kpmi
Previous issues
- Page Path
- HOME > Browse Articles > Previous issues
- Volume 8(2); April 2001
-
- A Scheme for Enhancing the Machinability of Sintered Materials
- J Korean Powder Metall Inst. 2001;8(2):83-90.
- 85 View
- 0 Download
- Binder Removal by Supercritical CO_2 in Powder Injection Molded WC-Co
- J Korean Powder Metall Inst. 2001;8(2):91-97.
- 75 View
- 0 Download
-
Abstract
PDF - The conventional debinding process in metal injection molding is very long time-consuming and unfriendly environmental method. Especially, in such a case of injection molded parts from hard and fine metal powder, such as WC-Co, an extremely long period of time is necessary in the conventional slow binder removal process. On the other hand, supercritical debinding is thought to be the effective method which is appropriate to eliminate the aforementioned inconvenience in the prior art. The supercritical fluid has high diffusivity and density, it can penetrate quickly into the inside of the green metal bodies, and extract the binder. In this paper, super-critical debinding is compared with wicking debinding process. Wax-based binder system is used in this study. The binder removal rate in supercritical CO_2 have been measured at 65°C, 75°C in the pressure range from 20 MPa to 28 MPa. Pores and cracks in silver bodies after sintering were observed using SEM When the super-critical CO_2 debinding was carried out at 75°C, almost all the wax (about 70 wt% of binder) was removed in 2 hours under 28 MPa and 2.5 hours under 25 MPa.
- Effect of Crystal Structures on the Sensing Properties of Nanophase SnO_2 Gas Sensor
- J Korean Powder Metall Inst. 2001;8(2):98-103.
- 86 View
- 0 Download
-
Abstract
PDF
- Metallic tin powder with diameter less than 50 nm was synthesized by inert gas condensation method and subsequently oxidized to tin oxide (SnO_2) along the two heat-treatment routes. The SnO_2 powder of single phase with a tetragonal structure was obtained by the heat-treatment route with intermediate annealing step-wise oxidation, whereas the SnO_2 powder with mixture of orthorhombic and tetragonal phases was obtained by the heat-treatment route without intermediate annealing (direct oxidation). SnO_2 gas sensors fabricated from the nano-phase SnO_2 powders were investigated by structural observations as well as measurement of electrical resistance. The SnO_2 gas sensors fabricated from the mixed-phase powder exhibited much lower sensitivity against H_2 gas than those fabricated from the powder of tetragonal phase. Reduced sensitivity of gas sensors with the new orthorhombic phase was attributed to detrimental effects of phase boundaries between orthorhombic and tetragonal phases and many twin boundaries on the charge mobility.
- Synthesis of \beta-FeSi_2 Powder by Mechanical Alloying Process
- J Korean Powder Metall Inst. 2001;8(2):104-109.
- 43 View
- 0 Download
-
Abstract
PDF
- The semiconducting beta-FeSi_2 compound has been recognized as a thermoelectric material with excel-lent oxidation resistance and stable characteristics at elevated temperature. In the present work, we applied mechanical alloying(MA) technique to produce beta-FeSi_2 compound using a mixture of elemental iron and silicon powders. The mechanical alloying was carried out using a Fritsch P-5 planetary mill under Ar gas atmosphere. The MA powders were characterized by the X-ray diffraction with Cu-K alpha radiation, thermal analysis and scanning electron microscopy. The single beta-FeSi_2 phase has been obtained by mechanical alloying of Fe_33Si_67 mixture powders for 120 hrs or for 70 hrs coupled with the subsequent heat treatment up to 700°C. The grain size of beta-FeSi_2 powders analyzed by Hall plot method was 44nm.
- Thermoelectric Properties of PbTe Sintered Body Fabricated by Mechanical Alloying Process
- J Korean Powder Metall Inst. 2001;8(2):110-116.
- 85 View
- 0 Download
-
Abstract
PDF
- Abstract To investigate the effect of mechanical alloying process to thermoelectric properties of PbTe sintered body, Pb-Te mixed powder with Pb : Te : 1 : 1 composition was mechanically alloyed using tumbler-ball mill. Thermoelectric properties of the sintered body were evaluated by measuring of the Seebeck coefficient and specific electric resistivity from the room temperature to 500°C. Sintered body of only mechanically alloyed PbTe powder showed p-type behavior at the room temperature, and occurred type transition from p-type to n-type at about 300°C. PbTe sintered body which was fabricated using heat treated powder in H_2 atmosphere after mechanical alloying showed stable n-type behavior under 500°C. N-type PbTe sintered body fabricated by mechanical alloying process had 4 times higher power factor than that fabricated by the melt-crushing process. Application of a mechanical alloying process to fabricate of n-type PbTe thermoelectric material seemed to be useful to increase the power factor of PbTe sintered body.
- Effects of Pores on the Microstructure of Melt-Processed YBa_2Cu_3O_7-x Oxides
- J Korean Powder Metall Inst. 2001;8(2):117-123.
- 76 View
- 0 Download
-
Abstract
PDF
- Formation of pores in melt-processed YBa_2Cu_3O_7-x (123) oxides and its effect on the microstructure were studied. Spherical pores with a size of a few tens of microns were formed due to the evolution of oxygen gas during melting of a 123 oxide. Some of pores were converted into liquid pockets by liquid filling, but others remained unfilled. The liquid pockets were converted into spherical 123 regions with a lower Y_2BaCuO_5 (211)density through the peritectic reaction during subsequent cooling, while the pores were entrapped into the periteictically grown 123 grains. The spherical 123 regions often consists of a residual melt due to the unbalanced peritectically reaction.
- Effects of SiC Cluster on Mechanical Properties of the 2024A1/SiC_p Composites
- J Korean Powder Metall Inst. 2001;8(2):124-130.
- 46 View
- 0 Download
-
Abstract
PDF
- A centrifugally atomized 2024A1/SiC/sub p/ composites were extruded to study effect of clusters on mechanical properties, and a model was proposed that the strength of MMCs would be estimated from the load transfer model approach that taken into consideration of the clusters. This model has been successfully utilized to predict the strength and fracture toughness of MMCs. The experimental and calculated results show coincidence and that the fracture toughness decreases with increasing the volume fraction of particles. On the basis of experimental observations, we suggest that the strength and fracture toughness of particle reinforced MMCs may be calculated from; σ/sub y/=σ/sub m/V/sub m/+σ/sub r/(V/sub r/-V/sub c)-σ/sub r/V/sub c/, K/sub IQ/=σ/sub Y/((3πt)((r/sub r//V/sub r/)(r/sub c//V/sub c/))/sup 1/2/)/sup 1/2/, respectively.
- Suppression of Abnormal Grain Growth in Barium Titanate by Atmosphere Control
- Byoung-Ki Lee, Sung-Yoon Chung, Yang-Il Jung, L. Suk-Joong
- J Korean Powder Metall Inst. 2001;8(2):131-135.
- 65 View
- 0 Download
-
Abstract
PDF
- The ferroelectric properties of barium titanate strongly depend on its microstructure, in particular, grain size and distribution. During sintering, BaTiO_3 usually exhibits abnormal grain growth, which deteriorates considerably the ferroelectric properties. A typical technique to suppress the abnormal grain growth is the addition of dopants. Dopant addition, however, affects the ferroelectric properties and thus limits the application of BaTiO_3. Here, we report a simple but novel technique to prevent the abnormal grain growth of BaTiO_3 and to overcome the limitation of dopant use. The technique consists of stepwise sintering in a reducing atmosphere and in an oxidizing atmosphere. The materials prepared by the present technique exhibit uniform grain size and high dielectric properties. The technique should provide opportunities of having BaTiO_3-based materials with superior ferroelectric properties.
First
Prev
TOP
