• 한국분말재료학회지
• /
• 제15권5호
• /
• pp.371-377
• /
• 2008

This paper deals with the phase analysis of $MgB_2$ bulk using spark plasma sintering process after ball milling. Mg and amorphous B powders were used as raw materials, and milled by planetary-mill for 9 hours at argon atmosphere. In order to confirm formation of $MgB_2$ phase, DTA and XRD were used. The milled powders were fabricated to $MgB_2$ bulk at the various temperatures by Spark Plasma Sintering. The fabricated $MgB_2$ bulk was evaluated with XRD, EDS, FE-SEM and PPMS. In the DTA result, reaction on formation of $MgB_2$ phase started at $340^{\circ}C$. This means that ball milling process improves reactivity on formation of $MgB_2$ phase. The $MgB_2$ MgO and FeB phases were characterized from XRD result. MgO and FeB were undesirable phases which affect formation of $MgB_2$ phase, and it's distribution could be confirmed from EDS mapping result. Spark Plasma Sintered sample for 5 min at $700^{\circ}C$ was relatively densified and it's density and transition temperature showing super conducting property were $1.87\;g/cm^3$ and 21K.

• 한국분말재료학회지
• /
• 제22권5호
• /
• pp.344-349
• /
• 2015

$MgB_2$ bulk superconductors are synthesized by the solid state reaction of ($MgB_4$+xMg) precursors with excessive Mg compositions (x=1.0, 1.4, 2.0 and 2.4). The $MgB_4$ precursors are synthesized using (Mg+B) powders. The secondary phases ($MgB_4$ and MgO) present in the synthesized $MgB_4$ are removed by $HNO_3$ leaching. It is found that the formation reaction of $MgB_2$ is accelerated when Mg excessive compositions are used. The magnetization curves of $Mg_1+_xB_2$ samples show that the transition from the normal state to the superconducting state of the Mg excessive samples with x=0.5 and x=0.7 are sharper than that of $MgB_2$. The highest $J_c-B$ curve at 5 K and 20 K is achieved for x=0.5. Further addition of Mg decreases the $J_c$ owing to the formation of more pores in the $MgB_2$ matrix and smaller volume fraction of $MgB_2$.

• 한국초전도ㆍ저온공학회논문지
• /
• 제19권2호
• /
• pp.19-24
• /
• 2017

Cryogenic milling which is a combined process of low-temperature treatment and mechanical milling was applied to fabricate high critical current density $(J_c)MgB_2$ bulk superconductors. Liquid nitrogen was used as a coolant, and no solvent or lubricant was used. Spherical Mg ($6-12{\mu}m$, 99.9 % purity) and plate-like B powder (${\sim}1{\mu}m$, 97 % purity) were milled simultaneously for various time periods (0, 2, 4, 6 h) at a rotating speed of 500 rpm using $ZrO_2$ balls. The (Mg+2B) powders milled were pressed into pellets and heat-treated at $700^{\circ}C$ for 1 h in flowing argon. The use of cryomilled powders as raw materials promoted the formation reaction of superconducting $MgB_2$, reduced the grain size of $MgB_2$, and suppressed the formation of impurity MgO. The superconducting critical temperature ($T_c$) of $MgB_2$ was not influenced as the milling time (t) increased up to 6 h. Meanwhile, the critical current density ($J_c$) of $MgB_2$ increased significantly when t increased to 4 h. When t increased further to 6 h, however, $J_c$ decreased. The $J_c$ enhancement of $MgB_2$ by cryogenic milling is attributed to the formation of the fine grain $MgB_2$ and a suppression of the MgO formation.

• 한국초전도ㆍ저온공학회논문지
• /
• 제19권1호
• /
• pp.42-46
• /
• 2017

The effects of the heat treatment temperature ($600^{\circ}C-1050^{\circ}C$) on the formation of $MgB_2$ and the superconducting properties have been examined. The self-synthesized $MgB_4$ and commercial Mg powders were used as raw materials for the formation of $MgB_2$. The superconducting critical temperatures ($T_cs$) of $MgB_2$ bulk superconductors prepared at $600^{\circ}C-850^{\circ}C$ were as high as 37-38 K regardless of the heat treatment temperature. However, because $MgB_4$ is more stable than $MgB_2$ at above $850^{\circ}C$, no superconducting signals were detected in the susceptibility-temperature curves of the samples prepared above $850^{\circ}C$. As for the critical current density ($J_c$), the sample heat-treated at a low temperature ($600^{\circ}C$) for a prolonged period (40 h) showed a Jc higher than those prepared at $650^{\circ}C-850^{\circ}C$ for a short period (1 h). The FWHM (full width at half maximum) result showed that the grain size of $MgB_2$ of the $600^{\circ}C$ sample was smaller than that of the other samples. The high $J_c$ of the $600^{\circ}C$sample is attributed to the presence of large numbers of grain boundaries, which can act as flux pinning centers of $MgB_2$.

• 한국초전도ㆍ저온공학회논문지
• /
• 제23권3호
• /
• pp.45-50
• /
• 2021

In this study, the effect of the size of B powder on the critical current density (J_c) of MgB₂ prepared by an in situ reaction process was investigated. Various combinations of B powders were made using a micron B, ball-milled B and nano B powders. Micron B powder was reduced by ball milling and the milled B powder was mixed with the micron B or nano B powder. The mixing ratios of the milled B and micron or nano B were 100:0, 50:50 and 0:100. Non-milled micron B powder was also mixed with nano powder in the same ratios. Pellets of (2B+Mg) prepared with various B mixing ratios were heat-treated to form MgB₂. T_c of MgB₂ decreased slightly when the milled B was used, whereas the J_c of MgB₂ increased with increasing amount of the milled B or the nano powder. The used of the milled B and nano B power promoted the formation MgB₂ during heat treatment. In addition to the enhanced formation of MgB₂, the use of the powders reduced the grain size of MgB₂. The use of the milled and nano B powder increased the J_c of MgB₂. The highest J_c was achieved when 100% nano B powder was used. The J_c enhancement is attributed to the high volume fraction of the superconducting phase (MgB₂) and the large grain boundaries, which induces the flux pinning at the magnetic fields.

• 한국초전도ㆍ저온공학회논문지
• /
• 제18권4호
• /
• pp.9-14
• /
• 2016

The effect of the size and shape of magnesium(Mg) powder on the formation of $MgB_2$ and the critical current density($J_{c,}$) of $MgB_2$ bulk was studied. As a precursor for the formation of $MgB_2$, Mg and $MgB_4$ powder, which was synthesized through the reaction of boron (B) with Mg powders, was used. $MgB_4$ was mixed with Mg powders of various sizes, pressed into pellets and heat-treated at $650^{\circ}C-750^{\circ}C$ in flowing argon gas. The XRD analysis of the heat-treated $MgB_2$ samples showed that the volume fraction of $MgB_2$ was the highest as 92.74 % when spherical Mg powder with an average size of $25.7{\mu}m$ was used, whereas the volume fraction was the lowest as 79.64 % when plate-like Mg powder with a size of $34.1{\mu}m$ was used. The superconducting transition temperature ($T_c$) of $MgB_2$ was not sensitive to the characteristics of the Mg powders used. All of the prepared $MgB_2$ samples showed a high $T_c$ of 38.3 K and a small superconducting transition width of 0.2 K-0.5 K. $J_c$ (5 K and 1 T) of $MgB_2$ was the highest as $3.93{\times}10^4A/cm^2$ when spherical Mg powder with a size of $25.7{\mu}m$ was used, whereas $J_c$ was the lowest as $2.18{\times}10^4A/cm^2$when plate-like Mg powder with a size of $34.1{\mu}m$ was used. The relationship between the $J_c$ of $MgB_2$ and the characteristics of the Mg powders used was explained in terms of the volume fraction of $MgB_2$ and the apparent density of the $MgB_2$ pellets.

• 한국초전도ㆍ저온공학회논문지
• /
• 제16권1호
• /
• pp.19-22
• /
• 2014

The effects of Mg content on the pore formation, density and critical properties were investigated in in-situ reacted $MgB_2$ superconductors. The $Mg_{1+x}B_2$, (x=-0.2, 0.0, 0.05, 0.3, 1.0) bulk samples with different Mg contents were heat-treated at $900^{\circ}C$ for 1 h in an Ar atmosphere. The dimensional changes of a pellet's mass and volume after heat-treatment were measured. After heat-treatment process, the sample mass was decreased by Mg evaporation, but the sample volume was expanded by pore formation at the Mg site; therefore, the apparent density was decreased. Spherical pores the same as Mg particles were developed after heat-treatment in all samples, and the pore density was increased with increasing Mg content. As the x of Mg content was increased to 1.0, the apparent density of $Mg_{1+x}B_2$ samples was decreased due to a relatively larger reduction in a mass change. The critical current density of Mg excessive sample of x=0.05 showed the highest values over the applied magnetic fields because the excessive Mg may compensate Mg loss and enhance grain connectivity.

• Progress in Superconductivity
• /
• 제9권1호
• /
• pp.74-79
• /
• 2007

We characterized the highly refined boron precursor powders which were attrition milled for different milling times. $MgB_2$ powder precursor was formed from elemental crystalline Mg and amorphous B powder. The microstructure was investigated by SEM. SEM results indicate that the size of the milled powders was reduced with increasing milling time, which were varied from 0 to 8 hours. We also studied thermal behavior of the starting precursor by DSC as a function of milling time. The thermal behavior of the powder precursors was influenced by milling time. In order to determine the thermal events at DSC peaks, we annealed the milled powder mixture at $600^{\circ}C$ and $650^{\circ}C$ under protective gas and then analyzed the formation of $MgB_2$ by the XRD. We observed that superconducting $MgB_2$ phase was formed at lower temperature by the longer high energy milling. These results show that the high energy milling of the boron precursor powder can improve the reactivity for the formation of $MgB_2$.

• 한국초전도ㆍ저온공학회논문지
• /
• 제21권1호
• /
• pp.18-24
• /
• 2019

This study was carried out to investigate the effect of milling of boron (B), which is one of raw materials of $MgB_2$, on the critical current density ($J_c$) of $MgB_2$. B powder used in this study is semi-amorphous B (Pavezyum, Turkey, 97% purity, 1 micron). The size of B powder was reduced by planetary milling using $ZrO_2$ balls (a diameter of 2 mm). The B powder and balls with a ratio of 1:20 were charged in a ceramic jar and then the jar was filled with toluene. The milling time was varied from 0 to 8 h. The milled B powders were mixed with Mg powder in the composition of (Mg+2B), and the powder mixtures were uniaxially pressed at 3 tons. The powder compacts were heat-treated at $700^{\circ}C$ for 1 h in flowing argon gas. Powder X-ray diffraction and FWHM (Full width at half maximum) were used to analyze the phase formation and crystallinity of $MgB_2$. The superconducting transition temperature ($T_c$) and $J_c$ of $MgB_2$ were measured using a magnetic property measurement system (MPMS). It was found that $B_2O_3$ was formed by B milling and the subsequent drying process, and the volume fraction of $B_2O_3$ increased as milling time increased. The $T_c$ of $MgB_2$ decreased with increasing milling time, which was explained in terms of the decreased volume fraction of $MgB_2$, the line broadening of $MgB_2$ peaks and the formation of $B_2O_3$. The $J_c$ at 5 K increased with increasing milling time. The $J_c$ increase is more remarkable at the magnetic field higher than 3 T. The $J_c$ at 5 K and 4 T was the highest as $4.37{\times}10^4A/cm^2$ when milling time was 2 h. The $J_c$ at 20 K also increased with increasing milling time. However, The $J_c$ of the samples with the prolonged milling for 6 and 8 h were lower than that of the non-milled sample.

• 식물조직배양학회지
• /
• 제28권6호
• /
• pp.329-333
• /
• 2001

배지, 절편체의 종류 그리고 생장조절제가 약용작물인 구기자나무 (Lycium chinese Mill. 'Cheongyang')의 캘러스 유도 및 신초 분화조건에 미치는 영향을 조사하였다. 배지 종류별 기관분화는 신초 분화율, 신초 발생수 및 생체중이 MS, B5, WPM 배지 순으로 양호하였다. 구기자나무의 캘러스 유도는 절간 절편체보다 잎 절편체가 좋았고, 캘러스는 0.5mg/L NAA와 0.2 mg/L BA가 첨가된 MS배지에서 효율적으로 유도되었다. 신초 분화에 관여하는 cytokinin으로는 TDZ보다는 BA가 좋았으며 BA 단일처리보다는 NAA와의 혼용처리에서 더 양호하였다. 신초분화는 0.01 mg/L NAA와 0.2 mg/L BA가 첨가된 MS 배지에서 가장 좋았다