Jo May 27, 2022
NTC (Negative Temperature Coefficient) thermistors are used in many electrical and electronic products including mobile phones, PCs and their peripherals, secondary batteries, LCDs, etc. for measurement, adjustment, compensation and time delay of their temperature, and voltage adjustment and noise reduction.
For most temperature sensing applications the NTC thermistors are made of spinel manganite.
Thick film thermistors are of several types such as sandwich, multilayer, segmented and interdigitated, and the methods of calculating resistivity and B value for these types are different.
Multilayer NTC thermistors are made up of inner metal electrodes and paralleled layers of NTC thermistor ceramics. In the process of manufacturing, stresses are produced between the components of multilayer NTC thermistors due to mechanical, thermal and electrical loads.
These stresses in multilayer electronic components can be simulated through the finite element method (FEM) and calculated by analytical models and measured by X-ray diffraction (XRD).
Compared to other methods, FEM proves to be more convenient for analysis of more detailed overall stresses in multilayer electronic components under various loads.
Up to now, scientists have often used two-dimensional models for considering the details of multilayer thermistors, but these models cannot fully display the distribution of stresses of multilayer thermistors.
Therefore, 3D finite element analyses (FEA) are more suitable for further study into the effects of the design parameters of multilayer thermistors on the stresses.
The research results show that manipulation of the length of the lateral margin most significantly influences the maximum principal stress experienced in multilayer thermistors during soldering process. In addition, the number of inner electrodes also contributes to the tensile stress that occurs in the soldering process.
More information about this can be found in the paper “The Simulation Study on Internal Stress in Multilayer Thermistors during Soldering Process” presented by Yu Nam Chol, a researcher at the Science Engineering Institute, to the SCI Journal “Solid State Electronics Letters”.
...
Jo May 24, 2022
Materials selection is a multi-criteria decision-making (MCDM) whereby materials designers and engineers have to select optimal material among two or more alternatives based on two or more criteria.
Many MCDM methods are applicable to the materials selection. These include rank sum ratio (RSR), simple additive weighting (SAW), weighted product method (WPM), analytic hierarchy process (AHP), technique for order preference by similarity to ideal solution (TOPSIS), VIse Kriterijumska Optimizacija Kompromisno Resenje (VIKOR) method, elimination and et choice translating reality (ELECTRE) method, preference selection index (PSI) method, preference ranking organization method for enrichment evaluations (PROMETHEE), grey relational analysis (GRA), complex proportional assessment (COPRAS) method, range of value method (ROVM), EXPROM, etc.
As materials selection may vary with the weights of materials selection criteria, the weights of materials selection criteria play an important role in materials selection by the MCDM methods.
Up to now, analytic hierarchy process (AHP) and entropy weighting method have widely been used to determine the weights of materials selection criteria. The AHP method is a subjective method while the entropy weighting method is an objective method. While objective weighting methods such as the entropy method fail to reflect the opinions of the materials designers and engineers, the AHP method can do it.
This is why the AHP method has mostly been applied to determine the weights of the materials selection criteria in many materials selection problems.
However, it still has some drawbacks. The major drawbacks to the AHP are as follows:
First, it is difficult to meet the consistency requirement of the pairwise comparisons.
Second, the pairwise comparison matrix (PCM) can hardly be constructed.
The solution is the introduction of a PCM construction method based on a new simplest questionnaire and a new modifying method of inconsistent PCM based on CR decrements.
Three indices are introduced to evaluate the effectiveness of the modifying method of inconsistent PCM.
• CR decrement ΔCR(A,B):
• Deviation index DI(A,B):
• Consistency ratio improving rate CRIR(A,B):
The results of analysis show that the simplest questionnaire helps materials designers and engineers to perform pairwise comparison judgments and the construction of PCM simply, easily and concisely without confusion, even if they have no knowledge and experience about the AHP method and that the modifying method based on CR decrements improves the consistency of inconsistent PCM better and faster by modifying a smaller number of elements with a smaller amount of modification.
This indicates that the simplest questionnaire and the modifying method could be widely used for calculating the weights of materials selection criteria or materials property indices in materials design and applications such as materials selection and optimization.
You can find more information about this in the paper “Materials selection criteria weighting method using analytic hierarchy process (AHP) with simplest questionnaire and modifying method of inconsistent pairwise comparison matrix” presented by Yang Won Chol, a researcher at the Faculty of Materials Science and Technology, to the SCI Journal “Proc IMechE Part L: Journal of Materials: Design and Applications” 2022, Vol. 236(1) 69–85.
...
Jo May 21, 2022
Ho Tong Chol, a researcher at the Faculty of Electrical Engineering, has developed DC-DC chopper experimental equipment, a part of a power electronics integrated laboratory table. It is designed to enable experiments on typical DC-DC converters ― Buck, Boost and Buck-Boost.
It can easily get an operational waveform at any point thanks to the terminals placed on the front-plate, it is supported by AC 220V as main power source, and the circuit diagrams of the front-plate are intuitive and plain enough to provide users with convenient environment for experiments on a certain converter.
This equipment, composed of power supply, control block, driving circuit, main circuit, load and filter, is useful for raising or dropping static DC 12V and for measuring some parameters such as duty cycle, voltage and current ripple rate, instantaneous value, mean value, etc.
The control block using current typed PWM integrated circuit SG3525 compares the carrier in 32.5kHz with DC voltage(0~3.3V) before it generates a PWM pulse at that comparison position. This process is observed at the terminals.
Additionally, since it includes a soft-starting circuit, you can increase the duty cycle smoothly so that no shock current flows into the main switch even when it starts on the maximum duty cycle.
The control range of duty cycle is 0.2 to 0.8.
The control block is also available for the control of a transformer isolated DC-DC converter since it has two ground separated power supplies, which is useful for the experiment on a half-bridge DC-DC converter.
The driving circuit guarantees a reliable and safe switch of the main device. The driving circuit is fed from unipolar power source.
The main circuit consists of a main switching device, a reactor, FRD, a capacitor and a current sensing resistor. The load is a resistor in 200Ω and it can also include a filter when necessary.
The newly-developed experimental equipment is useful to students, researchers and teachers who major in power electronics.
...
Jo May 18, 2022
Sim Jae Rim, a section head at the Faculty of Materials Science and Technology, has established a way of making WC-Co-B4C system electrode material for electric spark alloying and an alloying process by this material so that the service life of tools and machine parts could get more than 2.5 times as long.
B4C in the material has a good effect on the formation of alloying layers since it plays the role as flux which decreases the amount of oxide scale made during electric spark alloying. It also decreases the amount of oxides of low hardness and helps the formation of carbides of high hardness.
The reasonable electric conditions for electric spark alloying by WC-Co-B4C system electrode material are short circuit current I=9A, U=24V, C=420㎌ and t=2min/cm2. The thickness and hardness of an electric spark alloying layer by the new material are higher, that is, 90㎛ and 23.5GPa respectively, compared to 70㎛ and 19.5GPa by previous WC-Co system electrode material.
Electric spark alloying by WC-Co-B4C system electrode material forms alloying layers of high hardness on the surfaces of tools and machine parts, which leads to the lengthening of their working life.
...
Jo May 16, 2022
The research team led by Ri Ju Hyok, a section head at the Faculty of Heat Engineering, has designed and built a cryogenic liquid tank for storage of oxygen liquid.
The tank is manufactured to store oxygen liquid produced by cryogenic distillation process for a certain period and to supply it in either liquid or gas form.
It is composed of an inner body for storage of oxygen liquid, an outer body, a pipeline for injection and exhaustion, safety factors including checker valves, safety valves, pressure control valves, a level meter and manometers and a self-evaporator for increasing the pressure inside the tank.
The space between the inner and outer bodies is filled with thermal insulation material expanded perlite and made vacuous to reduce evaporation loss and to keep cool.
The problem of preserving the cold to minimize cold loss by a heat bridge was solved, which decreased daily loss to only 0.3~0.8%.
The degree of vacuum in the insulated space is 1.3Pa, and the total heat conductivity of the expanded perlite is 0.017W/(m•K).
With all these advantages it can also be used for storage of other cryogenic liquids like nitrogen, argon, etc. as well as oxygen.
...
Jo May 13, 2022
Kim Myong Il, a vice dean at the Faculty of Communications, has invented an indoor antenna for mobile communication.
Compared with previous antennas, this antenna has several advantages ― it ensures enough matching, gain and horizontal omni pattern in a wide band of frequencies; it is easy to feed, produce and assemble; it is light, good-looking and easy to install.
Consisting of a cone monopole, a double cone reflection panel, a curved plastic protection lid and coaxial feeding structure, the antenna assumes horizontal omni pattern in a wide range of frequency such as GSM, WCDMA, LTE, etc. With the gain of 3dBi, it can provide indoor mobile communication service in areas such as the underground of a building where mobile communication service cannot reach.
As a wide-range radiator, a cone monopole has top and bottom panels made of metal (Oblique angle is 25.5, radii are 12mm and 21mm respectively.), with an impedance of 50Ω and a wide radiation area. So it is of nearly omni directional pattern on the horizontal plane.
Vertical radiation can be controlled by a double cone reflection panel consisting of a primary cone reflection panel, a plain reflection panel, an upper cone reflection panel for coaxial feeding and it can bring a gain of 3dBi.
The inner conductor of a feeding coaxial cable is connected to the bottom panel and the outer conductor is linked to the reflection panel. The impedance of the antenna can be controlled by inserting an inclined line between the cone monopole and the reflection panel. Therefore, the impedance of the antenna remains constant in a wide frequency range. What is more, it is easy to install on the wall and it is covered tightly with a radome made of glass fiber plastic in order to prolong the life span and give a good-looking appearance.
In a wider frequency range(756~1 063/1 670~2 777MHz) than in the frequency range of former wire monopoles, the gain is 3.2dBi, which is 1.1dBi higher than former antennas (2.1dBi), so the service bound gets 1.4 times wider than before. The antenna is light, easy to install and meets aesthetic requirements.
...
