Thermal conductivity measurements

logo_smallAlthough cellular glass can have many applications, 99.999999% is going to thermal insulation. A good bench-marking of thermal insulation needs good equipment to measure and well trained operators. In this post, I give a short introduction with emphasis on the assumptions, which are made when a certain thermal conductivity is published.

Quality control (QC) needs a fast but reliable measurement for a decent quality assurance (QA). Indeed, the more measurements are done, the better the distribution of the thermal conductivity values around the average can be described. This becomes awarded with an improved declared thermal conductivity.

A very fast equipment is the hot wire method. This transient method is measuring in fact the thermal conductivity and volumetric heat capacity. The method is sensitive for only a small area of the sample (a few cm³) and in that way very sensitive to inhomogeneities and contact resistance. For a cellular material with cells on the scale of a mm, this looks not the appropriate method for the QA of cellular glass. On the other hand, it looks like a fine system to gather the thermal data needed to calculate the annealing curve.

TaurusThe heat flux meter method (HFM) is a more reliable method because the sensor measures typically up to 30 x 30 cm from the sample and the full thickness is taken into account. Typical samples are measured in a few hours with an accuracy up to 0.5%. The system uses a heat flow sensor, built as a many serially connected thermocouples. A sample is installed between a hot and cold sink. The sensor is mounted on the cold or hot sink or both. Because some heat leak to the outside can not be avoided, the system needs to be calibrated. HFM systems built in a cabinet with a stable and controlled temperature are not dependent on the laboratory conditions and have my favor. They need less calibration and more time is available for the actual measurements.

Taurus and Netzsch have both reliable systems, when properly calibrated, need about 2 hours to measure a 4 cm thick sample. Both give confident measurements in a bench marking but I would also put some attention on the robustness of the equipment. Measuring cellular glass can be very hard for this kind equipment due to the scratching surfaces and the unavoidable dust.

The quality of a HFM-measurement depends strictly on the quality of the calibration procedure and so on the quality of the calibration samples. It is well known that a calibrated sample changes slightly after each measurement due to thickness changes. Each confident cellular glass producer should have access to a system, which allows a perfect absolute measurement. In that way, confident calibration samples can be used.

NetzschThe Guarded Hot Plate method guarantees that all the measured heat flows through the sample and does not escape to the outside. This is done by working with a guard ring, kept on the same temperature as the actual measuring range. This system needs much more time to measure one sample (about 12 hours) but does not need calibration if certified thermocouple wire and self calibrated volt meters are used. Such a complicated system should be able to measure a large temperature range. In that perspective, I am charmed by the GHP 456 Titan® ,if adapted for larger samples.

The very small error of the GHP-system is calculated with finite elements by L. Troussart in “Analysis of errors in Guarded Hot Plate measurements as compiled by the finite element method”.


Patent study: RU2004000415, a STES patent

logo_smallThis STES patent looks very interesting because they make a comparison with the best cellular glass today on the market at that time. Like we already mentioned in another post about NEOPORM,  they claim the best quality without remelting the glass, which should induce a huge cost reduction.

The abstract is written in English and French, but the content was only Russian. I used GOOGLE TRANSLATE which gives a poor understandable English. (Russian patent).

I understood that it is possible to start from waste glass with different compositions. Water glass is added in a rather high percentage and the mixture is stirred. After some time, a gasifier, containing carbon (I gamble on glycerin) is added and the stirred mixture is heated to 530°C until all water has disappeared. The mixture is further ground to a fine powder with a maximum grain size of 15 micron. This powder is put in molds and heated to about 800°C for 90 minutes.

While for the highest quality cellular glass, it is needed to melt the recycled glass with additional raw materials, this patent discloses a method to obtain this quality without remelting at high temperatures. It is well known that a melting furnace is a huge investment and has only a short life between 5 and 15 years (depending on the oxidizing components in the glass)  while the extra heating is about 1250-550°C=700°C. But we also know that the measured thermal conductivity depends strongly on the used methodology. I am not sure we are comparing the “same thermal conductivity”.

I would be surprised that Andrei’s patent is fake and therefore I am hopeful that cheaper high quality cellular glass can be put on the market while the investor will be pleased. On top of that, the problem of the unsorted piles of waste glass in Russia will be solved. In the next days, I will have a good translation of the patent.

The cold war has given us not only two very different cultures but also two different very competitive processes for cellular glass. And free competition is all what a fair market needs, although free is relative in this case, the patent is valid for another 10 years.

Patent study: the old dicer

logo_smallIndustrial historical research is always fascinating. Although you know the instrument, it is nice to find the original patent. In this case, this brilliant invention was patented in 1953. There is still a lot of handwriting on the patent, found by Google Scholar, also a fantastic instrument.

The patent explains first which problem is solved with the invention. Indeed, the powdered batch does not sinter homogeneously but in small and large parts, all irregular shaped. After sintering, when the foaming starts and the parts are expanding, it happens easily that two parts touch and are also lifting each other, inducing a fold because everything is confined in a mold. A fold in a glass foam disturbs the cellular structure and so the mechanical stability, reducing largely the productivity.

By using this dicer, regular blocks of powder are formed, which will also sinter in a regular homogeneous shape. During foaming, the sintered blocks touch but lifting is less probable and so the formation of the fold. By working with small blocks, the heat can better penetrate into the little blocks for a more homogeneous sintering and foaming.

By using a dicer, we can expect an improved productivity by less folds and faster sintering. The patent also mentions (column 5, line 31) that the patent is not only valid in a mold but also on a belt conveyor. The dicer could be replaced by rotating devices like for example a series pizza knifes.

dicerThis patent is a typical example of very intelligent research: simple with enormous excellent consequences. After so many years (61 years), it is still used by Russians and Chinese cellular glass producers. The picture showed a “diced” block from the STESS factory in Russia.

Patent study: the ceramic belt

logo_smallPolydros is already working a long time with what they call a ceramic belt. Their last patent was expired in 2008 and is now public domain information, free for everybodyto use.

The idea is to fill a typical lehr belt with clay by pressing the substance in the holes. After a drying time, the clay is fired and becomes a ceramic solid. This solid does not fall out of the belt during turning of the belt. It allows to transport powder or granulates through a high temperature furnace if the used steel for the belt is also temperature resistant. For that reason, it can be used to foam glass like applied by POLYDROS.

The development of this belt by POLYDROS was a fundamental step in the continuous foaming of glass.