Process Engineering

Mariano Paganelli, Expert Sistem Double Beam Optical Dilatometry Solutions www.expertsystem- A Novel Tool f or the Research. solutions.com Optical dilatometers use a beam of to measure the dimensional Paper presented at Abstract changes and so the sample is not Qualicer 2004, in touch with the measuring system. Thanks to the new double optical dilatometer, the ceramic scientist has the Castellón (E) There are several methods for possibility to follow the behaviour of the sample during the heat treatment measuring the dimensional without interfering with the process. The applications brake the limits of changes of the sample with a beam traditional dilatometry in many fields of research: of tight and they can be • Incoherent materials, like the expansion and contraction of the raw glaze summarised in two categories: applied on the surface of a ceramic ware, or the behaviour of an reflected beam and direct beam. incoherent granular flit The first optical dilatometer was • Softened materials, like the behaviour of a glass above the transition tem- invented by Abbe and Fizeau in the perature, where the surface tension starts to pull the edges and to make second half of the 19th century. This the sample shorter design uses a reflected beam of • Sintering kinetics: studying the relationship between time and monochromatic light and the temperature during a sintering process measurement of the displacement • Extremely thin samples, like tape cast substrates can be analysed with is carried out by courting the regard to thermal expansion and sintering behaviour. interference fringes between the forward going beam and the reflected team. After the Abbe Introduction iour. There are several! types of invention, many improvements were achieved on the original The thermo mechanical analysis "Thermo Mechanical Analyser” for design and now a day there are plays a very important role in the the measurement of different many models on the market, which ceramic field, and it is used to mechanical properties as a function use new and design. But in check the thermal expansion of temperature. coefficient of bodies and glazes to The simplest ones, commonly order to measure thermal ensure an optimal match of thermo called dilatometers, measure the expansion up to high temperatures mechanical behav- dimensional changes as a function the sample has to be set on a of temperature. The instruments of sample holder inside the fumace this family may differ in the way and also the sample holder has its they measure dimensional changes own thermal expansion. In order to of the sample. One very relevant achieve a good accuracy it is difference is if there is contact with necessary to measure the the sample or not. expansion of the sample holder and Ali the mechanical or electronic to subtract it from the actual dilatometers use a push rod in expansion of the specimen. touch with the sample to transfer The best way to do is to split the the dimensional change of the laser beam into two beams of light, sample from the internal of the which are reflected by the top of the fumace to the transducer (dial sample and by the sample holder gauge, differential transformer, or by both ends of the specimen capacitive) (Fig. 1). (Fig. 2). Measuring the sample from Fig. 1 (Lop) The use of a push rod in both ends the measure is absolute Scheme of a mechanical contact with the sample and there is no need for any traditional requires to correct the measured correction. This is the most Dilatometer data to take in account the accurate way of measuring ther- With a push-rod and expansion of the push rod, but now mal expansion and it may reach a A displacement a day, with computerised systems, nano-metric resolution. This is the transducer this operation is carried out type of instrument used by the sup- automatically. As long as the pliers of standard materials for the System is kept calibrated, the thermal expansion. For example, measurements are very reliable, the National Institute for Standards even if sometimes the correction and Technology (NIST) uses a amounts for twice as much as the Fizeau double beam interferometer measured value. to certify the thermal expansion of The push rod dilatometer can their Standard Reference Materials follow the sample only as long as it for thermal expansion. is rigid enough not to be deformed This method proved to be very Fig. 2 by the pressure of the rod. When a accurate, with a resolution of a Scheme of a double sample is too thin to be pushed or fraction of the wave length of the Beam loo fragile to be pulled, or it is incident light, but it is limited by the interferometric almost incoherent or softened, it reflectance of the surface of the dilatometer cannot! be measured using this specimen, if the specimen is not kind of dilatometer. it is necessary reflective, or it becomes not to use a non contact measuring reflective during the test, it is then system, necessary to use a mir- E 50 cfi/Ber. DKG 81 (2004) No. 6-7 Process Engineering

Fig. 3 (left) Scheme of a double beam optical dilatometer with two direct beams of light

Fig. 4 (right) Picture of a sample of molten glaze on the measuring system of the “Misura ODLT” double beam horizontal optical dilatometer. The sample is completely free to ror, which is put in touch with the move specimen using a refractory push The Double Direct able to obtain a perfectly fiat filed and it is not in touch rod. Managing in this way, this Beam Optical Dilatometer over a large area CCD, and with anything. method looses the advantage of The idea of a double beam optical working with this magnification is being non contact and becomes dilatometer is not new, but many close to impossible. substantially similar to the attempts made in the past flailed The solution to this problem is to electronic dilatometer. because of the limitation imposed move the optical path in order to The direct beam measuring by the image production and follow the expansion or the System overcomes this problem image processing technology. Now contraction of the sample. In order because the specimen is a day, the performance of digital to do this it is necessary to use two measured by the image that it cameras and personal computers linear translation stages equipped projects on a CCD when it is enable the design of double beam with a step motor controlled by the lighted by a direct beam of light. optical dilatometer at competitive computer and able to make the Using a beam of light with short cost. shift in a very short time with very wave length and a very high As a first approach it may seem high accuracy resolution CCD it is possible to reasonable to use only one This new design, patented by achieve a good resolution. For camera with a large CCD to collect Expert System Solutions, allows to example, using a blue light with a the image of the sample as a build optical dilatometers with wave length below 0,5 µm, it is whole. This kind of design looks unprecedented features: using a possible to have an image with an cheaper at a first sight but sample 50 mm long placed actual resolution of 0,5 µm per becomes immediately unfeasible horizontally in the fumace it can pixel of the camera (not an thinking at the resolution required measure expansion with a interpolated resolution but an for a thermal expansion resolution of one part over actual resolution). With a sample of measurement on small size 100.000, or using a sample 15 mm 50 mm of length the resolution samples. In fact, to achieve a long placed vertically inside the becomes one part over 100 000. resolution of 0r5 µm on a sample fumace it can measure sintering or Using two beams of light, which 50 µm long, ensuring a resolution bioating process with no limitation illuminate both the ends of the of the measurement of one part in dimensional changes (Fig. 5). specimen it is then possible to over 100.000 it would be One relevant feature of this kind of achieve an absolute measurement necessary to use a CCD with dilatometers is the fact that the of the dimensional changes of the 100.000 pixels per line, which is calibration curve it is not sample during the heat treatment not yet available on the market necessary. Once the magnification (Fig. 3). and it may well not be available for of the two optical paths is carefully The specimen is completely free to a while. established, there b no need for expand or contract, there is no The use of two beams of tight further calibration, unless the measuring System in touch with it, implies the use of two microscopes instrument is mechanically taken Fig. 5 but only a sample holder, to keep it and two digital cameras. Focusing apart. In fact, the magnification of Scheme of the in place inside the fumace. The the image of the tip of the sample the optical system does not double beam displacement of the sample holder on the CCD of the camera with the change with time or with the vertical is not relevant for the result of the maximum of the magnification number of test performed. To Optical measurement because the optical achievable using visible light, prove it, it is possible to check the dilatometer system is watching both the ends means to be able to see only few calibration using a Standard “Misura ODHT” of the sample (Fig.4) hundred microns of the sample. A Reference Material supplied by sample 50 mm or 50.000 microns NIST. long which reaches an expansion Fig. 6 shows the perfect match of 1 % will expand 500 micron. between the curve measured on a Using blue fight with wave length certified sample and the curve of 478 nanometres and magnifying obtained plotting the certified data up to 0,5 micron per pixel, the from NIST. image will shift 1.000 pixel. Since the sample is free on the sample New Fields of Research holder, it may move in one direction only, going out of the field Now the ceramic scientist has the of view of a video camera with 1 possibility to follow the behaviour mega pixel (1.000 x 1.000). The of the sample during the heat use of line cameras is unfeasible treatment without interfering with because the measurement will be the process. The applications strongly affected by edge brake the limits of imperfection. The use of larger area CCD implies to be E 52 cfi/Ber. DKG 81 (2004) No. 6-7 Process Engineering

traditional dilatometry in many Fig. 6 fields of research: Thermal expansion of * Incoherent materials, like the the Standard Reference expansion and contraction of an Material Incoherent granular frit, as SRM 738 from NIST applied on a raw tile as measured with a * Softened materials, like the double beam optical behaviour of a glass above the dilatometer transition temperature, where the (blue line) in surface tension starts to pull the comparison with edges and to make the sample certified data (red shorter dotted line).

* Sintering kinetics; studying the

relationship between time and Fig. 7 (below) temperature during a sintering Samples of incoherent process granular frit: before (in * Extremely thin samples, like ther- front) and after the mal expansion and sintering thermal expansion and behaviour of the extremely thin sintering test. layer of glaze as applied on a ceramic tile. study of the transition from solid to liquid even in the glassy state. The Thermal Expansion on measurement of thermal expansion Incoherent Materials is substantially identical to the measurement obtained with a push Ceramic is the science of sintering rod dilatometer as long as the powders and powders can be sample is still rigid. When the incoherent. Optical dilatometry can temperature overcomes the glass be used to check the behaviour of transition, the viscosity drops uncompacted powders which have exponentially and the sample no mechanical strength, like a layer becomes soft. With a non contact of granular frit. system it is possible to continue Thermal expansion of a granular frit can only be measured with no con- tact because, with no addition of a Fig. 8 binder, the material is completely Thermal expansion incoherent. The thermal expansion and sintering of an test with traditional dilatometers on incoherent granular these raw materials is normally car- frit (blue curve), ried out adding a clay as binder, but derivative (green), managing in this way the behaviour time (red). changes completely. Using an optical dilatometer it is possible to get an accurate measurement of the thermal expansion even on a granular frit which was held together simply by wetting with a drop of water. Fig. 7 shows the samples before (in front) and after the thermal expansion test. The result of the measurement in Flg. 8 clearly shows the sintering and melting behaviour of the material. Fig. 9 Measurement of Thermal Thermal expansion up Expansion above the to the malting of part of the curve above the Temp- glass transition erature up to Melting temperature keeps going up to the Most of the sintering mechanisms melting of the sample. which evolve during the heat treat- ment in traditional imply the development of a liquid phase. The presence of the liquid phase during sintering makes the material substantially viscous. The dimensional changes during the sintering process can be studied only if the measuring system does not interfere with the sintering mechanism itself. The non contact measurement ìs ideal for the cfi/Ber. DKG 81 (2004) No. 6-7 E 53 Process Engineering

Fig. 10 of the thermal expansion of the Glaze samples after the thermal expansion molten glass (Fìg. 11). Knowing test: the edges of the this it becomes possible to samples become calculate the density of the molten round because the glass at any temperature and this glaze is melting. is a crucial Information for the calculation of the surface tension of the molten glass using the method of the sessile drop analysis. This instrument, developed by Expert System Solutions is the only one available on the market which can perform the measurement of the surface tension using the sessile drop Fig. 11 analysis up to 1600°C. Fig. 12 shows the dialog window of the The slope of the curve above the transition software which performs the temperature is the analysis of the profile of the sessile thermal expansion drop according the equation coefficient of the liquid written by Young and Laplace at phase. Extrapolating the beginning of the 19th century. this part of the curve it is possible to know the increase in volume of Sintering of a the glass at any Class temperature. Powder The study of the sintering kinetics is an ideal application for the optical dilatometer. The relationship between sintering speed at constant heating rate versus temperature or shrinkage versus time at constant Fig. 12 temperature can be exactly deter- Dialog window of the mined. Only with a non contact drop analysis instrument dilatometer it becomes possible to developed by Expert follow the sintering behaviour of a System Solution, which ceramic material when the driving apply the Young-Laplace force of the sintering process is the method to measure the surface tension of liquid surface tension of a liquid phase. This study is yielding extremely from room temperature up to 1600°C. valuable information which can be used to define the best heat treatment for each specific Fig. 13 material. Pressed sample of The sample of compacted glass recycled glass powder on powder (Fig. 13) was first heated the sample holder of the with constant heating rate up to “Misura ODHT” double complete melting. This test allows beam vertical optical to define the temperature of dilatometer. maximum sintering speed, calculating the derivative of the sintering curve. Designing a second heat treatment with a permanence at that temperature the measurement up to the the time necessary can be complete melting of the glass. identified to achieve a stable con- Fig. 9 shows the measurement on dition of sintering, preventing the different glass and glaze samples melting or the bloating of the mate- while Fig. 10 shows the samples rial (Fig. 14).As expected, the after measurement. sintering speed ìs decreasing The graph reaches a peak point exponentially against time at and then starts falling down. The constant temperature, while ii is reduction in size of the samples is increasing exponentially versus given by the surface tension of the temperature at constant heating glass which makes the tip rounded rate. and the sample shorter. The part of the curve above the transition point is hundreds of Extremely Thin Sample: degrees long and this allows the Layer of Glaze as Applied extrapolation on a Ceramic Tile

The behaviour of the raw glaze can be followed up to complete

E 54 cfi/Ber. DKG 81 (2004) No. 6-7 Process Engineering

Fig. 14 Sintering of a glass poder. The blue curve is the first, carried out Fig. 15 Experimental lethod to collect the glaze layer

at constant heating rate up to bloating. The maximum sintering rate is as applied on the glaze line. A small ape of ashless

identified at 701°C. the red curve is a second test carried out with the laboratory filter is set on the tile before the same heating rate up to 700°C, then holding this temperature for 10 application. After the application it is visible the

minutes. The sintering proceeds with no sign of bloating. relied given by the paper under the glaze.

melting, up to the point it becomes sample holder for thin samples (Fig. shorter because the surface tension 16) and the measurement is is rounding the edges of the started. The filter paper will burn sample. out leaving no ashes and with no The samples were obtained laying interference with the measurement. an ashless filter for chemical From the result of the test (Fig. 17) analysis on the ceramic tile before we can clearly see that the layer of the glaze application, directly on the glaze undergoes a sudden glazing line Flg. 15. The amount of shrinkage in the initial phase of glaze and the nature of the deposit sintering. Since the material is of glaze particles is exactly the incoherent and not viscous, this same as in the industrial shrlnkagemay produce an application. extensive microcracking on the sur- The filler is then removed from the face of the glaze. The scars of surface of the tile and it is cut in a these small cracks will make the suitable size for the measurement surface of the glaze slightly inside the optical dilatometer hammered, even thought the (typically 50 mm long, 5 mm wide). application was perfectly smooth. The sample is then set on the Fig. 16 The layer of glaze is carefully removed from special the surface of the tile, still on the filter paper. The

sheet is set on a sample holder inside the fumace of the double beam horizontal optical dilatometer.

Conclusions This is only overview of the new research possibility made available by the new technique of double direct beam optical dilatometry. Ceramic science is the ideal field of application of this testing method because of the specific characteristic of the ceramic materials. It is now possible to follow all the stages of the ceramic process with no interference due to the presence of a measuring system in touch with the material. Fig. 17 Thermal expansion and sintering of an extremely thin layer of glaze as applied on a ceramic tile (blue curve), derivative (green), time(red).

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