.