Jo Jun 2, 2022
Here is Kyongru-dong like a beautiful gem true to the meaning of its name in the centre of Pyongyang, capital of the DPRK.
Everything here including the bus stops, underground crossings, street lamps, etc. blends well with the Pothong riverside and the terraced residential district, which gives us an impression of being on the fairyland.
Walking up the exquisite stone steps, we could notice the plate bearing Block No. 12 in big fonts attached on the front door, which led us into the house of Merited Scientist, Candidate Academician, Prof. PhD. Ri Mu Chol, a section head at the Bioengineering Institute.
He and his son PhD. Ri Hwi Song, a researcher at the Faculty of Biology and Medicine Engineering, welcomed us, saying that they still could hardly believe this dreamlike reality.
The living room, study, kitchen, storeroom, …
Every element in the dwelling including the arrangement and colour of all the furnishings and lighting was on the highest level in the aspects of practicability, aesthetics and environment.
We could enjoy the beauty of the Pothong River flowing placidly in front of the spacious balcony where some fresh and green young pine trees were growing like a piece of painting.
Indeed, it was like a dream that ordinary working people are living in such luxurious dwellings in Kyongru-dong with fine scenery and good living conditions.
This is not a dream, but the reality in the DPRK.
We were totally absorbed in looking round the rooms when Ri Mu Chol told us as follows.
“It is really unbelievable that I am honoured to live here. I couldn’t believe that this is my own house even when I was handed over the entrance certificate and even when I was looking round it after the completion ceremony. But it turned out to be the reality, not a dream. Ordinary educators, scientists, workers, writers and artists like me became the owners of these new dwellings.”
This is not the remark from his heart alone.
It is from the hearts of all educators and scientists including Ri Tong Ju, a section head at the Faculty of Mechanical Science and Technology, O Su Yong, a section head at the Faculty of Heat Engineering, Kwon Chang Dok, a section head at the Faculty of Mining Engineering, Ri Mu Chol, a section head at the Bioengineering Institute, Son Ryong Chol, a section head at the Faculty of Materials Science and Technology, Jong Kwang Chon, a researcher at the Faculty of Materials Science and Technology, Min Chol U, a researcher at the Robotics Institute, Kim Chol Ryong, dean of the Faculty of Electrical Engineering, etc. It is from the hearts of all the people across the country.
It is not a dream, but reality.
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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”.
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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.
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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.
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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.
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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.
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