PTC (positive temperature coefficient) thermistors based on BaTiO₃ semiconducting ceramics are important devices for ferroelectric materials.

Due to their great temperature coefficient of resistance, barium titanate and its related compounds are ideal candidates for producing various PTC components. Particularly, Ba_1-xSr_xTiO₃(x=0~1) ceramic material-based PTC thermistors have nonlinear characteristics around the Curie point, which can be enhanced by the addition of some minor impurities such as Cu and Mn.

Jo Myong Chol, a researcher at the Institute of Semiconductor, has investigated the effects of Mn addition on the dielectric properties and microstructure formation of BaTiO₃-based ceramic chips and the effects of heating rate on PTCR response of barium-strontium titanate (BST) ceramics doped with Mn dopants.

Mn addition works as a sintering agent, which influences the densification behavior of the overall ceramic material. The dielectric constant increases to as high as 5 800 (at 25℃) and the dielectric loss decreases by 1% when Mn-content is added properly. This shows that the phase formation, density, microstructure and electrical properties of co-doped BST ceramics containing sintering additives are strongly affected by heating rate.

The highest densification during sintering occurs at the heating rate of 5 or 20℃/min. There is a general trend of decrease in effective resistivity with increasing heating rate, which could be attributed to the decrease in grain size. However, when the heating rate went from 5 to 20℃/min, there was a slight increase in resistivity, which may have been due to the increased proportion of crystallized secondary phases in the most rapidly heated sample, but the reasons for this are uncertain. Thus, it can be concluded that 3 and 5℃/min are the best heating rates for favorable PTCR effects.

From olden times, the Korean people have celebrated Jongwoldaeborum (lunar January 15) as one of the great folk holidays, and they made it a traditional practice to make several kinds of dishes including ogokbap, yapbap, laver-wrapped rice, nine kinds of seasoned edible grass, noodle, etc. and enjoy them on this day.

Ogokbap is boiled rice admixed with four other staple cereals.

The ingredients usually included five kinds of cereal―white rice, hulled glutinous millet, polished millet, hulled barley and adzuki beans, but the five kinds of cereal were not fixed. Here is a recipe for ogokbap.

First, adzuki beans are soaked in water.

Second, hulled glutinous millet and polished millet are washed.

Then, white rice, adzuki beans and hulled barley are boiled together. After some time of boiling, hulled glutinous millet and polished millet are placed upon the mixture.

Finally, the fire is weakened for enough steaming.

What is important is to add less water for glutinous rice than for common boiled rice.

Mixing is an important technique for improving product quality in various manufacturing processes. Various mixers are used in the mixing processes of many fields including iron works, food industry, cosmetics and construction.

Among various types of mixers, a twin spiral mixer has the advantage of small volume and continuous mixing. In a twin spiral mixer, the left and right spiral wings attached to two axes rotating in opposite directions alternately work by advancing the material in one direction. First, the left spiral wing rotates and contacts material particles, advancing them and sending some of them toward the right spiral wing. The right spiral wing then approaches the position where it can contact the particles. Next, the left spiral wing releases the particles, and the right spiral wing comes into contact with them, advancing them and sending some toward the left spiral wing. Thus, in the twin spiral mixer, mixing is achieved by a simple cross action of the spiral wings that send some particles toward each other. From this mixing principle, the mixing degree in the twin spiral mixer is determined by the number of wings that interact with particles. In addition, due to the single mode of mixing, the attainable mixing degree is not high, and therefore, the length of the mixer must be long inevitably to increase the mixing degree.

Generally, various methods of structural and working parameters optimization have been widely used to improve the performance of machines and equipment. However, these methods are not sufficient to fundamentally change the performance characteristics and they have certain limitations. In the twin spiral mixer, it is also difficult to achieve the improvement in the mixing degree by optimizing the structural parameters, since the mixing process is realized by a simple cross action of the spiral wings.

Ri Song, a researcher at the Faculty of Mining Engineering, evaluated the mixing degree by simulating the mixing process of a twin spiral mixer by DEM (discrete element method) and then, based on this, he proposed a plan of increasing the mixing degree by installing inverse spiral wings at regular intervals in a twin spiral mixer to change the mixing mode.

Comparing the mixing degree for the conventional twin spiral mixer and the twin spiral mixer with inverse spiral wings, it was 0.779 7 for the conventional mode, 0.801 3 for the 2:1 mode, 0.81 for the 3:1 mode and 0.790 7 for the 4:1 mode, which means the 3:1 mode was the best, and the outputs from simulations were 89.1W, 429.7W, 287.8W and 151.5W, respectively.

You can find more details in his paper “Study on improving the mixing degree of a twin spiral mixer by changing the mixing mode using inverse spiral wings” in “Journal of the Brazilian Society of Mechanical Sciences and Engineering” (SCI).