초음파가 분체 시료의 충진에 미치는 영향 해석

Abstract

Powder compacting technique using ultrasound has been used in various industrial fields, such as pharmaceutical, metal metallurgy, and environmental radioactive sample preparation. Such powder compacting technique needs to be compacted with sufficient pressure during the manufacturing process of the product for various purpose such as increasing density or increasing durability. Machines for increasing the density by applying pressure, such as a pressing machine or a compaction machine, use ultrasound to increase the compaction rate in the compaction process. Ultrasonic compaction is widely used in various fields such as metallurgy, ceramics and pharmaceuticals. For example, it is known that the compaction with ultrasound in manufacturing process of tablets has brought to increase the relative density of tablets by decreasing the porosity in the tablets and it is utilized increasing the drug efficiency per unit volume or the dissolution time of the drug. However, the theoretical interpretation of the compaction effect by ultrasound is insufficient. Therefore, in this study, we investigate the theoretical and experimental compaction effects in the powder sample using ultrasound by using friction coefficient reduction effect. Furthermore, in order to derive the optimum driving condition, analysis of compaction effect by ultrasonic vibration under various static pressure and initial height is required. The density of the compacted powder depends on the Janssen equation using the angle of repose and the pressure depends on the effective friction coefficient. If the ultrasound emitted from the transducer is applied to the powder sample, ultrasonic vibration reduces the friction coefficient inside the sample. Reduction of the effective friction coefficient caused by ultrasound leads to the increase of elastic modulus by decreasing porosity in the powder. Therefore, the compaction rate is improved. In order to experimentally confirm the compaction rate by the ultrasonic effect, the sample is compacted using Langevin type ultrasonic transducer, which has a resonant frequency of 28.8 kHz, and an acoustic intensity of 3793 W/m2. As a result, the friction coefficient inside of the compacted powder with the ultrasonic vibration of 28.8 kHz decreases by three times compared with that of the compacted sample without ultrasound and the compaction mass increases by about 10%. To find the optimal height using compaction with ultrasound, the acoustic properties of the initial height of the sample are compared. The speed sound according to the initial height of the sample compacted with ultrasound is about 12% faster at 3.5 cm, and the compaction rate according to the initial height is also the largest at about 30% at 3.5 cm. Thus, the optimum height in a given range is 3.5 cm. In addition the optimum static pressure is derived using the reduction of the effective friction coefficient. The reduction of the effective friction coefficient is always greater than 1 within the given static pressure range. Therefore, the friction coefficient of the sample compacted with ultrasound is smaller within the given static pressure range. Also, as the applied static pressure increases, the sound speed and the received voltage amplitude of the compacted sample is larger. This shows that the compaction rate increases as the static pressure increases within the range. We use numerical method to check the inside of the sample and to verify the properties of the sample perfectly homogeneous. The sound speed of the sample, which was assumed to be perfectly uniform using numerical method is 70 ～ 400 m/s larger than the sound speed of the actual sample compacted with ultrasound. In addition, it is possible to visually check the internal state when the sample was compacted using ultrasound. Also, we can confirm that the compaction rate is high when the powder sample was compacted using ultrasound using numerical method. To validate the suggested method, we actually applied the suggested method to the environmental solid radioactive sample. In the conventional sample compaction method, the sample is compacted according to the force applied by a technician using an acrylic pestle. The density of the compacted sample in the container is thus inhomogeneous. Therefore, the result of radionuclide analysis of the compacted samples by the suggested method is compared with that by the conventional method. The compacted sample put in a high-purity germanium detector (HPGe), and the radionuclides are measured by -ray spectroscopy. In the suggested method, the number of counters corresponding to the energy levels of radionuclides increased. This means that there was a high probability of finding radionuclides. In addition, radionuclide Ra-223 can only be detected in the sample compacted with ultrasound. As a result, the suggested method found various radionuclides and increased the probability of finding radionuclides.