Jo Jun 28, 2022
NTC (Negative Temperature Coefficient) thermistors are widely used in industrial applications for temperature measurement, control and compensation. One important problem in the use of NTC thermistors is to overcome aging phenomenon. Their electrical properties change with time particularly when the temperature increases and it is the main reason why they are unable to be used at temperatures above 150℃.
Aging of electrical properties is attributable to non-equilibrium states that exist inherently in semiconducting NTC ceramics.
Sol-gel auto combustion is an advanced process that is widely used in nano-powder synthesis such as magnetic material.
Yu Nam Chol, a researcher at the Science Engineering Institute, successfully prepared Fe0.8Mn1.54Ni0.66O4 nano-ceramics by sol-gel auto combustion and observed the microstructure and phase of these samples by using Scanning Electron Microscope and X-Ray Diffraction. The results showed that powdered Fe0.8Mn1.54Ni0.66O4 ceramics pre-fired at 800℃ and sintered at about 1 050℃ had more compact and uniform spinel structure. In addition, the investigation of the electrical characteristics of these thermistors manufactured at different sintering temperatures proved that the sample sintered at 1 200℃ was suitable for the improvement of the properties of thermistors. It was also shown that the thermistor annealed for 72 hours at 450~550℃ in N2 atmosphere had aging ratio of less than 0.7%.
The details about this are carried in his paper “Preparation of Fe-Mn-Ni-O Nano-Ceramics by sol-gel auto combustion for NTC thermistor” presented to the SCI Journal “International Journal of Materials”.
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Jo Jun 26, 2022
Ri Yong Ho, a section head at the Faculty of Heat Engineering, has developed an instrument that can measure and analyze vibration of rotating machinery such as steam turbine generators real-time.
As a steam turbine generator is composed of rotors that are supported by their pedestals, vibration state of all the pedestals should be measured and analyzed at the same time by a multi-channel instrument.
The multi-channel vibration instrument acquires vibration signals of up to 16 channels at high speed at the same time and transmits them to the computer so that one can measure and analyze vibration state of all the pedestals of turbine generators, diagnose vibration fault and decrease vibration level by taking necessary measures like balancing.
The multi-channel vibration instrument is composed of analog signal processing circuit, key-phase pulse input circuit, digital signal processing circuit, USB communication circuit and power supply circuit.
The analog signal processing circuit does overvoltage protection, filtering and amplification of vibration signals, and then sends them to the digital signal processing circuit.
Digital signal processing is performed by a microchip, in which AD conversion of 16-channel analog signals, measurement of the rotating speed and vibration phase by key-phase pulse, saving of vibration waveforms and communication control take place.
The acquired vibration waveforms of 16 channels are transmitted immediately to the microchip which has USB communication function by means of parallel communication mode and then they are real-time transmitted to computers through USB ports.
In the power supply circuit, AC 220V power is converted to stable direct current voltages 5V and ±12V and then supplied to every signal processing circuit.
The computer performs digital filtering and integral of multi-channel vibration waveforms received from the vibration instrument, transforms vibration acceleration signals into vibration velocity signals and vibration displacement signals, calculates vibration amplitude to measure vibration level, and then makes vibration analyses such as spectrum analysis by fast Fourier transformation, phase analysis and trend analysis.
Besides, it can perform one-plane balancing and two-plane balancing by method of influence coefficient, so field balancing of steam turbine generators can be realized.
We can measure pedestal vibration by linking this instrument with piezoelectric acceleration sensors and electromagnetic speed sensors, and shaft vibration is also measurable when it is connected with eddy displacement sensors.
The measurement range is 0~200㎛ in the case of pedestal vibration and 0~500㎛ in the case of shaft vibration, and the measurement accuracy is 2%.
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Jo Jun 25, 2022
The research group led by Pae Ho Rim, a section head at the Faculty of Shipbuilding and Ocean Engineering, has developed an experiment device for measuring natural frequency of a ship hull.
The device measures the natural frequency of a ship hull model by the principle that resonance occurs when natural frequency is coincident with the frequency of forced vibration caused by decentration mass.
The device consists of ship hull model, vibration generator, unit for motor velocity control, frequency sensor, acceleration sensor, amplification and integration circuit, communication interface, user interface, etc.
The model is 1.2 metres long, 0.1 metres wide and 0.1 metres high. The cross section is rectangular and the bow and stern assume a pyramidal form.
The user interface shows the graphs and values of real-time signals of revolution, displacement, acceleration and velocity.
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Jo Jun 22, 2022
Cast aluminum alloys are quite widespread and they find more and more applications in modern industry such as aerospace, navigation and railway transport thanks to their good mechanical property, castability, corrosion resistance, etc.
Quality indices are a measure to quantify quality of the alloy on the basis of its mechanical properties.
The indices are Q, QR, QC, QD(Q0), QE, etc. Each one has its own unique features and individual one cannot display the overall performance of cast aluminium alloys.
The most representative indices to evaluate their overall performance can be chosen by two methods ― method of correlation analysis between quality indexes and mechanical properties and method of correlation analysis and rank deviation analysis between overall performance scores obtained from quality indices and MCDM method.
The analysis result showed that Q0 proved to be the most representative one of all the indices Q, QR, QC and Q0, and that Q0 can be used instead of MCDMs for evaluation of overall performance of cast aluminum alloys and selection of optimal cast aluminum alloys.
Detailed information about this can be found in the paper “ Method to evaluate overall performance of cast aluminum alloys using most representative quality index instead of MCDM method” presented by Yang Won Chol, a researcher at the Faculty of Materials Science and Technology, to the SCI Jounal “International Journal of Metalcasting”.
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Jo Jun 14, 2022
Kim Ryul, a researcher at the Faculty of Electrical Engineering, has realized selection of crack insulators from a pile of 10kV pin insulators by data processing of thermal shock sample breaking test and a corona short-circuit nondestructive test device.
Compared with previous selection methods by electric penetration breakdown test, this method provides more safety and lower production cost. In addition, as it uses nondestructive electric tests to sort out crack insulators, the reliability of production pile of insulators can be further improved.
This method is constructed with several stages: determination of size of 10kV pin insulator piles and samples, thermal shock sample destructive test, state decision of pass or pass failure of production piles by processing test data, determination of acceptance probability by quality control software and of the number of faulty insulators, and selection of crack pin insulators by the corona short-circuit nondestructive selection device.
This thermal shock test costs less and provides 1.4 times higher accuracy than the previous electric penetration breakdown test. To estimate cracks in 10kV pin insulator piles, previous binomial distribution was replaced with noncentral t-distribution, which reduced the number of samples to half and improved the accuracy of acceptance probability of piles and the calculation of the number of crack insulators by 1.2 times. Instead of electric penetration breakdown test devices, corona nondestructive test devices were used, which made it possible to select 100% of crack insulators from passed piles.
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