• 한국재료학회지
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• 제34권2호
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• pp.116-124
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• 2024

In this study, we investigated the microstructure and piezoelectric properties of 0.96(K_0.456Na_0.536)Nb_0.95Sb_0.05-0.04Bi_0.5(Na_0.82K_0.18)_0.5ZrO₃ (KNNS-BNKZ) ceramics based on one-step and two-step sintering processes. One-step sintering led to significant abnormal grain (AG) growth at temperatures above 1,085 ℃. With increasing sintering temperature, piezoelectric and dielectric properties were enhanced, resulting in a high d₃₃ = 506 pC/N for one-step specimen sintered at 1,100 ℃ (one-step 1,100 ℃ specimen). However, for one-step 1,115 ℃ specimen, a slight decrease in d₃₃ was observed, emphasizing the importance of a high tetragonal (T) phase fraction for superior piezoelectric properties. Achieving a relative density above 84 % for samples sintered by the one-step sintering process was challenging. Conversely, two-step sintering significantly improved the relative density of KNNS-BNKZ ceramics up to 96 %, attributed to the control of AG nucleation in the first step and grain growth rate control in the second step. The quantity of AG nucleation was affected by the duration of the first step, determining the final microstructure. Despite having a lower T phase fraction than that of the one-step 1,100 ℃ specimen, the two-step specimen exhibited higher piezoelectric coefficients (d₃₃ = 574 pC/N and k_p = 0.5) than those of the one-step 1,100 ℃ specimen due to its higher relative density. Performance evaluation of magnetoelectric composite devices composed of one-step and two-step specimens showed that despite having a higher g₃₃, the magnetoelectric composite with the one-step 1,100 ℃ specimen exhibited the lowest magnetoelectric voltage coefficient, due to its lowest k_p. This study highlights the essential role of phase fraction and relative density in enhancing the performance of piezoelectric materials and devices, showcasing the effectiveness of the two-step sintering process for controlling the microstructure of ceramic materials containing volatile elements.

• 대한금속재료학회지
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• 제49권11호
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• pp.868-873
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• 2011

In this study, the fabrication of ultra fine grained Mo bulk was conducted. $MoO_3$ nanopowders were prepared by a high energy ball-milling process and then reduced at the temperature of $800^{\circ}C$ without holding time in $H_2$ atmosphere. The particle size of Mo nanopowder was ~150 nm and grain size was ~40 nm. The two-step process was employed for the sintering of Mo nanopowder to obtain fine grain size. The densification over 90% could be obtained by the two-step sintering with a grain size of less than 660 nm. For higher density, modified two-step sintering was designed. 95% of theoretical density with the grain size of 730 nm was obtained by the modified two-step sintering.

• 한국세라믹학회지
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• 제35권7호
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• pp.659-664
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• 1998

구조재료용 요업체로 널리 쓰이는 질화규소 요업체를 2단계 가스압 소결 방식에 의해 치밀화시켰을 때의 효과를 상압조절, 단순 가스압소결 또는 HIP 처리시와 비교하였다. 상압 소결시에는 $1800^{\circ}C$ 이상에서 분해가 일어나지만 질소 가스에 의한 가스압 소결시에는 소결 온도를 더 높일 수 있어서 ${\beta}-Si_3N_4$ 침상 조직의 발달을 이룰 수 있었다. 질소 가스압이 2MPa 정도이면 $1890^{\circ}C$까지도 상압소결시에 비해 분해가 현저히 억제될 수 있어서, 폐기공이 형성될 수 있는 단계까지는 분해만 억제시킬 수 있는 정도의 비교적 낮은 압력으로 치밀화시키고, 이어서 10MPa의 높은 압력을 가하는 소위 2단계 가스압 소결시 경도와 인성이 증진되는 것을 확인할 수 있었다.

• 한국분말재료학회지
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• 제17권3호
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• pp.235-241
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• 2010

Mo nanopowder was synthesized by ball-milling and subsequent hydrogen-reduction of $MoO_3$ powder. To fabricate ultra fine grained molybdenum, two-step sintering and spark plasma sintering process were employed. The grain size of specimen by two-step sintering and spark plasma sintering was around $0.6\;{\mu}m$ and $0.4\;{\mu}m$, respectively. Mechanical properties of ultra fine grained Mo with relative density of above 90% were significantly improved at room and high temperatures comparing to commercial bulk Mo of 99% relative density. This result was mainly explained by the grain size refinement due to diffusion-controlled sintering.

• Journal of Electrochemical Science and Technology
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• 제8권2호
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• pp.146-154
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• 2017

In this study, copper (II) oxide was prepared for use in a copper electroplating solution. Copper chloride powder and copper (II) oxide are widely used as raw materials for electroplating. Copper (II) oxide was synthesized in this study using a two-step chemical reaction. Herein, we developed a method for the preparation of copper (II) oxide without the use of sintering. In the first step, copper carbonate was prepared without sintering, and then copper (II) oxide was synthesized without sintering using sodium hydroxide. The optimum amount of sodium hydroxide used for this process was 120 g and the optimum reaction temperature was $120^{\circ}C$ regardless of the starting material.

• 한국세라믹학회지
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• 제30권4호
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• pp.309-315
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• 1993

Gas pressure sintering is a promising process in various densification methods of high strength Si3N4 ceramics. Environmental influences on gas pressure sintering of Si3N4 was investigated with the variationof packing powder, specimen container and N2 gas pressure. The specimens had higher density, larger weight loss and inhomogeneous color in graphite specimen container than in SN26 crucible. The variations of sintering densities in various packing powders (Si3N4, SN26, AlN, BN) were very small but SiC powder was synthesised in graphite crucible with Si3N4 packing powder, aluminium oxynitride compounds were synthesised in SN26 crucible with AlN packing power. Also N2 gas pressure over 20kg/$\textrm{cm}^2$ reduced the densification of Si3N4 in one step-gas pressure sintering. As the result of two step-gas pressure sintering at 700kg/$\textrm{cm}^2$ for 15min., relative density of 99.9% and 3-point bending strength of 1090MPa and dense microstructure of 3~4${\mu}{\textrm}{m}$ grain size were obtained.

• 한국분말재료학회지
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• 제19권5호
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• pp.373-378
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• 2012

Sintered bulks of $ZrO_2$ nanopowders were fabricated by magnetic pulsed compaction (MPC) and subsequent two-step sintering employed in this study and the formability effects of nanopowder on mixing condition, pressure and sintering temperature were investigated. The addition of PVA induced and increase in the formability of the sintered bulk. But cracked bulks were obtained on sintering with addition of over 10 wt% PVA due to generation of crack during sintering. The optimum compaction pressure during MPC was 1.0 GPa and mixing conditions included using 5.0 wt% PVA. The optimum processing condition included MPC process, followed by two-step sintering (first at 1000 and then at $1450^{\circ}C$). The sintered bulks with the diameter of 30 mm under these conditions were found to have non crack, ~99% density.

• 한국재료학회:학술대회논문집
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• 한국재료학회 2008년도 추계학술발표대회 및 제15회 신소재 심포지엄
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• pp.38.1-38.1
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• 2008

• 한국분말재료학회지
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• 제6권3호
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• pp.246-250
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• 1999

The effects of the ${\beta}-Si_3N_4$ starting particle size and $\alpha$/$\beta$ phase transformation during sintering process on the microstructure evolution of Yttrium $\alpha$-Sialon ceramics were investigated. As-received ${\beta}-Si_3N_4$ powder (mean particle size: 0.54$\mu$m) and classified ${\beta}-Si_3N_4$ powder(mean particle size: $0.26\mu{m}$) were used as starting powders. With decreasing the starting particle size, the growth of elongated grains was enhanced, which resulted in the whisker -like microstructure with elongated grains. These results were discussed in relation to the two-dimensional nucleation and growth theory. In the specimen heat treated at $1600^{\circ}C$ for 10h before sintering at $1950^{\circ}C$for 1h under 40atm(2-step sintering), the grain size was smaller than of the 1-step sintering at 195$0^{\circ}C$ for 1h. However, bimodal microstructure evolution were not not remarkable in both sample, which is ascribed to the $\alpha$-phase contents existing in ${\beta}-Si_3N_4$ starting powder.

• 한국정밀공학회:학술대회논문집
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• 한국정밀공학회 2012년도 추계학술대회 논문집
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• pp.95-96
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• 2012