• Journal of Korean Tunnelling and Underground Space Association
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• v.21 no.3
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• pp.323-345
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• 2019

It is essential to predict ground conditions ahead of a tunnel face in order to successfully excavate tunnels using a shield TBM. This study proposes a forward prediction method for a mixed soil ground and/or a ground containing core stones by using electrical resistivity and induced polarization exploration. Soil conditioning in EPB shield TBM is dependent upon the composition of mixed soils; a special care need to be taken when excavating the core-stoned soil ground using TBM. The resistivity and chargeability are assumed to be measured with four electrodes at the tunnel face, whenever the excavation is stopped to assemble one ring of a segment lining. Firstly, the mixed ground consisting of weathered granite soil, sand, and clay was modeled in laboratory-scale experiments. Experimental results show that the measured electrical resistivity considerably coincides with the analytical solution. On the other hand, the induced polarization has either same or opposite trend with the measured resistivity depending on the mixed ground conditions. Based on these experimental results, a method to predict the mixed soil ground that can be used during TBM tunnel driving is suggested. Secondly, tunnel excavation from a homogeneous ground to a ground containing core stones was modeled in laboratory scale; the irregularity of the core stones contained in the soil layer was modeled through random number generation scheme. Experimental results show that as the TBM approaches the ground that contains core stones, the electrical resistivity increases and the induced polarization fluctuates.

• Journal of the Korean Magnetics Society
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• v.14 no.3
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• pp.99-104
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• 2004

We have studied to understand the barrier and interface qualities and structural changes through measuring temperature dependent spin-polarization as functions of plasma oxidation time and annealing time. Magnetic tunnel junctions consisting of SiO2$_2$/Ta 5/CoFe 17/IrMn 7.5/CoFe 5/Al 1.6-Ox/CoFe 5/Ta 5 (numbers in nm) were deposited and annealed when necessary. A 30 s,40 s oxidized sample showed the lowest spin-polarization values. It is presumed that tunneling electrons were depolarized and scattered by residual paramagnetic Al due to under-oxidation. On the contrary, a 60s, 70 s oxidized sample might have experienced over-oxidation, where partially oxidized magnetic dead layer was formed on top of the bottom CoFe electrode. The magnetic dead layer is known to increase the probability of spin-flip scattering. Therefore it showed a higher temperature dependence than that of the optimum sample (50 s oxidation). temperature dependence of 450 K annealed samples was improved when the as-deposited one compared. But the sample underwent 475 K and 500 K annealing exhibits inferior temperature dependence of spin-polarization, indicating that the over-annealed sample became microstucturally degraded.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.12 no.1
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• pp.36-41
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• 2002

Abstract The stoichiometric and double-ordered perovskite $Sr_2FeMoO_6$ (SFMO) polycrystalline ceramics were fabricated by sintering at above $900^{\circ}C$ in $H_2$(5%)/Ar reductive ambient. SMO polycrystals showed good ferromagnetic properties andmagnetrotesistqnce ratios of about 15 % at 8K and 3 % at room temperature. Amorphous SFMO thin films were deposited on $LaA1O_3$ and $SrTiO_3$ single crystal substrates using rf sputtering method with the SFMO polycrystalline ceramic target. Double-ordered perovskite polycrystalline SFMO thin films were fabricated by solid state crystallization by annealing the deposited amorphous films at above $680^{\circ}C$ in $H_2$(5%)/Ar reductive ambient. SFMO thin films exhibited ferromagnetic behavior. Their magnetroresistance ratios, however, were only 0.3~0.5% at 8K and disappeared with increasing the measuring temperature. This was attributed to the absence of magnetic spin tunneling between grains due to the porous structure and non-stoichiometric composition of the deposited films.

• Journal of Korean Tunnelling and Underground Space Association
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• v.22 no.1
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• pp.135-144
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• 2020

Since the depth of tunneling with tunnel boring machine (TBM) becomes deeper and deeper, the expense for site investigation for coring and geophysical survey increases to obtain the sufficient accuracy. The tunnel ahead prediction methods have been introduced to overcome this limitation in the stage of site investigation. Probe drilling can obtain the core and borehole images from a borehole. However, the space in TBM for the probe drilling equipment is restricted and the core from probe drilling cannot reflect the whole tunnel face. Seismic methods such as tunnel seismic prediction (TSP) can forecast over 100 m ahead from the tunnel face though the signal is usually generated using the explosive which can affect the stability of segments and backfill grout. Electromagnetic methods such as tunnel electrical resistivity prospecting system (TEPS) offer the exact prediction for a conductive zone such as water-bearing zone. However, the number of electrodes installed for exploration is limited in small diameter TBM and finally the reduction of prediction ranges. In this study, the theoretical equations for the electrical resistivity survey whose electrodes are installed in the face and side of TBM to minimize the installed electrodes on face. The experimental tests were conducted to verify the derived equations.

• Korean Journal of Materials Research
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• v.13 no.6
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• pp.394-397
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• 2003

MTJ devices are fabricated using metal shadow masks and switching characteristics are investigated under 2 dimensional magnetic field. When the hard axis field is less than $\pm$ 16 Oe, switching behavior is similar to that based on the Stoner-Wohlfarth model. As the hard axis field is larger than $\pm$ 16 Oe, deviation from the expectation by Stoner-Wohlfarth model is observed. These phenomena are induced by the generation of multi-domain and inhomogeneous magnetization reversal.

• Proceedings of the Korean Magnestics Society Conference
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• 2002.12a
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• pp.174-175
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• 2002

A ramp-type tunneling magnetoresistance (TMR) junction having structure NiO(60 nm)/pinned Co(10 nm)MiO(60 nm)/barrier SrTiO$_3$(2-10 nm)/free NiFe(10 nm) with the 15 degree slope was investigated. We obtained nonlinear I(V) characteristics for ramp-type tunneling junctions that have distinctive difference with and without applied magnetic field. In the barrier SrTiO$_3$ thickness of 4 nm, the TMR was about 52% at a bias voltage of 50 mV. (omitted)

• Journal of the Korean Magnetics Society
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• v.12 no.3
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• pp.99-102
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• 2002

We fabricated TMR devices which have doubly oxidized tunnel barrier using plasma oxidation method to form homogeneously oxidized AlO tunnel barrier. We sputtered 10 $\AA$-bottom Al layer and oxidized it with oxidation time of 10 sec. Subsequent sputtering of 13 $\AA$-Al was performed and the metallic layer was oxidized for 50, 80, and 120 sec., respectively. The electrical resistance changed from 500 Ω to 2000 Ω with increase of oxidation time, while variation of MR ratio was little spreading 27∼31 % which is larger than that of TMR device of ordinary single tunnel barrier. We calculated effective barrier height and width by measuring I-V curves, from which we found the barrier height was 1.3∼1.8 eV sufficient for tunnel barrier, and the barrier width (<15.0 $\AA$) was smaller than physical thickness. Our results may be caused by insufficient oxidation of Al precursor into A1$_2$O$_3$. However, doubly oxidized tunnel barriers were superior to conventional single tunnel barrier in uniformity and density. Our results imply that we were able to improve MR ratio and tune resistance by employing doubly oxidized tunnel barrier process.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2008.06a
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• pp.145-145
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• 2008

갈륨-질화물(GaN) 기반의 고속전자이동도 트랜지스터(high electron mobility transistor, HEMT)는 최근 마이크로파 또는 밀리미터파 등의 차세대 고주파용 전력소자로 각광받고 있다. AlGaN/GaN HEMT는 이종접합구조(heterostructure) 로부터 발생하는 이차원 전자가스(two-dimensional electron gas, 2DEG) 채널을 이용하여 높은 전자 이동도, 높은 항복전압 및 우수한 고출력 특성을 얻는 것이 가능하다. AlGaN/GaN HEMT에서 ohmic 전극 부분과 채널이 형성되는 부분과의 거리에 의한 저항의 성분을 줄이고 전자의 터널링의 확률을 증가시키기 위해서 recess된 구조가 많이 사용되고 있다. 그러나 이 구조에서는 recess된 소스와 드레인에 의해 AlGaN층의 제거로 AlGaN층의 두께에 영향을 미치며 그에 따라 채널에 생성되는 전자의 농도를 변화시키게 된다. 본 논문에서는 소스와 드레인의 Trench 구조를 제안하였다. ohmic 전극 부분과 채널간의 거리의 감소로 특성을 향상시켜서 recess 구조의 장점이 유지된다. 그리고 recess되는 소스와 드레인 영역에서 AlGaN층을 전체적으로 제거하는 것이 아니고 Trench 즉 일부분만 제거하면서 AlGaN층의 두께의 변화에 따른 문제점도 줄일 수 있다. 따라서 이러한 전극 부분을 Trench구조화 시킨 AlGaN/GaN HEMT의 DC특성을 $ATLAS^{TM}$를 이용하여 전산모사하고 최적화된 구조를 제안하였다.

• Journal of Korean Tunnelling and Underground Space Association
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• v.23 no.2
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• pp.93-106
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• 2021

In general, segment lining tunnel refers to a tunnel formed by connecting precast concrete segments as a ring and connecting such rings to each other in the longitudinal direction of the tunnel. As the structural properties of the segment lining is highly dependent on the behavior of the segment joints, thus correct modelling of joint behavior is crucial to understand and design the segment tunnel lining. When the tunnel is subjected to ground loads, the segment joint behaves like a hinge that resists rotation, and when the induced moment exceeds a certain limit of the rotation then it may enter into non-linear field. In understanding the effect of the segment joint on the lining behavior, a moment-rotation relationship of the segment joint was explored based on the Japanese practice and Janssen's approach commonly used in the actual design. This study also presents a method to determine the rotational stiffness of joint refer to the bearing strength. The rotation of the segment joint was estimated in virtual design conditions based on the existing models and the proposed method. And the sectional force of the segment lining and joint were calculated along with the estimated rotation. As the rotation at the segment joint increases, the joint contact area decreases, so the designer have to verify the segment joint for bearing strength as well. This paper suggests a consistent method to determine the rotational stiffness and bearing strength of joints.

• 한국지구물리탐사학회:학술대회논문집
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• 2006.06a
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• pp.109-112
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• 2006

Successful field test results for high-current time-lapse electrical imaging in marine sediments area are discussed. Because field trial by commercially available equipments were failed, self-developed system which supports transmitting current up to 5 ampere was used. Some weak zones due to local fractures were detected, but the weak zone effect in this area by time-lapse measurements from sea level change was minor.