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Beyond IMT-2000 시스템을 위한 OFDM 무선 전송 기술
조용수,장경희,Cho, Y.S.,Chang, K.H. 한국전자통신연구원 2002 전자통신동향분석 Vol.17 No.1
본 고에서는 Beyond IMT-2000 시스템에서 Mobile Access의 무선 전송 기법으로 OFDM 방식의 적용 가능성에 대하여 살펴본다. 현재 사용되는 CDMA 방식은 고속 전송 시 chip rate가 크게 증가하기 때문에, Beyond IMT-2000 시스템의 Mobile Access에서 요구하는 2∼100Mbps의 고속 데이터 전송률을 만족시키기가 용이하지 않다. 반면, OFDM 방식은 주파수 효율이 높고, 주파수 선택적 페이딩 채널에 쉽게 대처할 수 있으며, FFT를 사용하여 고속으로 구현할 수 있기 때문에 고속 데이터 전송 시 적합하다. 본 고에서는 OFDM 방식을 사용한 기존의 통신 시스템의 특징을 살펴본 후, Beyond IMT-2000 시스템 Mobile Access의 전송방식으로 OFDM을 적용할 경우의 기술적인 고려사항과 모의실험 결과에 대하여 서술한다.
Fe68.5Co5M₃Cu₁Si13.5B9(M=Nb, Mo, Mn, Cr)계 초미세결정립합금의 결정립 크기에 다른 자기적 특성의 변화
조용수(Y. S. Cho),김택기(T. K. Kim) 한국자기학회 1991 韓國磁氣學會誌 Vol.1 No.1
초급냉법중 단롤법으로 제작한 비정질 Fe_(68.5)Co_5M₃Cu₁Si_(13.5)B_9(M=Nb, Mo, Mn, Cr)계 합금을 결정화온도 이상의 온도에서 열처리하여 결정화 시킨 후, 결정립 크기에 따른 항자력, 투자율 및 교류자기이력손실을 조사하였다. Fe_(68.5)Co_5M₃Cu₁Si_(13.5)B_9(M=Nb, Mo, Mn, Cr)합금중 M=Mo, Nb조성에서 초미세결정립이 형성되며 약 10 ㎚의 결정립 크기에서 가장 우수한 연자기적 특성을 나타낸다. 그러나 결정립의 크기가 10 ㎚보다 작거나 15 ㎚이상 커지면 연자기 특성이 열화된다. 결정립크기가 10㎚이하에서 연자기특성이 열화되는 것은 결정화 초기 결정립계에 존재하는 것으로 판단되는 Fe rich 비정질상에 의한 것으로 고찰된다. Amorphous Fe_(68.5)Co_5M₃Cu₁Si_(13.5)B_9(M=Nb, Mo, Mn, Cr) alloys were prepared by using rapidly quenching techinque and were annealed above their crystallization temperatures. Coercive force, initial permeability and AC power loss of the annealed Fe_(68.5)Co_5M₃Cu₁Si_(13.5)B_9(M=Nb, Mo, Mn, Cr) alloys have been studied systematically. Nanocrystallines are formed in the annealed alloys which include Mo and Nb. Remarkably improved soft magnetic properties are obtained in the alloys whose average grain size is around 10㎚. However, soft magnetic properties of the alloys are degraded when grain size is less than 10㎚ or larger than 15㎚. It is considered that the degradation of soft magnetic properties in the alloys whose average grain size is less than 10 ㎚ is due to the Fe-rich amorphous phase retained at grain boundary during the initial crystallization process.
Fe - B - Al계 비정질합금의 자왜 및 자기탄성효과
조용수(Y. S. Cho),김윤배(Y. B. Kim),김창석(C. S. Kim),김택기(T. K. Kim) 한국자기학회 1993 韓國磁氣學會誌 Vol.3 No.2
Saturation magnetostriction and magnetoelastic properties of amorphous Fe_(82)B_(18-x)Al_x and Fe_(80)B_(20-x)Al_x alloys have been studied. Saturation magnetostriction of the alloys has increased according to the increase of Al content. The amorphous Fe_(82)B₁₄A₄ alloy shows the highest saturation magnetostriction of 45 ppm among the alloy systems. The ratio of maximum magnetic induction change to tensile stress of this alloy is about 0.026 Tㆍ㎟/N, and it is considered to be applicable for a high resolution mechanical sensor.
고포화자화 (Fe, Co) - Zr - B - Cu계 초미세결정립합금의 자기특성
조용수(Y. S. Cho),김동환(D. H. Kim),김택기(T. K. Kim) 한국자기학회 1993 韓國磁氣學會誌 Vol.3 No.3
Structure and magnetic properties of the Fe_(85-x)Co_xZr_7B_7Cu₁ alloys crystallized over their crystallization temperature have been invastigated. Nanocrystalline composed of α-Fe(Co) of about 10 ㎚ has heen obtained in all rangs of x by annealing below 600 ℃. It is found that average grain size of the alloys abruptly increase at annealing temperature over 600 ℃ and the degradation of magnetic properties occurs. The nanocrystalline Fe_(85-x)Co_xZr_7B_7Cu₁ alloys show not only lower optimum annealing temperature, but smaller grain size than Fe-Zr-B nanocrystalline. It is seems to be due to addition of Cu to Fe-Zr-B alloys. The nanocrystalline Fe_(80)Co_5Zr_7B_7Cu₁ alloy annealed at 540 ℃ during 1 hr shows the most improved magnetic properties among the prepared alloys. Saturation magnetization, permeability and power loss(f=50 ㎑, Bm=0.2T) of the alloy is 157.3 emu / g (1.5 T), 1.8×10⁴ and 13 W / ㎏, respectively.
(Fe, Co) - B - Al - Nb 초미세 결정립합금의 제조 및 자기적 특성
조용수(Y. S. Cho),김윤배(Y. B. Kim),김창석(C. S. Kim),김택기(T. K. Kim) 한국자기학회 1993 韓國磁氣學會誌 Vol.3 No.3
Magnetic properties of (Fe, Co)-B-Al-Nb alloys prepared by rapid solidification process have been studied for the development of new type Fe based nanocrystalline alloy. Phases with α-Fe(Co) crystallities in amorphous matrix has been obtained directly from the rapid solidification by substituting 10 at. %Al for transition metal in (Fe._(85)Co._(15))_(80)B_(20) amorphous alloy. Addition of 2~6 at. % Nb to (Fe._(85)Co._(15))_(70)B_(20)Al_(20) alloy decrease saturation magnetostriction below 6 ppm, and improves soft magnetic properties. Heat treatment effects decrease with increasing Nb content. (Fe._(85)Co._(15))_(70)B_(18)Al_(10)Nb₂ alloy annealed at 400 ℃ during 1 hr shows structure composed of ultra-fine grains of about 8 ㎚ and the most improved magnetic properties among the prepared alloys. Saturation magnetic flux density, power loss and permeability(f=50 ㎑, Bm=0.2 T) of the alloy are 1.1 T, 12 W / ㎏ and 2.5×10⁴ respectively.
조용수(Y. S. Cho),김윤배(Y. B. Kim),김창석(C. S. Kim),김택기(T. K. Kim) 한국자기학회 1994 韓國磁氣學會誌 Vol.4 No.2
In order to invetigate the effect of Pr additive on the magnetostriction of amorphous Fe-B alloys, amorphous Fe_(86-x)B₁₄ Pr_x (2≤x≤8 at. %) alloys were prepared by a rapid solidification process. As the Pr content increased in the as-prepared amorphous Fe_(84)B₁₄Pr₂ alloy annealed at 300℃ for 2 hr increased to 70 ppm Ac power loss and permeability(f= 50㎑, Bm = 0.1 T) of the annealed amorphous Fe_(84)B₁₄Pr₂ alloy were 15 W/㎏ and 5.5 × 10³, respectively.
고잔류자화 α-Fe기 Nd-Fe-B 초미세결정립 합금의 자기특성
조용수(Y. S. Cho),김윤배(Y. B. Kim),박우식(W. S. Park),김창석(C. S. Kim),김택기(T. K. Kim) 한국자기학회 1995 韓國磁氣學會誌 Vol.5 No.1
The effects of Nb and Cu additives as will as substitutional Co into Nd₄Fe_(85.5)B_(10.5) melt-spun alloy were studied aiming for finding a α-Fe based Nd-Fe-B composite alloys with high energy product. The addition of Nb and Cu to Nd₄Fe_(85.5)B_(10.5) decreased the average grain size and increased the coercivity up to 207㎄/m(2.6kOe). Furthermore, the substitution of Co for Fe in Nd₄Fe_(82)B_(10)Nd₃Cu₁ alloy resulted in the decrease of the average grain size (< 2O㎚) and improved the hard magnetic properties. The remanence, coercivity and energy product of optimally annealed Nd₄Fe_(74)Co_8B_(10)Nb₃Cu₁ alloy were 1.345, 219㎄/m(2.75kOe) and 95.5kJ/㎥ (12MGOe), respectively.
Ndx(Fe0.9Co0.1)90-xB6Nb₃Cu₁(x=3, 4, 5) 초미세결정립합금의 자기특성
조용수(Y. S. Cho),김만중(M. J. Kim),천정남(Z. N. Qian),김택기(T. K. Kim),박우식(W. S. Park),김윤배(Y. B. Kim) 한국자기학회 1995 韓國磁氣學會誌 Vol.5 No.5
Magnetic properties of Nd_x(Fe_(0.9)Co_(0.1))_(90-x)B_6Nb₃Cu₁(X = 3, 4, 5) melt-spun alloys with 6 at% B content were studied aiming for finding out a new α-Fe based Nd-Fe-B nanocrystalline alloy with good hard magnetic properties. Nd_x(Fe_(0.9)Co_(0.1))_(90-x)B_6Nb₃Cu₁ melt-spun alloys prepared by RSP crystallized to nanocrystalline phase. An optimally annealed Nd₃(Fe_(0.9)Co_(0.1))_(87)B_6Nb₃Cu₁ melt-spun alloys had larger volume ratio of α-Fe (Co) than that of higher Nd content alloy and showed high remanence of about 1.6 T. On the contrary, the increase of Nd content in Nd_x(Fe_(0.9)Co_(0.1))_(90-x)B_6Nb₃Cu₁ alloys gave rise to gradual increase of an amount of Nd₂(Fe, Co)₁₄B phase and improved coercivity. An optimally annealed Nd_5(Fe_(0.9)Co_(0.1))_(85)B_6Nb₃Cu₁ alloy showed the most improved hard magnetic properties. The remanence, coercivity and energy product of the alloy were 1.35 T, 219 ㎄/m (2.75 kOe), and 129 kJ/㎥ (16.2 MGOe), respectively.
민석홍(S. H. Min),조용수(Y. S. Cho),하태권(T. K. Ha) 한국소성가공학회 2014 한국소성가공학회 학술대회 논문집 Vol.2014 No.10
High temperature deformation behavior of cast 310S stainless steel has been investigated in this study by performing tensile and compression tests at temperatures from 900 to 1200℃. Rectangular ingots of which the dimensions were 350×350×100 in millimeter were cast using vacuum induction melting. Phase equilibrium was calculated using the FactSage<SUP>Ⓡ</SUP>, thermodynamic software and database. Thermal expansion coefficient was also measured on the ingot in the temperature range from room temperature to 1200℃. Tensile strength of cast 310S stainless steel was 9 MPa at 1200℃, which is a little higher than that of a wrought 310S. With temperature decreased, tensile strength increased rapidly and reached up to 72 MPa at 900℃. Elongation also increased with temperature decreased. Microstructure observation revealed that σ phase was precipitated along the grain boundary and within the matrix over 1200℃, which is detrimental to high temperature elongation.