• Journal of the korean academy of Pediatric Dentistry
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• v.28 no.4
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• pp.673-682
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• 2001

The use of resin composites has continued to increase over the last several years. In spite of their growing popularity, composites continue to exhibit a number of undesirable characteristics. One of the major deficiencies of composite restorative resins is their inadequate resistance to wear. Of the multitude of factors that have been associated with wear, subsurface degradation within the restoration is considered to be one. The aim of this study was to evaluate the resistance to degradation of four commercial composite resins in an alkaline solution. This solution with a high concentration of hydroxyl ions is a convenient medium for accelerated degradation of silane coupling and filler particles. The brands studies were Definite($Degussa-H\ddot{u}ls$ AG, Germany), Prodigy(Kerr, USA), Pyramid(Bisco, USA) and Synergy(Coltene, Swiss). Preweighed discs of each brand were exposed to 0.1N NaOH solution at $60^{\circ}C$. After 14 days they were removed, neutralized with HCl, washed with water and dried. Resistance to degradation was evaluated on the basis of following parameters : (a) mass loss(%)-determined from pre-and post-exposed specimen weights : (b) Si loss(ppm)-obtained from ICP-AE analysis of solution exposed to specimens; and (c) degradation $depth({\mu}m)$-measured microscopically (SEM) from polished circular sections of exposed specimens. The results were follows: 1. Mass loss of Synergy was $1.24{\pm}0.002%$, it was the highest, there was no significant difference among the materials. 2. The degree of degradation layer depth of Synergy was $107.83{\pm}2.52{\mu}m$, it was the highest, there was no significant difference among any other materials than Synergy. 3. There was no difference among the four materials in Si loss. 4. The correlation coefficient between mass loss and degradation depth was relatively high(r=0.06, p<0.05). 5. There was no coefficient correlation between Si loss and mass loss, the degree of degradation layer depth and Si loss. 6. When observed with SEM, destruction of bonding is observed between resin matrix and filler.

• Journal of the Korea Institute of Building Construction
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• v.16 no.1
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• pp.9-15
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• 2016

The part of a building with the biggest energy loss is the exterior and many studies are actively conducted to reduce the energy loss on that part. However, most studies consider the window frames and insulation materials, but many studies do not discuss the concrete that takes more than 70% of the exterior. In order to minimize the energy loss of buildings, it is necessary to enhance the concrete's insulation performance and studies need to be conducted on this. Therefore, this study used a micro foam cell admixture, calcined diatomite powder, and lightweight aggregates as a part of a study to develop a type of concrete with improved insulation performance that has twice higher thermal conductivity compared to concrete. It particularly secured the porosity inside concrete to lower thermal conductivity. As a result of the experiment, the slump and air capacity showed fair results, but all mixtures containing micro foaming agent showed 14.3~35.1% lower mass per unit of volume compared to regular concrete. Compressive strength decreased slightly due to the materials used to improve the insulating performance, but it all satisfied this study's target strength(24MPa). Thermal conductivity was up to twice higher than that of regular concrete.

• KOREAN JOURNAL OF CROP SCIENCE
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• v.27 no.1
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• pp.1-10
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• 1982

Degree of grain shattering which is of varietal character is an important determinant for the magnitude of field loss of grains during harvest and threshing. Seven Indica \times Japonica progeny varieties and four Japonica varieties were subjected to measurements of tensile strength of grains, degree of grain shattering when panicles were dropped at 1.5m above concrete floor, and moisture content of grains (wet basis) during a period 35 to 63 days after heading. In addition, two varieties were tested for the relation of tensile strength of grains to the magnitude of field loss of grains in actual binder harvest. The 11 varieties differed conspicuously in tensile strength of grains and the degree of grain shattering: the weakest average tensile strength of grains of a variety was about 90g and the strongest about 250g with varying standard deviation of 30 to 60g. Three Indica \times Japonica varieties and one Japonica variety shattered I to 30% of the grains under the falling test. The threshold tensile strength of grains allowing grain shattering was estimated to be 180g on average for a sampling unit of 10 panicles, but only the grains having tensile strength weaker than 98g within the samples shattered. A decrease in average tensile strength by 10g below the threshold value corresponded to an increase of 3 to 5% in grain shattering. Most varieties did not change appreciably the tensile strength of grains and degree of grain shattering with delay in time of harvest and showed a negative correlation between the tensile strength and the moisture content of grains. The average tensile strength of grains was negatively correlated linearly with field loss in binder harvest. The average tensile strength for zero field loss in binder harvest was estimated to be 174g and a decrease in the average tensile strength by 10g corresponded to an increase of 40kg per hectare in field loss of grains. Instead of the average tensile strength of grains, the percentage of grains having tensile strength weaker than 100g is recommended as a criterion for the estimation of field loss of grains during harvesting operations as well as a basis of variety classification for grain shattering, since the standard deviation of tensile strength of grains varies much with variety and time of harvest, and individual grains having tensile strength stronger than 98 did not shatter practically.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.21 no.9
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• pp.604-609
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• 2020

There are various methods of finishing concrete surfaces, and when considering workability, the spray method is effective, but rebound occurs. The allocation of rebound occurrence control should be adjusted according to the materials used. Thus, a basic study was conducted on multiple techniques for reducing the rebound incidence that are suitable for surface finishing materials containing a photocatalyst. A prior study derived the reduction effect and optimal mix ratio for photocatalytic performance. Based on that study, the rebound reduction was verified according to the specifications of the content and the mechanical durability characteristics of the mixed materials. Rebound, compressive strength, flexural rigidity, and table flow tests were done. The flow was fixed at 170±10 mm considering the workability of the mortar spray equipment. For the experimental variables, the rebound number was adjusted to the silica sand variables relative to the cement weight, and silica sands No. 5 and No. 7 were used. The results show the highest compression strength in the final S-1 variable, and the amount of rebound was minimized. These results were sufficiently filled with the bindings of the silica pores, which increased the binding force between the aggregates, resulting in a lower amount of rebound.

• Proceedings of the Materials Research Society of Korea Conference
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• 2011.05a
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• pp.42.1-42.1
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• 2011

높은 광흡수 계수를 갖는Cu(In,Ga) $Se_2$ (CIGS) 화합물 박막 소재는 고효율 태양전지 양산을 위해 가장 전도유망한 재료이나 상대적으로 매장량이 적은 In 및 Ga을 사용한다는 소재적 한계가 있다. $Cu_2ZnSnSe_4$ (CZTSe) 혹은 $Cu_2ZnSnS_4$(CZTS)와 같은 Cu-Zn-Sn-Se계 화합물 반도체는 CIGS 내 희소원소인 In과 Ga이 범용원소인 Zn 및 Sn으로 대체된 소재로써 미래형 저가 태양전지 개발을 위해 활발히 연구되고 있는데, 그 화합물 조합에 따라 0.8 eV부터 1.5 eV까지의 에너지 밴드갭을 갖는 것으로 알려져 있다. 스퍼터링법에 기반한 2단계 공정에 의해 3.2%의 CZTSe 및 6.7%의 CZTS 태양전지 효율 달성이 보고된 바 있으며, 최근 비진공 방식을 이용하여 제조된 $Cu_2ZnSn(S,Se)_4$ (CZTSSe) 태양전지가 9.6%의 변환효율을 생산하여 세계 최고기록을 갱신한 바 있다. 반면, 동시진공증발법에 의한 Cu-Zn-Sn-Se계 연구는 박막 조성 조절이 상대적으로 용이하다는 장점에도 불구하고, 상대적으로 공개된 연구결과의 양이 적으며 그 효율에 대한 보고는 특히 미미하다. 본 연구에서는 동시진공증발법에 의한 CZTSe 박막 연구 결과를 바탕으로 Sn 손실을 최소화하기 위한 진공증발 공정을 최적화하였으며, 이를 통해 CZTSe 박막 태양전지를 제조하고 그 특성분석을 통해 5% 이상의 변환효율을 달성하였다.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.17 no.6
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• pp.580-588
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• 1992

Plasma-spraying was conducted to produced the microwave absorbing surface films on the alumi-num-alloy used for the fuselage to protect the aircraft against the RADAR detction. The surface films were produced by plasma-splaying the mecharucally mixed composite powders of the silicon carblde and Ni-Zn ferrite. This M /W absorbers were designed experimentally and fabricated trialty, as a result of which the rolative frequency bandwidth of 7.6 to 8.4% were obtained under the tolerance limits of the re-flection coefficients lower than -6dB(absorption ratio 75%), and the maximum absorption thickness becomes 0.5 to 0.5.imm, which Is much thinner than that of the conventional ones.

• Journal of the Korea Institute of Information Security & Cryptology
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• v.13 no.6
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• pp.33-43
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• 2003

In this paper, we proposed a real-time watermarking algorithm to be combined and to work with a DWT(Discrete Wavelet Transform)-based image compressor. To reduce the amount of computation in selecting the watermarking positions, the proposed algorithm uses a pre-established look-up table for critical values, which was established statistically by computing the correlation according to the energy values of the corresponding wavelet coefficients. That is, watermark is embedded into the coefficients whose values are greater than the critical value in the look-up table which is searched on the basis of the energy values of the corresponding level-1 subband coefficients. Therefore, the proposed algorithm can operate in a real-time because the watermarking process operates in parallel with the compression procession without affecting the operation of the image compression. Also it improved the property of losing the watermark and the efficiency of image compression by watermark inserting, which results from the quantization and Huffman-Coding during the image compression. Visual recognizable patterns such as binary image were used as a watermark The experimental results showed that the proposed algorithm satisfied the properties of robustness and imperceptibility that are the major conditions of watermarking.

• Tunnel and Underground Space
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• v.31 no.6
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• pp.469-479
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• 2021

As a material for preventing spalling of concrete, the effectiveness of PP fiber has already been confirmed. However, it is necessary to consider the maximum temperature that occurs during a fire, and to solve the mixing problem and the strength reduction problem that occur depending on the mixing amount. In this study, the fire resistance performance of tunnel segment linings according to the PP fiber content and air volume under the RABT fire scenario was investigated. As a result, no spalling or cross-sectional loss occurred in all test specimens, and when the PP fiber content was small, the maximum temperature was relatively high and the maximum temperature arrival time was also fast. On the other hand, no trend was found for the maximum temperature and arrival time according to the difference in air volume. In the internal temperature distribution results for the PP fiber mixing amount of 0.75, 1.0, 1.5, and 2.0 kg/m³, the results of 0.75 and 1.0 kg/m³ showed similar temperature distribution, and the results of 1.5 and 2.0 kg/m³ were similar. It was confirmed that the internal temperature distribution tends to decrease at the same depth when the amount of PP fiber mixed is large, and it was confirmed that a remarkable difference occurred from the results of 1.0 kg/m³ and 1.5 kg/m³ of PP fiber mixed amounts.

• The Journal of Engineering Geology
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• v.34 no.3
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• pp.403-413
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• 2024

This study aims to investigate the material properties of specimens made from pyrophyllite and pyrophyllite-aluminum mixed powders using high-speed, high-pressure dynamic compression technology. The aluminum powder exhibited a highly uniform particle size distribution ranging from 10 to 100 ㎛, whereas the pyrophyllite powder displayed four distinct particle size distributions: 0.1~1 ㎛, 1~10 ㎛, 80~100 ㎛, and 200~1,000 ㎛. Using high-speed, high-pressure dynamic compression technology with a drop time of approximately 0.34~0.4 seconds and a dynamic load of about 207 tonf, it was possible to fabricate pyrophyllite and pyrophyllite-aluminum mixed powder specimens with a volume of about 548 mm². The Shore hardness measurement results showed that specimen BG100 had an average of 43.7, BG90 had an average of 33.2, and BG85, BG80, BG75, and BG70 had an average of 31.0, indicating that the specimen with the least aluminum content exhibited the highest Shore hardness value. The thermogravimetric analysis revealed mass losses at two points: the first mass loss occurred at around 270℃ with a loss of approximately 1.45%, and the second mass loss occurred at around 600℃, where BG70 and BG80 showed a mass loss of about 2.53%, and BG75, BG85, and BG90 showed a mass loss of about 3.43%. Scanning electron microscopy analysis indicated that the microstructure of the specimens was similar regardless of the mixing ratio, with three elements-O, Si, and Al-being detected in all specimens. The mapping analysis of BG90 revealed an oxygen weight ratio of 50.80%, silicon weight ratio of 37.36%, and aluminum weight ratio of 11.85%. In the case of BG85, the results were 43.09% oxygen, 43.50% silicon, and 13.41% aluminum; for BG80, the results were 44.83% oxygen, 40.30% silicon, and 14.87% aluminum; for BG75, the results were 44.71% oxygen, 35.49% silicon, and 19.80% aluminum; and for BG70, the results were 34.95% oxygen, 35.73% silicon, and 29.32% aluminum.

• Electrical & Electronic Materials
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• v.7 no.6
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• pp.534-542
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• 1994

Microstructures and dielectric properties of (Sr$_{1-x}$ .Ca$_{x}$)TiO$_{3}$+0.006Nb$_{2}$O$_{5}$ (0.05.leq.x.leq.0.2) ceramic were investigated. The specimens fired in a reducing atmosphere(N$_{2}$) were painted on the surface with CuO paste, and then annealed at 1100.deg. C for 2 hr. SEM and EDAX revealed that CuO penetrated rapidly into the bulk along the grain boundaries during the annealing. Grain size increased with increasing Ca content up to 15[mol%], but decreased with further addition. In the specimens with 10-15[mol%l of Ca, excellent dielectric properties were obtained as follows; dielectric constant <25000, dielectric loss(tan .delta[%]) <0.3[%] and capacitance change rate with temperature <.+-.[%], respectively. All the specimens in this study exhibited dielectric relaxation with frequency as a function of the temperature. The dispersive frequency was over 10$^{6}$ [Hz].z].