An optical dilatometer is a non-contact device able to measure

thermal expansion Thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to a change in temperature, usually not including phase transitions. Temperature is a monotonic function of the average molecular kinetic ...

s or

sintering Clinker nodules produced by sintering Sintering or frittage is the process of compacting and forming a solid mass of material by pressure or heat without melting it to the point of liquefaction. Sintering happens as part of a manufacturing ...

kinetics of any kind of materials, unlike traditional push rod dilatometer, it can push up to the dilatometric softening of the specimen. It is a device for measuring changes in the dimensions of a specimen, optically, the achieved resolution can result in greater values than those of a conventional pushrod dilatometer. A

monochromatic A monochrome or monochromatic image, object or color scheme, palette is composed of one color (or lightness, values of one color). Images using only Tint, shade and tone, shades of grey are called grayscale (typically digital) or Black and wh ...

light source, such as a

laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fir ...

, illuminates the specimen. Some of the light is reflected by the specimen and interferes with the incoming light, creating optical

interference fringe In physics, interference is a phenomenon in which two waves combine by adding their displacement together at every single point in space and time, to form a resultant wave of greater, lower, or the same amplitude. Constructive and destructive ...

s. As the specimen contracts or expands, there is a proportional movement of the interference fringes, which can be measured using a camera system. The measurement resolution is determined by the

wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...

of the light, and is typically 0.5 μm for blue light. Optical dilatometers are used to measure

. The optical dilatometer is in fact complementary to the traditional higher resolution push rod

dilatometer A dilatometer is a scientific instrument that measures volume changes caused by a physical or chemical process. A familiar application of a dilatometer is the mercury-in-glass thermometer, in which the change in volume of the liquid column is ...

when it comes to measure dimensional changes of materials, as a function of temperature, and no contact between specimen and instrument is required.

Structure and types

The most recent types of optical dilatometer use a direct-beam system so as to avoid contact-related problems by measuring the image that the specimen projects on an

image sensor An image sensor or imager is a sensor that detects and conveys information used to make an image. It does so by converting the variable attenuation of light waves (as they pass through or reflect off objects) into signals, small bursts of curr ...

, when irradiated by a

light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 tera ...

beam. By using a single beam of

with short

and a very high

resolution Resolution(s) may refer to: Common meanings * Resolution (debate), the statement which is debated in policy debate * Resolution (law), a written motion adopted by a deliberative body * New Year's resolution, a commitment that an individual mak ...

it is possible to achieve good resolutions, although not comparable to those of the standard dilatometers. For example, using a blue

with a

below 1500 micrometers, it is possible to achieve images with an actual

of nearly 1500 micrometers/pixel of the camera (not an interpolated resolution but an actual resolution). By using two

beams, which illuminate two small portions of material at the extremities of the sample, set perpendicularly to the beams direction, it is then possible to achieve an absolute

measurement Measurement is the quantification of attributes of an object or event, which can be used to compare with other objects or events. In other words, measurement is a process of determining how large or small a physical quantity is as compared ...

of longitudinal variation while heat-treating the specimen. There are two major types of optical dilatometers which are currently in use: *
Horizontal Optical Dilatometer:
'' the rod-specimen is laid horizontally on the sample holder and, during the thermal treatment in the oven, it is completely free to expand and contract. The displacement of the sample holder is not relevant for the result of the

because the optical system follows the horizontal movements of small portions of the specimen at its extremities. *
Vertical Optical Dilatometer:
'' the rod-specimen is set vertically on the sample holder (alumina thin plate) and while one camera “watches” the top of the plate the other follows the vertical displacements of the sample’s top edge. This system has been used so far to find the characteristic temperatures (phase transitions, maximum sintering speed, swelling, etc.) of ceramic materials. In fact, phase transitions are always associated to dimension variation of the material:

is accompanied by high shrinkages, related to the decrease of porosity - swelling usually appears at high temperatures. After

occurs and the

viscosity The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the inte ...

of the ceramics formulation strongly decreases and trapped gas is then free to exit.

History

The first optical dilatometer was invented by Abbe and Fizeau in the second half of 19th century. This design has a reflected beam of monochromatic

and the

of the displacement is carried out by counting the

s between the forward going beam and the reflected beam. After the Abbe invention, many improvements were achieved on the original design and there are now many models available on the market, which use modern

optics Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultraviole ...

and designs. Over the last five decades, interest has grown in the use of thermomechanical technologies for characterising materials in different fields of science and engineering. In particular, the use of optical methods has been playing a role in the field of

ceramic A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelain ...

s and have been used to measure the

coefficient of thermal expansion Thermal expansion is the tendency of matter to change its shape A shape or figure is a graphics, graphical representation of an object or its external boundary, outline, or external Surface (mathematics), surface, as opposed to other pro ...

of bodies and glazes in order to ensure an optimal match of their thermomechanical behaviour.

Applications and fields of research

In order to measure thermal expansion up to high temperatures, the material to be tested has to be set on a sample holder inside an oven which has its own thermal expansion. In order to achieve a good accuracy it is necessary to measure the expansion of the sample holder and to subtract it from the actual expansion of the specimen. The best approach is to split the laser beam into two beams of light, which are reflected by the top-edge of the sample and by the top-edge of the sample holder, or by both longitudinal edges of the specimen. By detecting the longitudinal variations at both the extremities of the specimen the measurement achieved is absolute and there is no need for further corrections. This is the most accurate way of measuring thermal expansion and it may attain nanometric resolution. This is the type of instrument used by the suppliers of certified standard materials. For example, the National Institute for Standards and Technology uses a Fizeau double-beam interferometer to certify the

of their

certified reference materials Certified reference materials (CRMs) are 'controls' or standards used to check the quality and metrological traceability of products, to validate analytical measurement methods, or for the calibration of instruments. A certified reference materi ...

. This method has proved to be very accurate, with a resolution of a fraction of the wavelength of the incident light, but it is limited by the

reflectivity The reflectance of the surface of a material is its effectiveness in reflecting radiant energy. It is the fraction of incident electromagnetic power that is reflected at the boundary. Reflectance is a component of the response of the electronic ...

of the surface of the specimen. If the specimen is not reflective, or it becomes non reflective during the test, it is then necessary to use a mirror, which is set in contact with the specimen by using a

refractory In materials science, a refractory material or refractory is a material that is resistant to decomposition by heat, pressure, or chemical attack, and retains strength and form at high temperatures. Refractories are polycrystalline, polyphase, ...

push rod. By doing so such method loses the advantage of being non contact and becomes substantially similar to the electronic

. Optical dilatometers are used along traditional dilatometers in ceramics for non-destructive testing of a sample's behavior during a thermal treatment. Optical dilatometers are used for

thermal analysis Thermal analysis is a branch of materials science where the properties of materials are studied as they change with temperature. Several methods are commonly used – these are distinguished from one another by the property which is measured: * D ...

of various types of materials, such as incoherent materials (expansion and contraction of an incoherent granular

frit A frit is a ceramic composition that has been fused, quenched, and granulated. Frits form an important part of the batches used in compounding enamels and ceramic glazes; the purpose of this pre-fusion is to render any soluble and/or toxic compo ...

, as applied, for example, on raw

tiles Tiles are usually thin, square or rectangular coverings manufactured from hard-wearing material such as ceramic, stone, metal, baked clay, or even glass. They are generally fixed in place in an array to cover roofs, floors, walls, edges, or o ...

) and

polymers A polymer (; Greek '' poly-'', "many" + ''-mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic an ...

(behaviour above the

glass transition temperature The glass–liquid transition, or glass transition, is the gradual and reversible transition in amorphous materials (or in amorphous regions within semicrystalline materials) from a hard and relatively brittle "glassy" state into a viscous or rubb ...

, where the

surface tension Surface tension is the tendency of liquid surfaces at rest to shrink into the minimum surface area possible. Surface tension is what allows objects with a higher density than water such as razor blades and insects (e.g. water striders) to f ...

starts pulling the edges and making the sample shrink), as well as for analysis of various processes in materials manufacturing, for example,

kinetics,

and

behaviour of thin glaze layer or

polymer A polymer (; Greek '' poly-'', "many" + ''-mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...

thin films.

References

Length, distance, or range measuring devices

Structure and types

History

Applications and fields of research

See also

References