• Elastomers and Composites
• /
• v.27 no.3
• /
• pp.161-173
• /
• 1992

A series of polyurethane block copolymers based on hydroxyterminated poly(dimethyl siloxane), poly(propylene glycol) and poly(tetramethylene glycol) soft segments of molecular weights 1,809, 2,000 and 2,000, respectively, were synthesized. The hard segments consisted of 4,4'-diphenylmethane diisocyanate and 1,4-butanediol as the chain extender. Samples with different molar ratios were prepared. We tried to synthesize poly(dimethyl siloxane)-based polyurethane(PDMS-PU) containing a hard block as major fraction and a soft block as minor fraction for preparing toughened rigid systems. After a study of the pure PDMS-PU, poly(propylene glycol)-based polyurethane(PPG-PU) and poly(tetramethylene glycol)-based polyurethane(PTMG-PU), (mixed polyol)-based block copolymers and blends between PDMS-PU, PPG-PU and PTMG-PU were prepared, and characterized by means of differential scanning calorimetry, tensile testing and scanning electron microscopy. In (mixed polyol)-based PU and in lower hard segment content blends, macro-phase separation was shown, but blends with higher hard segment contents showed significant reduction in amounts of phase separation.

• Journal of Powder Materials
• /
• v.8 no.1
• /
• pp.55-60
• /
• 2001

Mechanical alloying technique was applied to prepare hard magnetic $Sm_2Fe_{17}N_x$ compound powders. Staring from pure Fe and Sm powders, the formation process of hard magnetic $Sm_2Fe_{17}N_x$ phase by mechanical alloying and subsequent solid state reaction was studied. As milled powders were found to consist of Sm-Fe amorphous and $\alpha$-Fe phases in all compositions of $Sm_xFe_{100-x}$(x = 11, 13, 15, 17). The effects of starting composition on the formation of $Sm_2Fe_{17}$ intermetallic compound was investigated by heat treatment of mechanically-alloyed powders. When Sm content was 15 at.％, heat-treated powders consisted of nearly $Sm_2Fe_{17}$ single phase. For preparation of hard magnetic $Sm_2Fe_{17}N_x$ powders, additional nitriding treatment was performed under $N_2$ gas flow at 45$0^{\circ}C$. The increase in the coercivity and remanence was proportional to the nitrogen content which increased drastically at first and then increased gradually as the nitriding time was extended to 3 hours.

• Macromolecular Research
• /
• v.10 no.6
• /
• pp.325-331
• /
• 2002

Phase behavior of a PEO-PPO-PEO (Pluronic P103) triblock copolymer in water is investigated using small-angle neutron scattering (SANS), small-angle X-ray scattering (SAXS), dynamic light scattering (DLS) and rheology. Pluronic P103 shows apparent two gel states in different temperature regions. The first sol-to-gel transition at a lower temperature (i.e., the hard gel I state) turns out to be the hexagonal microphase as evidenced by the combined SANS and SAXS and the frequency dependence of both G′ and G" in rheology. In contrast to the hard gel I, the second sol-to-gel transition (i. e., the hard gel II state) at a higher temperature represents the block copolymer micelles in somewhat disordered state rather than the ordered state seen in the hard gel I. Moreover, turbidity change depending only on the temperature with four distinct regions is observed and the large aggregates with size larger than 5,000 nm are detected with DLS in the turbid solution region. Based upon the present study, two different gelation mechanisms for aqueous PEO-PPO-PEO triblock copolymer solutions are proposed.

• Korean Journal of Applied Biomechanics
• /
• v.32 no.2
• /
• pp.49-55
• /
• 2022

Objective: This study aims to verify the front squat motions using by two different surfaces, thereby elucidating the grounds for effective training environment that can minimize the risk of injury. Method: Total of 10 healthy male crossfit athletes were recruited for this study (age: 32.30 ± 3.05 yrs., height: 173.70 ± 5.12 cm, body mass: 82.40 ± 6.31 kg, 1RM: 160 ± 13.80 kg). All participants are those who know how to do front squats well with more than five years of crossfit athlete experience. All participants have sufficient experience in front squats on two types of surface which are soft surface (SS) and hard surface (HS). In each surface, participant perform 10reps of the front squat with 80% of the pre-measured 1RM. A 3-dimensional motion analysis with 8 infrared cameras and 2 channels of EMG was performed in this study. Paired sample t-test was used for statistical verification between two surfaces. The significant level was set at α=.05. Results: The significantly decreased rectus femoris muscle activation was found in SS compared with that of HS on phase 1 (p<.05). Also, ROM of ankle joint was significantly increased in the SS compare with that of HS on phase 1 (p<.05). The muscle activity ratio of gluteus maximus/rectus femoris showed a significant difference only in SS compared with that of HS on phase 1 (p<.05). Conclusion: In conclusion, Korean crossfit boxes should consider the use of hard surface, which has a relatively less risk of injury than soft surface, in selecting flooring materials. For the Crossfit athletes, they are also considered appropriate to train on a relatively hard surface.

• Journal of the Korean Ceramic Society
• /
• v.48 no.2
• /
• pp.173-177
• /
• 2011

Decomposition promoting factors and decomposition mechanism in the zinc decomposition process of waste hard metals which are composed mostly of tungsten carbide and cobalt were evaluated. Zinc volatility amount was suppressed and zinc valatilization pressure was produced in the reaction graphite crucible inside an electric furnace for ZDP. Reaction was done for 2 h at $650^{\circ}C$, which 100 % decomposed the waste hard metals that were over 30 mm thick. As for the separation-decomposition of waste hard metals, zinc melted alloy formed a liquid composed of a mixture of ${\gamma}-{\beta}1$ phase from the cobalt binder layer (reaction interface). The volume of reacted zone was expanded and the waste hard metal layer was decomposed-separated horizontally from the hard metal. Zinc used in the ZDP process was almost completely removed-collected by decantation and volatilization-collection process at $1000^{\circ}C$.

• Korean Journal of Materials Research
• /
• v.30 no.12
• /
• pp.672-677
• /
• 2020

Expensive PCBN or ceramic cutting tools are used for the processing of difficult-to-cut materials such as Ti and Ni alloy materials. These tools have a problem of breaking easily due to their high hardness but low fracture toughness. To solve this problem, cutting tools that form various coating layers are used in low-cost WC-Co hard material tools, and researches on various tool materials are being conducted. In this study, WC-5, 10, and 15 wt%Ni hard materials for difficult-to-cut cutting materials are densified using horizontal ball milled WC-Ni powders and pulsed current activated sintering method (PCAS method). Each PCASed WC-Ni hard materials are almost completely dense, with a relative density of up to 99.7 ~ 99.9 %, after the simultaneous application of pressure of 60 MPa and electric current for 2 min; process involves almost no change in the grain size. The average grain sizes of WC and Ni for WC-5, 10, and 15 wt%Ni hard materials are about 1.09 ~ 1.29 and 0.31 ~ 0.51 µm, respectively. Vickers hardness and fracture toughness of WC-5, 10, and 15 wt%Ni hard materials are about 1,923 ~ 1,788 kg/mm2 and 13.2 ~ 14.3 MPa.m1/2, respectively. Microstructure and phase analyses of PCASed WC-Ni hard materials are performed.

• Transactions of the Korean Society of Mechanical Engineers
• /
• v.7 no.4
• /
• pp.398-403
• /
• 1983

In this investigation, the effect of microstructural factors on the impact energy was studied, using dual phase steels with soft phase of ferrite encapsulated by hard phase of martensite. It was found that the effect of the ferrite grain size on the impact energy decreases as the strength ratio increases, these results are due to the micro-brittlefracture facet by the difficulty of cleavage crack formation in the ferrite grain.

• KIEE International Transaction on Electrical Machinery and Energy Conversion Systems
• /
• v.2B no.3
• /
• pp.115-124
• /
• 2002

This paper presents an active auxiliary edge-resonant DC link snubber with a ringing surge damper and a three-phase voltage source type zero voltage soft-switching inverter with the resonat snubber treated here for the AC servo motor driver applications. The operation of the active auxiliary edge-resonant DC link snubber circuit with PWM voltage is described, together with the practical design method to select its circuit parameters. The three-phase voltage source type soft-switching inverter with a single edge-resonant DC link snubber treated here is evaluated and discussed for the small-scale permanent magnet (PM) type-AC servo motor driver from an experimental point of view. In addition to these, the AC motor stator current and its motor speed response for the proposed three-phase soft-switching inverter employing Intelligent Power Module(IPM) based on IGBTS are compared with those of the conventional three-phase hard-switching inverter using IPM. The practical effectiveness of the three-phase soft-switching inverter-fed permanent magnet type AC motor speed tracking servo driver is proven on the basis of the common mode current in a novel type three-phase soft-switching inverter-fed AC motor side and the conductive noise on the mains terminal interface voltage as compared with those of the conventional three-phase hard-switching inverter-fed permanent magnet type AC servo motor driver for the speed tracking applications.

• Journal of the Microelectronics and Packaging Society
• /
• v.22 no.3
• /
• pp.45-50
• /
• 2015

The goal of this research is the create novel magnets with no rare-earth contents, with larger energy product by comparison with currently used ferrites. For this purpose we developed nano-sized hard-type/soft-type composite ferrite in which high remanent magnetization (Mr) and high coercivity (Hc). Nano-sized Ba-ferrite, Ni-Zn ferrite and $BaFe_{12}O_{19}/Ni_{0.5}Zn_{0.5}Fe_2O_4$ composite ferrites were prepared by sol-gel combustion method by use of glicine-nitrate and citric acid. Nanocomposite ferrites were calcined at temperature range $700-900^{\circ}C$ for 1h. According to the X-ray diffraction patterns and FT-IR spectra, single phase of NiZn-ferrite and Ba-ferrite were detected and hard/soft nanocomposite ferrite was indicated to the coexistence of the magnetoplumbite-structural $BaFe_{12}O_{19}$ and spinel-structural $Ni_{0.5}Zn_{0.5}Fe_2O_4$ that agreed with the standard JCPDS 10-0325 data. The particle size of nanocomposite turn out to be less than 120 nm. The nanocomposite ferrite shows a single-phase magnetization behavior, implying that the hard magnetic phase and soft magnetic phase were well exchange-coupled. The specific saturation magnetization ($M_s$) of the nanocomposite ferrite is located between hard ($BaFe_{12}O_{19}$) and soft ferrite($Ni_{0.5}Zn_{0.5}Fe_2O_4$). The remanence (Mr) of nanocomposite ferrite is much higher than that of the individual $BaFe_{12}O_{19}$ and $Ni_{0.5}Zn_{0.5}Fe_2O_4$ ferrite, and $(BH)_{max}$ is increased slightly.

• Proceedings of the Korean Powder Metallurgy Institute Conference
• /
• 2003.10a
• /
• pp.39-40
• /
• 2003

1) Using a developed high-frequency induction heated sintering method, the rapid densification of WC-Co hard materials was accomplished using ultra fine powders with 260 nm size within 1 minute. 2) The relative density of the composite was 99.5% for the applide pressure of 60MPa and the induced current for 90% output of total capacity. 3) The grain size of WC-Co hard materials is about 260nm and the average thickness of the binder phase determined is about 11nm. The fracture toughness and the hardness of this work 12 $MPa{\cdot}nm^2$, respectively. 4) Using pressureless sintering, we produced dense WC-Co hard materials with a relative density of 97% without applying pressure.