GB1600062A - Temperatureresponsive light gate device and method of measuring temperature - Google Patents

Description

(54) TEMPERATURE-RESPONSIVE LIGHT GATE DEVICE AND METHOD OF MEASURING TEMPERATURE (71) I, STANLEY BENNETT ELLIOTT, a citizen of the United States of America, of 7125 Conelly Boulevard, Bedford, Ohio, United States of America, do hereby declare the invention for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a temperatureresponsive light gate device and to a method of measuring temperature with such a device.

In accordance with one aspect, the present invention provides a temperature responsive light gate device to be exposed to a source of illumination providing a light beam, comprising (1) a non-ferroelectric material which, at a predetermined temperature, is triggered without a change in the chemical composition of the material, from an anisotropic birefringent state to an isotropic, non-birefringent state and (2) means to intensify observable changes in the brightness and intensity of the light coming from the material when the light beam contacts the material, comprising a polarizer for polarizing the light beam before contacting the material and an analyzer for analyzing the beam coming from the material, whereby the device passes a relatively greater amount of light when the material is at a temperature in which it is in its birefringent state and a relatively lesser amount of light or no light when triggered by a change of temperature into its non-birefringent state.

In accordance with a further aspect the present invention provides a method of measuring temperature with a temperature responsive light gate device, comprising the steps of (1) providing a light beam from a source of illumination; (2) polarizing the light beam; (3) bringing the polarized beam into contact with means providing observable changes in temperature, including a non-ferroelectric material which, at a predetermined temperature, is triggered without a change in the chemical composition of the material, from an anisotropic, birefringent state to an isotropic, non-birefringent state, whereby the polarized beam is doubly refracted when said material is at a temperature in which it is in its birefringent state and not doubly refracted when said material is at a temperature in which it is in its nonbirefringent state; and (4) analyzing the light beam coming from the material to provide a visual indication of whether said material is above or below said predetermined temperture.

The non-ferroelectric heat-sensitive material that is used in the present invention is birefringent at a first temperature and nonbirefringent at a second temperature, the change from the birefringent to the non hire ringent states being accomplished without a change in the chemical composition of the material. The change in birefringence is detected by passing the light beam through a polarizer and passing the polarized beam through the material to an analyzer. In one embodiment the polarizer for polarizing the light beam may be a circular polarizer, in which case the polarized beam is passed through the material to a suitable mirror and is then reflected back along its original path, the polarizer thus serving also as the analyzer.The material is also translucent at a first temperature and clear at a second temperature and the abrupt observable change in light coming from a light beam which contacts the material (due to a change in temperature) is readily observed by the unaided eye or by a device such as a photocell or other light detecting means for detecting observable changes in the brightness and intensity of the light coming from the composition when the light beam contacts the composition.

Thus the light gate device of the present invention in a specific embodiment may include also a source of illumination providing a light beam as an integral part of the device, and light detecting means for detecting observable changes in the brightness and intensity of the light coming from the material when the light-beam contacts the material In general, such light detecting means can be electrical means, e.g. a photoresponsive solid state electronic device which is responsive to observable changes in the brightness and intensity of the light coming from the heat-sensitive material, the wight coming from the material when it is in its birefringent state being of sufficient brightness and intensity to provide a visual signal to activate the light-detecting means.Such a device may also include electrical control means, responsive to said electrical means, for controlling electrical power to activate an electrical circuit.

In one embodiment, crystals which are birefringent in the solid form are displayed between a polarizer and analyzer so that changes in birefringence may be reac@ily observed. When the melting point of compound is reached its crystal structure is destroyed and so is its birefringence. Sucn a system will be referred to as a solid liquid system. By selection of suitable bire@ringent crystalline compounds, having varying meit- ing points, a series of highly visual displays mav be readilv assembled to comprise fin efffcient thermometer.

In another embodiment, crystals which are birefringent in the solid form below or above what I choose to call the "crystal con- version temperature" are displayed between a polarizer and an analyzer (preferably crossed) so that changes in biref ingence may readily observed. When the crystal conver- sion temperature of the compound is reached the birefringent crystal structure converts to a nonbirefringent crystal structure. Such a system will be referred to as a solid solid system. By selection of birefring cut, crystalline compounds having varying crvstal conversion temperatures.

readily observable displays may be readily assembled to comprise an efficient thermometer.

The present invention is predicated on the use of polarized light for the amplification of the signal generated by compounds wnose change in optical properties are directly a result of their temperatures. For not only is there a brilliant white to blue-black, go-no go visual output, but it is instantaneous.

That is, at the moment of "crystal conversion" (the point at which the solid under goes the birefringent/nonbirefringent change) in the solid/solid system or melting in the solidlliquid system there is a go-no go amplified signal available. This is in contrast to such attempts as having dyes dissolve in or materials react with a melted compound to give an amplified signal indicating that a change has indeed taked place. For many purposes a sequential read-out of a number of temperature sensing plaques is desirable, and the thermometer" is exposed to the heating source for but a brief time, before a reading is taken. if dyes must dissolve or compounds react to form colored reaction products in order to be sure that a certain temperature has indeed been reached. serious errors may result.Amplification using polarizers eliminates all these errors and uncertainty, for it is as fast as the melting or "crystal conversion" itself.

Ordinarily the change in appearance from brilliant white to blue-black using polarizers signifies a change from a birefringent crystal structure to an isotropic state. Theoretically at least, there is another state which might be termed a "pseudoisotrspie" state. In such an instance a thin film of the signalling compound would appear blue-black bet ween crossed polarizers. That is. it would appear isstropic to any ordinary visual inspection. But this pseudoisotropy would be due to a uniform alignment of the optical axes of the molecules comprising the film in relation to the plane of the film. Thus, if it were possible to somehow examine the opti @ai properties of the film in the plane of the film, it would be found that such properties are different from those perpendicuiar to the film.This special case is mention for, thougn truly isotropic films generally form, I mean to inc@ude also those instances where pseudoisotropic films form for their signailing properties are equivalent.

The solid solid system is especiaily useful because the presence or absence of biref ringence is an inherent thermai property of the selected molecuies and so is quite stable against drift. Solid liquid systems, however, which depend on the melting point of particular systems, must be formulated with care and protected from any deleterious effects wnich would introduce impurities.

For the melting points of materials are susceptible to substantial variations when impuritles are present.

Drdinarily it is desirable to seai or encapsulate the indicating chemical compositions within hermetically sealed transparent, thermally conducting shells to orctect the materials from possicly deleterious gases such as oxygen, water vapor, etc. by preciuding them. However, there are some compositions which are substantially inert to such gases and these need not be protected unless desired.

There are many accurate thermometers available whose operation depends on the expansion and contraction of various fluids, solids, or gases. There are others which depend on various electrical effects. Many are distinguished by a lack of ruggedness and an inapility to readily be formed inte dispiays of varying size- whether very large for industrial situations or very mail as for use on an integrated circuit where the temperature of a solid state component must be monitored Others may indicate temperature by the formation of color bodies in an indicating "melt" at a particular temper- ture, but this irreversible feature is not always desirable.Still others may indicate temperature irreversibly through the fusion of opaque waxes or similar materials at a particular temperature or more usually over a temperature range to a melt which stays clear even on cooling. Liquid crystals also may be used to indicate temperatures, but they are so expensive as to limit thermometers based on them to certain specialized uses. Further, liquid crystal compositions are susceptible to drift from the effect of trace impurities such as moisture.

In contrast to the industrial devices described above, it is possible by means of the present invention to make termomet- ers and other temperature-responsive devices which are rugged, inexpensive, easily read, and stable. Further, since the melting or crystal conversion temperatures of appropriately chosen compounds are verv sharp, go/no go indication is readily secured.

By contrast, the reaction of compounds to form color bodies, for example, does not occur solely at a particular temperature. The use of linear or circular polarizing materials as amplifiers of the optical transitions from birefringent to nonbirefringent is especially useful in emphasizing the go/no go aspect of the optical signal.

Thus, in one embodiment of the invention there is provided a light gate device in which at least two discrete areas which are adjacent in the same plane or optically spaced in parallel planes are occupied with materials whos birefringence terminates at different temperatures, whereby the device serves as a thermometer to measure different temperatures. In one form of such a device, at least two discrete areas spaced in parallel planes are occupied with materials in intimate contact and whose birefringence terminates at different temperatures. Thus, the present invention provides in certain embodiments a visual-type thermometer in which a number of delineated areas change sequentially from blue-black to brilliantly white as the temperature changes.

In another embodiment there is provided a visual temperature alarm in which a relatively large area changes from blue-black to brilliantly white as the temperature varies from some desired point. In such a device suitable warning legends may appear to the viewer after the temperature has deviated from the desired range.

As a variant of this, a temperaturesensitive compound which is birefringent below a selected temperature but which is nonbirefringent above the temperature is deposited on a suitable substrate. This may be sealed into a double-pane window of the type used for insulating purposes and in which the space is moisture-free, and the whole placed between crossed polarizers to form a temperature-sensitive system. If direct sunlight falls onto the system the temperature rises and the system no longer transmits an undeisrably high amount of light. If desired, of course, the crossed polarizers may be within the sealed double-pane window or comprise the pane surfaces.Or single pane windows may be coated, with appropriate encapsulation of the chemical composition if its structure requires preclusion of gases such as oxygen and water vapor, the polarizing members being appropriately placed in relation to the temperature-sensing coating.

Still another variant would utilize an edge-sealed "sandwich" of crossed olariz- ing sheets, the interior of one or botch sheets being coated with one or more coatings which have different temperature response points. The net effect of having multiple, thin coatings having differing birefringent/nonbirefringent temperatures is that the amount of light transmitted changes gradually as the temperature shifts. Suitable dyes which absorb heavily in the infra red but only slightly in the visible range may be coated onto the substrate before the temperature-responsive materials are coated onto it or the dyes may be incorporated directly into the responsive composition. Such a sandwich structure comprises a kind of automatic "Venetian blind" when the hot sun strikes it directly if it is installed close to a window within a structure.

For such applications as these, it is preferred that the temperature-sensitive material should be adapted to undergo at least 100 cycles of birefringence and nonbirefringence.

It It is of great importance to create devices for controlling the immense output of light which accompanies nuclear explosions. For both military and civilian applications a mode is needed for instantaneously blocking the great burst of radiation characteristic of such phenomena. Photochromic dyes have been used for controlling such radiation but the offer many problems for they degrade with time. In contrast, many of the materials of the present invention comprise materials of great stability. Used with crossed polarizers, the solid/solid transition materials are of exceptional interest for with them there is not the time lag born of absorbing sufficient heat to melt the composition.If carbon tetrabromide is used, for example, at the moment the composition reaches approximately 117"F. crystal conversion occurs and the polarizing pair blocks light. Infra red absorbing dyes which absorb visible light only slightly can be used with such systems to provide "light gates" for goggles, aircraft windows, and a variety of windows which should block radiation in the event of a nuclear explosion.

The invention will now be further described with reference to the accompanying drawings, in which: FIGURE 1 is a somewhat schematic view of a typical transmission-type light gate device of this invention; FIGURE 2 is a view of a transmissiontype device using a single piece of polarizing material; FIGURE 3 is a view of a transmissiontype device designed to efficiently check the temperature of a gas stream normal to the polarizer and analyzer; FIGURE 4 is a view of a typical reflection-type device of this invention; FIGURE 5 is a view of a typical transmission-type device using sequential temperature series plaques; and FIGURE 6 illustrates another embodiment of a typical transmission type such as shown in FIGURE 1; In a representative embodiment of the visual-type temperature-responsive light gate device of the present invention shown in FIGURE 1, a light beam from light 5 which may be a window, a tungsten lamp, a fluorescent lamp, etc. passes through polarizer 1 where the light beam is polarized. The beam then passes through transparent or translucent substrate 2 which may be glass or some isotropic plastics material such as cellulose triacetate, on which is deposited crystal layer 3. The beam passing through 2 and 3 then encounters analyzer 4 whose polarizing axis is usually at right angles to the polarizing axis of 1 so as to result in what is generally termed a "dark field".

If the coating 3 or substrate 2 is in its nonbirefringent mode, little light passes through the analyzer 4 and the system appears "dark field" to viewer 6. However, if the temperature changes sufficiently, coating 3 becomes birefringent. When a light beam enters a birefringent or, as it is sometimes called, double refracting material, it is divided into two components, one defined as an extraordinary ray and the other as an ordinary ray, each vibrating in a direction at right angles to the other and traversing the birefringent material with a different velocity to thereby introduce a phase difference therebetween. As said beam is thereby resolved into two components, one of which is retarded with respect to the other, said beam is generally referred to as being elliptically polarized.The two components emerging from the birefringent material and entering the second sheet of polarizing material 4 are resolved into one plane-polarized beam again. But a phase difference has been introduced between the two parts of the same beam, and so the necessary conditions for interference are present. With a white light source brilliantly colored light beams will emerge from analyzer 4 if the coating 3 crystallizes in large crystals. If the crystals are very small there is a mixing of colors and the crystal mass appears white. But in either case the field which was previously a blueblack passing very little light now glows brilliantly. The system may be hermetically sealed in the absence of any undesirable gases such as oxygen or moisture.

FIGURE 2 is essentially the same as FIGURE 1 but better adapted to mass production in that a single piece of polarizing material is folded at 45" to its polarizing axis. This forms two leaves 1 and 4 whose polarizing axes are at right angles to one another and so creates a "dark field" condition when the viewer 6 interposes the folded layers between him and light source 5. A substrate coated with temperature-sensitive layer 3 is then inserted to create a temperature-responsive sandwich. Or, if desired, the layer 3 may be coated on one or both inner surfaces of 1 and 4 so as to eliminate the need for a separate substrate. If the particular temperature-responsive component is affected by moisture or other gases the system may be hermetically sealed in the absence of any undesirable gases.

Figure 3 typifies a transmission-type device useful for checking the temperature (and uniformity of heat and/or air distribution) of air emerging from ducts. Air stream 7 passes through apertures 8 pierced in polarizer 1 illuminated by lamp 5. The air stream then encounters temperaturesensitive layer 3 coated onto substrate 2. If the temperature-sensitive coating is affected deleteriously by such gases as oxygen or moisture it may be encapsulated or otherwise sealed behind a coating or shell nonpermeable to the as which it is desirable to exclude. Viewer 6 scans the system through analyzer 4 to determine the uniformity of birefringence.

FIGURE 4 typifies a reflection-type device in which light beams from a source 5 pass through polarizer 10 where they are polarized. They then pass through the temperature-sensitive layer 3 coated on substrate 2 to the polarization-conserving mirror 9. The mirror reflects the beam back through the polarizer 10 which now serves as an analyzer. As a variant of this system a circular polarizer may be used for 10 in place of the usual linear polarizer. Then, when the coating 3 is non-birefringent, no light will be reflected back through 10 because the circular polarizer has polarized the beam to a "right-handed" or "left- handed" helix form which cannot pass back through the circular polarizer 10. When coating 3 becomes birefringent, the polarization form of the light which is reflected from the mirror is altered and the returning light passes through the polarizer 10.

The substrate on which the temperature sensitive layer is deposited may be a smooth material such as isotropic glass if the device is to be operated in the horizontal plane.

However, when the device is vertical and the system is a solid/liquid type, the composition in the liquid condition may drain to the bottom of the plate under the influence of gravity. Under such circumstances, since roughening of the surface generally allows the solutions used to wet the substrate more thoroughly, plaques may be sandblasted or etched into the substrate, to give anchorage to the solutions and prevent their moving downward across the poorly wetting smooth surface. Such a unit is shown in FIGURE 5 where the coatings 3,3', 3", etc. are applied to etched areas on the substrate.Figure 5 illustrates a device in which there are a plurality of discrete areas which are adjacent in the same plane and which are occupied with compositions whose birefringence terminates at different temperatures, whereby the device can serve as a thermometer to measure different temperatures.

To further control drainage problems the solutions of temperature-sensitive compounds may be deposited before drying in quite small areas, of circular shape, for example, either on smooth or etched spots on the substrate. These "droplets" may be "printed" on the surface, for example, or they may be deposited by spraying through apertures in a mask over the substrate. The droplets of solution, analogous to the dots which comprise "half-tone" pictures, may be arranged to form plaques, temperature legends, warnings, etc.

Temperature-sensitive salt solutions may also be deposited in narrow channels or holes engraved into such substrates as transparent acrylic polymers. Such channels serve to hold the salt in its liquid form yet make effective displays when the compound has solidified and birefringence has appeared. The channels may form numbers indicating the particular temperature range of the salt filling the channels or may form rectangular display panels, etc.

Either compounds which indicate by way of solid/solid conversion of solid/liquid melting may be conveniently applied as aqueous or nonaqueous solutions. In such cases the compounds are conveniently compounded with wetting agents to lower their surface tension so that they may wet the chosen substrate. Since salts or other polar compounds are often selected for use as temperaturesensitive materials, the wetting agents are most suitably of a nonionic form. Further, to obtain effective but controlled wetting of the substrate, sufficient wetting agent is desirably compounded into the solution to obtain a surface tension near but not below the Critical Surface Tension of the substrate. That way a small contact angle is secured but wetting dies not proceed spontaneously across the entire surface so as to exacerbate drainage problems due to gravity.

Though generally good wetting is desired, in special cases poor wetting may be advantageous to create warning devices. Thus, a solution of a temperature-sensitive compound of the solid/liquid system having a naturally high surface tension may be sprayed onto a substrate in such a concentrated form that it dries almost immediately at the temperature and/or relative humidity present under spraying conditions. This temperature-sensitive film on its carrier substrate may then be displayed in typical transmission- or reflection-type temperature-responsive devices where an evenly illuminated, birefringent surface is maintained so long as the temperature remains below the melting point of the particular compound. Above the melting point, the crystals liquefy. The high surface tension of the liquid then causes it to pull to gether into droplets.Thus, even is the dangerously high temperature is subsequently lowered, a simple visual inspection of the film will reveal by the presence of the droplets that the danger point was indeed passed.

As another method of applying this invention, as shown in FIGURE 6, it is possible to disperse droplets of materials in solution, or droplets of melted materials, or finely divided solid materials, all of which are temperature-sensitive in appropriate vehicles of a type which might be called "lacquers." As would be expected, aqueous droplets would be dispersed in nonaqueous "lacquer," and finely divided particles of non-polar organic materials would be dispersed in an aqueous "lacquer" vehicle.

Such coatings 3 may then be applied to suitable substrates 2, where on evaporation of the solvents, the droplets or particles remain dispersed as small globules throughout a high viscosity, isotropic coating in which the sensing material is essentially insoluble. The substrate are placed between polarizing elements 1 and 4 to form temperatureresponsive devices of the type already described.

In general, where the temperatureresponsive material is provided as at least one layer on a transparent or translucent substrate it is preferred that the layer should have a thickness of at least 0.001 mm.

Compounds that of themselves sense changes in temperature without undergoing a chemical change may be grouped into two systems as was noted earlier. Compounds of the solid/solid system have the remarkable property of converting from the birefringent state to the nonbirefringent state at a particular temperature typical of the compound, which I have named the "crystal conversion temperature." I do not intend to be bound by theory, but is appears that heating the material weakens the intermolecular bonds which have maintained its "low temperature" crystal structure. Thus, rotation of the molecules or portions of the molecules may occur so that conversion to a different crystal form can occur.I particularly claim the use for signalling purposes of those conversions of crystal structure which are characterized by abrupt changes from the anisotropic state to the isotropic as the temperature passed through the crystal conversion temperature. Exceptionally vivid optical changes are noted when the birefringent/nonbirefringent change is monitored with crossed-field linear polarizers or a circular polarizer with a mirror.

Such unusual molecular configurations are not easily established precisely. How- ever, I theorize the molecular structures which exhibit the remarkable phenomenon are those of relatively high potential symmetry so that they tend to assume symmetrical structures as the rise of temperature reduces the bonding forces within the unit cell characteristic of the compound at temperatures below the crystal conversion point.

I have found that carbon tetrabromide, for example, can be melted on a suitable isotropic material such as a glass plate and cooled to give a thin, solid layer. If such a layer is viewed through crossed polarizers and the ambient temperature is raised, at a precisely reproducible temperature the brilliantly glowing field suddenly becomes blue-black as anisotropy disappears. On reducing the temperature below the crystal conversion point, which is about 117so., the birefringence appears just as suddenly.By comparison, the melting point of carbon tetrabromide is 198off. Carbon tetrachloride has a very low crystal conversion point so it may be mixed with carbon tetrabromide in various ratios so as to create a temperatureresponsive series covering the range below 117"F. Carbon tetraiodide, having a high crystal conversion point, may be comixed with the bromide in suitable ratios to create a high temperature series.

Ammonium nitrate, NH4NO3, as another example, has been found to have a crystal conversion point of about 260'F. By judicious choices of other individual compounds, or by comixing responsive compounds of similar polarity in various ratios, various temperature-responsive series may be prepared. As noted before, relatively symmetrical molecules are generally the types which are responsive, tetramethyl methane, tertiary butyl halides, the methyl and chloro penta- and hexa-substituted benzenes, pentaerythritol, and pentaerythritol tetraacetate being examples.

To prevent oxidation or the deleterious effect of moisture, or to prevent evaporation if the selected compounds have appreciable vapor pressures, it may prove desirable to hermetically seal the responsive materials within a nonpermeable outer shell. Depending on the structure desired, the temperature-responsive salts may be coated on one or both of the polarizers or on a separate substrate. Or a polarizationconserving mirror can be coated and a circular polarizer used for viewing, if visual amplification is desirable. If the compound is coated directly onto a mirror and viewed directly or through an isotropic cover plate, of glass for example, the birefringence/nonbirefringence conversion may then be observed directly.

From an esthetic and pragmatic standpoint it is desirable to coat the isotropic substrate with a very thin, evenly distributed layer of the selected compound or compounds. This is especially important if multi-coats are to be laid down so that the system might serve as a sun-screen for example, one layer after the other sequentially triggering as the temperature of the system rises. For such coating I have found it highly useful to compound the selected compounds with organic or inorganic polymers with which the materials are compatible after the carrier vehicle has evaporated. For salt-type compounds I prefer polymers containing repetitive oxygenbearing groups including the hydroxyl, the carboxyl, the sulfonic acid group, or mixtures thereof repetitively present along a substantially linear chain.Examples of these polymers which are preferably solid are as follows: methoxy group - methoxycellulose; polyether group - polyethylene oxide; hydroxyl group - polyvinyl alcohol, hydroxyethyl cellulose; carboxyl group - poly (methyl vinyl ether/maleic anhydride), poly (styrene/maleic anhydride), poly (ethylene/maleic anhydride), polyacrylic acid; sulfonic acid group - polyvinylsulfonic acid; pyrrolidone group - polyvinylpyrrolidone. Usually it is necessary to neutralize the carboxyl or sulfonic groups with an appropriate basic compound so that the system as a whole is neutral or slightly basic.

Further, other copolymers may be polymerized in the formation of these materials without materially altering their effectiveness so long as the polar groups dominate the polymers' structures. For example, polyacrylic acid may be modified by the inclusion of methacrylic acid during polymerization. Or other polymers can be copolymerized with the acrylic acid to produce highly acidic so-called "acrylic emulsions" which function much as the pure polyacrylic acids do.

I have found that the molecular weight of the polymer is not a critical matter, so long as the temperature-sensitive salt is adequately soluble in the polymer/solvent mixture. Because of the rheological requirements of the coating process and the need for appropriate physical qualities in the final solid film, the concentration of salt, polymer, and solvent are appropriately adjusted, depending on the nature of the components.

As would be expected, a less polar type of polymer usually proves best for compounding temperature-responsive materials having non-salt structures. Suitable examples include the following: polystyrene, polymethyl acrylate, ethylcellulose, and cellulose acetate. In such instances the solvents selected for the systems are also usually of limited polarity. N-methyl - 2-pyrrolidone, the lower alcohols, and the aromatic solvents illustrate types which are usually suitable alone or in admixture.

In some circumstances it may prove desirable to substitute very finely divided particles for polymers in order to bring about suitable adjustment of the rheology of the coating solutions containing the temperature-responsive compounds. Or the finely divided particles may be used in conjunction with dissolved or highly swollen polymers. The particles as a general class are distinguished by their high surface area and may include such materials as diatomaceous earths, pyrogenic silica or aluminium oxide, precipitated silica, silica sols, and similar materials.

A variety of modifying agents may be used to develop suitable specialized coatings, including finely divided inorganic particles for controlling opacity, plasticizers for modifying the mechanical properties of the polymeric binder, infra-red absorbing dyes to control the rate of heating of the film, surface-active compounds to facilitate smooth coating, etc.

If desired, instead of directly coating the substrate with compounded solutions of the type described, the temperature-responsive materials (either alone or mixed with suitable modifying agents) may be encapsulated within polymeric outer shells. Such encapsulation into very small spheres is now a common practice with a variety of materials.

After encapsulation the tiny spheres are then coated onto suitable substrates and dried to produce a system which performs much like that produced with compounded solutions.

I have just described typical solid/solid systems. As I noted earlier, there are also solid/liquid systems which function as highly useful visual signalling systems. The solid/liquid system depends on an entirely different phenomenon from that discovered and used as the basis for the solid/solid signalling system. In particular, it is based on the discovery that certain compounds are not only birefringent in the solid form but they retain this birefringent quality up to their melting points, at which time the birefringence disappears concurrently with the change in state from solid to liquid.

Various techniques have been used for determining the melting point of organic and inorganic compounds. Some techniques involved change in heat content, some change in volume, and others even the change in appearance when the finely divided powder being tested collapsed into a denser, usually darker melt at the melting point. But as my signalling materials I choose to use materials which are anisotropic in the solid state up to the melting point and which then change to isotropic liquids at the melting point. It is the opalescence of the anisotropic state shifting to the clarity of the isotropic state which I use for my signalling when the appropriate temperature has been reached. As noted before, linear polarizers or a circular polarizer and mirror are used to amplify this change from anisotropic to isotropic (and back to anisotropic on cooling).

ust as with the solid/solid systen, the system may be hermetically sealed to prevent any deleterious effects from oxygen, mois- ture, or to prevent evaporation of the indicators if desirable. Or if the temperatureresponsive compounds are quite stable and of very low vapor pressure they may be coated onto a substrate and left unsealed.

Because of the solid/liquid transition I have found that depositing a material in the form of droplets eliminates drainage problems in the vertical mode as I have described before. Or minute particles of the temperature-sensitive compound may be encapsulated in tiny inert polymeric shells and the spheres deposited on a suitable substrate.

Though I prefer pure materials for most temperature-responding purposes, the presence of other materials can be useful. Very pure materials generally melt very sharply.

The introduction of controlled quantities of other materials can produce a spread in melting which is quite useful for some thermal alarm devices.

Polymeric materials of varying polarity together with appropriate solvents can be used for compounding both salt-type and non-salt-type materials for solid/liquid systems just as with the solid/solid systems.

However, great care must be used to avoid choosing polymeric materials which might contribute a drift in melting point of the temperature-responsive compounds due to the effect of impurities dissolving out of the polymeric matnx into the indicator compounds. Just as before, infra-red absorbing dyes, opacifiers, and other compounding agents can be used so long as their effect in the melting points of the indicating compounds is carefully monitored.

Magnesium nitrate, hexahydrate, MgNO3.6H20, is an excellent example of a salt-type compound which is strongly birefrigent below its melting point of about 203so. but which passes into the nonbirefringent mode above this temperature. Benzophenone is a fine example of a non-salt compound which passes from a strongly birefringent solid below about 118 F. to the nonbirefringent mode when melted. By selecting suitable materials from the many pure organic and inorganic compounds available, a variety of excellent visual-type temperature-responsive devices can be assembled.

Indole is another example of a material exhibiting strong birefringence below its melting point, 125"F. and changing to a nonbirefringent liquid above it. The following is a good example of an homologous series of fatty acid compounds exhibiting good birefringent / nonbirefringent transitions at their melting points: caproic (24"F.), caprylic (61so.), capric (88"F.), laurlc (111"F.), Myristic (130 F.), Palmitic (145 F.), and stearic (157 F.). Mixtures of acids can be used to secure intermediate melting points. Many other similar series may be used, of course.

I have now used two inorganic compounds which are very effective in my systems: ammonium nitrate for use in solid/solid systems and magnesium nitrate hexahydrate for use in solid/liquid systems.

Although the first compound crystallizes in the anhydrous form, the second customarily is hydrated. However, both compounds are so very soluble in water that they tend to deliquesce if exposed to air of a high enough relative humidity. However, unlike one of the 5 stems described in my U.S. patent 3,776,038, deliquescence is not an essential part of this invention. Indeed, in many circumstances such adventitious moisture as might accidentally be sorbed during manufacture of devices using these materials could be deleterious to their most efficient and precise function. Thus, I have specified that the systems utilizing materials such as these salts be sealed to preclude moisture pick-up (or loss, as well, in the case of hydrated compounds such as magnesium nitrate hexahydrate).Compounds such as the fatty acids do not tend to pick up moisture from the air and so they may be left unsealed if desired.

Summarizing, U.S. 3,776,038 claims the use of both solid/solid and solid/liquid indicating compounds in which the acquisition or loss of moisture, leading to a change in the chemical composition of the temperature-sensitive material is an absolutely essential part of their signalling function. In the present invention water may be present in the signalling compositions through accidental pick-up of moisture or because of water of crystallization in the compound selected. But the pick-up or loss of water has nothing to do with the signalling function in the present invention.Thus, with the present invention, if magnesium nitrate hexahydrate is placed in an isotropic sealed tube with no gas above it and heated it melts at about 203so. When viewed between crossed polarizers, at the melting point the composition which has appeared brilliantly white until then abruptly appears blue-black. If the same magnesium nitrate hexahydrate is used to practice the hygrometric / thermometric aspects of U.S.

3,776,038 the salt is spread thinly on a substrate and placed in a sealed system containing a gas of controlled humidity. As the system is cooled the relative humidity rises until it reaches the deliquescent point of the salt at which point the salt dissolves in the water removed from the gas. If viewed through crossed polarizers the composition which has appeared brilliantly white until then appears abruptly as blue-black on dissolving. In the present invention the triggering temperature is an essential function of the signalling compound selected. In U.S.

3,776,038 in the standard mode the triggering temperature is a function of both the compound and the humidity of the material encapsulated above the signalling compound or within its microstructure.

Many crystalline compounds can exist in various crystal forms, changing from one form having a particular melting point to another form which may have a melting point substantially different. The change can be brought on by temperature changes or it often can be induced or accelerated through the inclusion of crystallization nuclei. Thus, through a judicious choice of indicating compounds irreversible indicators or indicating series can be formulated in which due to temperature changes crystals have formed which no longer revert to their earlier structure when the temperature changes again. Such indicators are especially useful in indicating when the equipment being monitored has deviated from the desired temperature range for any significantly long period. For such applications, the temperature-responsive material employed may, for example, be adapted to undergo at least one cycle from non-birefringent to birefringent but which becomes irreversibly birefringent if the birefringent state is maintained longer than approximately 72 hours.

WHAT I CLAIM IS: 1. A temperature responsive light gate device to be exposed to a source of illumma- of tion providing a light beam, comprising (1) a

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (25)

**WARNING** start of CLMS field may overlap end of DESC **. agents can be used so long as their effect in the melting points of the indicating compounds is carefully monitored. Magnesium nitrate, hexahydrate, MgNO3.6H20, is an excellent example of a salt-type compound which is strongly birefrigent below its melting point of about 203so. but which passes into the nonbirefringent mode above this temperature. Benzophenone is a fine example of a non-salt compound which passes from a strongly birefringent solid below about 118 F. to the nonbirefringent mode when melted. By selecting suitable materials from the many pure organic and inorganic compounds available, a variety of excellent visual-type temperature-responsive devices can be assembled. Indole is another example of a material exhibiting strong birefringence below its melting point, 125"F. and changing to a nonbirefringent liquid above it. The following is a good example of an homologous series of fatty acid compounds exhibiting good birefringent / nonbirefringent transitions at their melting points: caproic (24"F.), caprylic (61so.), capric (88"F.), laurlc (111"F.), Myristic (130 F.), Palmitic (145 F.), and stearic (157 F.). Mixtures of acids can be used to secure intermediate melting points. Many other similar series may be used, of course. I have now used two inorganic compounds which are very effective in my systems: ammonium nitrate for use in solid/solid systems and magnesium nitrate hexahydrate for use in solid/liquid systems. Although the first compound crystallizes in the anhydrous form, the second customarily is hydrated. However, both compounds are so very soluble in water that they tend to deliquesce if exposed to air of a high enough relative humidity. However, unlike one of the 5 stems described in my U.S. patent 3,776,038, deliquescence is not an essential part of this invention. Indeed, in many circumstances such adventitious moisture as might accidentally be sorbed during manufacture of devices using these materials could be deleterious to their most efficient and precise function. Thus, I have specified that the systems utilizing materials such as these salts be sealed to preclude moisture pick-up (or loss, as well, in the case of hydrated compounds such as magnesium nitrate hexahydrate).Compounds such as the fatty acids do not tend to pick up moisture from the air and so they may be left unsealed if desired. Summarizing, U.S. 3,776,038 claims the use of both solid/solid and solid/liquid indicating compounds in which the acquisition or loss of moisture, leading to a change in the chemical composition of the temperature-sensitive material is an absolutely essential part of their signalling function. In the present invention water may be present in the signalling compositions through accidental pick-up of moisture or because of water of crystallization in the compound selected. But the pick-up or loss of water has nothing to do with the signalling function in the present invention.Thus, with the present invention, if magnesium nitrate hexahydrate is placed in an isotropic sealed tube with no gas above it and heated it melts at about 203so. When viewed between crossed polarizers, at the melting point the composition which has appeared brilliantly white until then abruptly appears blue-black. If the same magnesium nitrate hexahydrate is used to practice the hygrometric / thermometric aspects of U.S. 3,776,038 the salt is spread thinly on a substrate and placed in a sealed system containing a gas of controlled humidity. As the system is cooled the relative humidity rises until it reaches the deliquescent point of the salt at which point the salt dissolves in the water removed from the gas. If viewed through crossed polarizers the composition which has appeared brilliantly white until then appears abruptly as blue-black on dissolving. In the present invention the triggering temperature is an essential function of the signalling compound selected. In U.S. 3,776,038 in the standard mode the triggering temperature is a function of both the compound and the humidity of the material encapsulated above the signalling compound or within its microstructure. Many crystalline compounds can exist in various crystal forms, changing from one form having a particular melting point to another form which may have a melting point substantially different. The change can be brought on by temperature changes or it often can be induced or accelerated through the inclusion of crystallization nuclei. Thus, through a judicious choice of indicating compounds irreversible indicators or indicating series can be formulated in which due to temperature changes crystals have formed which no longer revert to their earlier structure when the temperature changes again. Such indicators are especially useful in indicating when the equipment being monitored has deviated from the desired temperature range for any significantly long period.For such applications, the temperature-responsive material employed may, for example, be adapted to undergo at least one cycle from non-birefringent to birefringent but which becomes irreversibly birefringent if the birefringent state is maintained longer than approximately 72 hours. WHAT I CLAIM IS:

1. A temperature responsive light gate device to be exposed to a source of illumma- of tion providing a light beam, comprising (1) a

non-ferroelectric material which, at a predetermined temperature, is triggered without a change in the chemical composition of the material from an anisotropic, birefringent state to an isotropic, non-birefringent state and (2) means to intensify observable changes in the brightness and intensity of the light coming from the material when the light beam contacts the material, comprising a polarizer for polarizing the light beam before contacting the material and an analyzer for analyzing the beam coming from the material, whereby the device passes a relatively greater amount of light when the material is at a temperature in which it is in its birefringent state and a relatively lesser amount of light or no light when triggered by a change of temperature into its nonbirefringent state.

2. A device according to Claim 1, including also a source of illumination providing a light beam as an integral part of the device, and light detecting means for detecting observable changes in the brightness and intensity of the light coming from the material when the light-beam contacts the material.

3. A device according to Claim 2, wherein said light detecting means includes electrical means responsive to the light passed by the device when said material is in its birefringent state.

4. A device according to Claim 3, wherein said electrical means includes a photo-responsive solid state electronic device.

5. A device according to Claim 3 or Claim 4, including also electrical control means, responsive to said electrical means, for controlling electrical power to activate an electrical circuit.

6. A device according to any preceding claim, wherein the device is a transmissiontype in which the light beam passes through the polarizer, the polarized light beam passes through the material, and the emerging beam passes through the analyzer.

7. A device according to any one of Claims 1-5, wherein the device is a reflection type in which the light beam passes through the polarizer, the polarized light beam passes through the material, and the emerging beam is reflected by a mirror back through the material and the analyzer.

8. A device according to any preceding claim, wherein said material undergoes a solid-solid transition from its birefringent to its non-birefringent state.

9. A device according to any one of Claims 1-7, wherein said material is triggered at said predetermined temperature from an anisotropic solid transition state to an isotropic liquid state.

10. A device according to Claim 9, wherein said material comprises a substance selected from magnesium nitrate hexahydrate; benzophenone; and indole.

11. A device according to any preceding claim, wherein said material includes an inorganic compound having a high surface area.

12. A device according to any preceding claim, wherein said material is capsulated as small, approximately spherical globules within polymeric shells.

13. A device according to any preceding claim, wherein said material is adapted to undergo at least 100 cycles of birefringence and nonbirefringence.

14. A device according to any preceding claim, wherein said material is adapted to undergo at least one cycle from nonbirefringent to birefringent but which becomes irreversibly birefringent if the birefringent state is maintained longer than approximately 72 hours.

15. A device according to any preceding claim, wherein said material is provided as at least one layer on a transparent or translucent substrate.

16. A device according to Claim 15, wherein the thickness of the layer is at least 0.001 mm.

17. A device according to Claim 15 or Claim 16, wherein said material is dispersed as small globules throughout a high viscosity, isotropic coating in which the sensing material is essentially insoluble.

18. A device according to any preceding claim, in which at least two discrete areas which are adjacent in the same plane or optically spaced in parallel planes are occupied with materials whose birefringence terminates at different temperatures, whereby the device serves as a thermometer to measure different temperatures.

19. A device according to Claim 18, in which at least two discrete areas spaced in parallel planes are occupied with materials in intimate contact and whose birefringence terminates at different temperatures.

20. A device according to Claim 18 or Claim 19, in which said materials whose birefringence terrninates at different temperatures comprises compounds selected from the fatty acid series of caproic, caprylic, capric, lauric, myristic, palmitic and stearic acids, or mixtures thereof.

21. A method of measuring temperature with a temperature-responsive light gate device, comprising the steps of (1) providing a light beam from a source of illumination; (2) polarizing the light beam; (3) bringing the polarized beam into contact with means providing observable changes in temperature, including a non-ferroelectric material which, at a predetermined temperature, is triggered without a change in the chemical composition of the material from an anisotropic, birefringent state to an iso tropic, non-birefringent state, whereby the polarized beam is doubly refracted when said material is at a temperature in which it is in its birefringent state and not doubly refracted when said material is at a temperature in which it is in its non-birefringent state; and (4) analyzing the light beam coming from the material to provide a visual indication of whether said material is above or below said predetermined temperature.

22. A method according to Claim 21, in which said material is as defined in any one of Claims 8-17.

23. A method according to Claim 21 or Claim 22, in which said means providing observable changes in temperature include at least two discrete areas which are adjacent in the same plane or optically spaced in parallel planes and which are occupied with materials whose birefringence terminates at different temperatures; and in which the light beams coming from the materials are analyzed to provide a visual indication of the temperature to which the materials are being subjected.

24. A temperature responsive light gate device, according to Claim 1 and substantially as hereinbefore described with reference to Fig. 1, Fig. 2, Fig. 3, Fig. 4, Fig. 5 or Fig. 6 of the accompanying drawings.

25. A method of measuring temperature with a temperature-responsive light gate device, according to Claim 21 and substantially as hereinbefore described with reference to Fig. 1, Fig. 2, Fig. 3, Fig. 4, Fig.

5 or Fig. 6 of the accompanying drawings.