Thermochromatic device and thermochromatic display apparatus

A thermochromatic device in a thermochromatic display includes an insulating substrate, a color element, a heating element, a first electrode, and a second electrode, the color element and the heating element located on the insulating substrate being virtually integral but together are physically isolated and heat-insulated and allow such fast electrically-governed color changes that moving color images can be displayed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims all benefits accruing under 35 U.S.C. §119 from China Patent Application No. 201210493750.8, filed on Nov. 28, 2012 in the China Intellectual Property Office, disclosure of which is incorporated herein by reference.

BACKGROUND

1. Technical Field

The present disclosure relates to a thermochromatic device and thermochromatic display apparatus using the same.

2. Description of Related Art

Thermochromatic materials are materials that change their color in response to changes in temperature. Thermochromatic materials can be used to make a thermochromatic device. A thermochromatic device usually includes a heater made of ceramics, conductive glasses or metals. However, the speed of color change of the thermochromatic device is slow because the relatively high heat capacity per unit and slow heating speed of the heater.

In US20110149373A1 published on Jun. 23, 2011, Liu et al. discloses a thermochromatic device to overcome the above shortcomings. Referring toFIGS. 13and14, the thermochromatic device120of Liu et al. includes an insulating substrate102, a color element118, a heating element108, a first electrode110, and a second electrode112. However, the heating response speed of the thermochromatic device120is relatively slow, usually above 5 seconds, because the color element118and/or heating element108are in contact with the insulating substrate102. Thus, the response speed of the thermochromatic device120is relatively slow.

What is needed, therefore, is to provide a thermochromatic device having an improved color change speed and thermochromatic display apparatus using the same.

DETAILED DESCRIPTION

References will be made to the drawings to describe various embodiments of the present thermochromatic devices and thermochromatic display apparatus using the same.

Referring toFIG. 1, a thermochromatic device220of a first embodiment includes an insulating substrate202, a color element218, a heating element208, a first electrode210, and a second electrode212.

The color element218and the heating element208are combined with each other to form a composite226. The composite226is a free-standing structure. The term “free-standing structure” means that the composite226can sustain the weight of itself when it is hoisted by a portion thereof without any significant damage to its structural integrity, so that at least one part of the composite226can be free of contact with any other surface, to permit rapid temperature changes. In one embodiment, at least one part of the composite226can be suspended over the insulating substrate202and supported by the first electrode210and the second electrode212. In one embodiment, at least one part of the composite226can be suspended over the insulating substrate202and supported by two supporting elements spaced from each other. In one embodiment, at least one part of the composite226can be suspended over a recess defined in the insulating substrate202. The at least one part of the composite226is suspended means the at least one part of the composite226is free of contact with any other surface, to permit rapid temperature changes. In use, the thermochromatic device220is located in a vacuum or a space filled with inert gas.

In one embodiment, the heating element208forms a matrix, and the color element218is dispersed in the heating element208. In one embodiment, the color element218forms a matrix, and the heating element208is dispersed in the color element218. In one embodiment, both the heating element208and the color element218are layers and stacked on each other. For example, two heating elements208can be located on two opposite surfaces of a single color element218, that is, the single color element218is sandwiched between two heating elements208. For another example, two color elements218can be located on opposite surfaces of a single heating element208, that is, the single heating element208is sandwiched between two color elements218. For example, a plurality of color elements218and a plurality of heating elements208can be alternately stacked on each other to form a multi-layer structure.

The insulating substrate202may be made of rigid material or flexible material. The rigid material may be ceramic, glass, quartz, resin, silicon, silicon dioxide, diamond, or alumina. The flexible material may be flexible polymer, fiber, or synthetic paper. The flexible polymer can be polyethylene terephthalate (PET), polycarbonate (PC), polyethylene (PE), or polyimide (PI). When the insulating substrate202is made of flexible material, the thermochromatic device220can be folded into random shapes for and during use. The melting point of the insulating substrate202is equal to or higher than 200° C. A size and a thickness of the insulating substrate202can be chosen according to need. In one embodiment, the insulating substrate202is a PET film with a thickness of about 1 millimeter.

The heating element208is a free-standing structure and can be made of material such as metal, alloy, or carbon nanotubes. In one embodiment, the heating element208includes a carbon nanotube structure. The carbon nanotube structure includes a plurality of carbon nanotubes uniformly distributed therein, and the carbon nanotubes therein are combined by van der Waals attractive force therebetween. The carbon nanotube structure can be a substantially pure structure of carbon nanotubes. The carbon nanotubes can be used to form many different structures and provide a large specific surface area. The heat capacity per unit area of the carbon nanotube structure can be less than 2×10−4J/m2·K. In one embodiment, the heat capacity per unit area of the carbon nanotube structure is less than 1.7×10−6J/m2·K. As the heat capacity of the carbon nanotube structure is very low, a fast heating and fast cooling of the heating element208results in substantially instant temperature changes in the composite226, allowing a high heating efficiency and accuracy. As the carbon nanotube structure is substantially pure, the carbon nanotubes are not easily oxidized and the life of the heating element208will be relatively long. Further, the carbon nanotubes have a low density, about 1.35 g/cm3, so the heating element208is lightweight. As the heat capacity of the carbon nanotube structure is very low, the heating element208has a high response heating speed. Because the carbon nanotube has a large specific surface area, the carbon nanotube structure with a plurality of carbon nanotubes has a large specific surface area. When the specific surface area of the carbon nanotube structure is large enough, the carbon nanotube structure is self-adhesive and can be directly applied to a surface.

The carbon nanotubes in the carbon nanotube structure can be arranged orderly or disorderly. The term ‘disordered carbon nanotube structure’ includes, but is not limited to, a structure where the carbon nanotubes are arranged along many different directions, and the aligning directions of the carbon nanotubes are random. The number of the carbon nanotubes arranged along each different direction can be almost the same (e.g. uniformly disordered). The disordered carbon nanotube structure can be isotropic. The carbon nanotubes in the disordered carbon nanotube structure can be entangled with each other.

The carbon nanotube structure including ordered carbon nanotubes is an ordered carbon nanotube structure. The term ‘ordered carbon nanotube structure’ includes, but is not limited to, to a structure where the carbon nanotubes are arranged in a consistent manner, e.g., the carbon nanotubes are arranged approximately along a same direction and/or have two or more sections within each of which the carbon nanotubes are arranged approximately along a same direction (different sections can have different directions). The carbon nanotubes in the carbon nanotube structure can be single-walled, double-walled, or multi-walled carbon nanotubes.

The carbon nanotube structure can be a carbon nanotube film structure with a thickness ranging from about 0.5 nanometers to about 1 millimeter. The carbon nanotube film structure can include at least one carbon nanotube film. The carbon nanotube structure can also be a linear carbon nanotube structure with diameters ranging from about 0.5 nanometers to about 1 millimeter. The carbon nanotube structure can also be a combination of the carbon nanotube film structure and the linear carbon nanotube structure. It is understood that any carbon nanotube structure described can be used with all embodiments.

In one embodiment, the carbon nanotube film structure includes at least one drawn carbon nanotube film. A drawn carbon nanotube film can be drawn from a carbon nanotube array that is able to have a film drawn therefrom. The drawn carbon nanotube film includes a plurality of successive and orderly arranged carbon nanotubes joined end-to-end by van der Waals attractive force therebetween. The drawn carbon nanotube film is a free-standing film. Referring toFIGS. 2 to 3, each drawn carbon nanotube film includes a plurality of carbon nanotube segments143joined end-to-end by van der Waals attractive force therebetween. Each carbon nanotube segment143includes a plurality of carbon nanotubes145parallel to each other, and combined by van der Waals attractive force therebetween. As can be seen inFIG. 2, some variations can occur in the drawn carbon nanotube film. The carbon nanotubes145in the drawn carbon nanotube film have a preferred orientation. The carbon nanotube film can be treated with an organic solvent to increase the mechanical strength and toughness and reduce the coefficient of friction of the carbon nanotube film. A thickness of the carbon nanotube film ranges from about 0.5 nanometers to about 100 micrometers. In one embodiment, the heating element208is a single drawn carbon nanotube film with a length of 300 micrometers and a width of 100 micrometers. The carbon nanotubes of the heating element208extend from the first electrode210to the second electrode212. The drawn carbon nanotube film can be attached to surfaces of the insulating substrate202with an adhesive, by mechanical force, by the self-adhesive properties of the carbon nanotube film, or by a combination thereof. The response speed of the drawn carbon nanotube film is very high because of the very low heat capacity per unit area, the large surface area, and the large radiation coefficient. The single drawn carbon nanotube film with a length of 300 micrometers and a width of 100 micrometers can be heated to a temperature of 2000 Kelvin in 1 millisecond.

The carbon nanotube film structure of the heating element208can include at least two stacked drawn carbon nanotube films. In other embodiments, the carbon nanotube structure can include two or more coplanar carbon nanotube films, and can include layers of coplanar carbon nanotube films. Additionally, when the carbon nanotubes in the carbon nanotube film are aligned along one preferred orientation (e.g., as in the drawn carbon nanotube film), an angle can exist between the orientation of carbon nanotubes in adjacent films, whether the films are stacked on each other or arranged side by side. Adjacent carbon nanotube films can be combined by only the van der Waals attractive force. The number of layers of carbon nanotube films is not limited. However the thicker the carbon nanotube structure, the smaller the specific surface area will be. An angle between the aligned directions of the carbon nanotubes in two adjacent carbon nanotube films can range from about 0° to about 90°. When the angle between the aligned directions of the carbon nanotubes in adjacent stacked carbon nanotube films is larger than 0 degrees, a microporous structure is defined by the carbon nanotubes in the heating element208. The carbon nanotube structure in an embodiment employing these films will have a plurality of micropores. Stacking the carbon nanotube films will also add to the structural integrity of the carbon nanotube structure. In some embodiments, the carbon nanotube structure is a free standing structure.

In another embodiment, the carbon nanotube film structure includes a flocculated carbon nanotube film. The flocculated carbon nanotube film can include a plurality of long, curved, disordered carbon nanotubes entangled with each other. Further, the flocculated carbon nanotube film can be isotropic. The carbon nanotubes can be substantially uniformly dispersed in the carbon nanotube film. Adjacent carbon nanotubes are subject to van der Waals attractive force to form an entangled structure with micropores defined therein. It is understood that the flocculated carbon nanotube film is very porous. Sizes of the micropores can be less than 10 micrometers. The porous nature of the flocculated carbon nanotube film will increase specific surface area of the carbon nanotube structure. Further, due to the carbon nanotubes in the carbon nanotube structure being entangled with each other, the carbon nanotube structure employing the flocculated carbon nanotube film will have excellent durability, and can be fashioned into desired shapes with a low risk of breaking or cracking of the carbon nanotube structure. The flocculated carbon nanotube film, in some embodiments, will not require the use of the planar supporter18due to the carbon nanotubes being entangled and adhering together by van der Waals attractive force therebetween. The thickness of the flocculated carbon nanotube film ranges from about 0.5 nanometers to about 1 millimeter.

In another embodiment, the carbon nanotube film structure can include at least one pressed carbon nanotube film. The pressed carbon nanotube film can be a free-standing carbon nanotube film. The carbon nanotubes in the pressed carbon nanotube film are arranged along a same direction or arranged along different directions. The carbon nanotubes in the pressed carbon nanotube film can rest upon each other. Adjacent carbon nanotubes are attracted to each other and combine by van der Waals attractive force. An angle between a primary alignment direction of the carbon nanotubes and a surface of the pressed carbon nanotube film is 0 degrees to approximately 15 degrees. The greater the pressure applied, the smaller the resulting angle. When the carbon nanotubes in the pressed carbon nanotube film are arranged along different directions, the carbon nanotube structure can be isotropic. The thickness of the pressed carbon nanotube film ranges from about 0.5 nanometers to about 1 millimeter.

Carbon nanotube structures include linear carbon nanotube structures. In other embodiments, the linear carbon nanotube structures, including carbon nanotube wires and/or carbon nanotube cables, can be used.

The carbon nanotube wire can be untwisted or twisted. Treating the drawn carbon nanotube film with a volatile organic solvent untwists the carbon nanotube wire. Specifically, the organic solvent is applied to soak the entire surface of the drawn carbon nanotube film. During the soaking, adjacent parallel carbon nanotubes in the drawn carbon nanotube film will bundle together, due to the surface tension of the organic solvent as it volatilizes, and thus the drawn carbon nanotube film is shrunk into untwisted carbon nanotube wire. Referring toFIG. 4, the untwisted carbon nanotube wire includes a plurality of carbon nanotubes substantially oriented along a same direction (i.e., a direction along the length of the untwisted carbon nanotube wire). The carbon nanotubes are parallel to the axis of the untwisted carbon nanotube wire. More specifically, the untwisted carbon nanotube wire includes a plurality of successive carbon nanotube segments joined end to end by van der Waals attractive force therebetween. Each carbon nanotube segment includes a plurality of carbon nanotubes substantially parallel to each other, and combined by van der Waals attractive force therebetween. The carbon nanotube segments can vary in width, thickness, uniformity and shape. Length of the untwisted carbon nanotube wire can be arbitrarily set as desired. A diameter of the untwisted carbon nanotube wire ranges from about 0.5 nanometers to about 100 micrometers.

The twisted carbon nanotube wire can be formed by twisting a drawn carbon nanotube film using a mechanical force to turn the two ends of the drawn carbon nanotube film in opposite directions. Referring toFIG. 5, the twisted carbon nanotube wire includes a plurality of carbon nanotubes helically oriented around an axial direction of the twisted carbon nanotube wire. More specifically, the twisted carbon nanotube wire includes a plurality of successive carbon nanotube segments joined end to end by van der Waals attractive force therebetween. Each carbon nanotube segment includes a plurality of carbon nanotubes parallel to each other, and combined by van der Waals attractive force therebetween. Length of the carbon nanotube wire can be set as desired. A diameter of the twisted carbon nanotube wire can be from about 0.5 nanometers to about 100 micrometers. Further, the twisted carbon nanotube wire can be treated with a volatile organic solvent after being twisted. After being soaked by the organic solvent, the adjacent paralleled carbon nanotubes in the twisted carbon nanotube wire will bundle together, due to the surface tension of the organic solvent when the organic solvent volatilizes. The specific surface area of the twisted carbon nanotube wire will decrease, while the density and strength of the twisted carbon nanotube wire will be increased.

The carbon nanotube cable includes two or more carbon nanotube wires. The carbon nanotube wires in the carbon nanotube cable can be twisted or untwisted. In an untwisted carbon nanotube cable, the carbon nanotube wires are parallel with each other. In a twisted carbon nanotube cable, the carbon nanotube wires are twisted with each other.

The heating element208can include one ore more linear carbon nanotube structures. The plurality of linear carbon nanotube structures can be laid next to each other, crossed with each other, woven together, or twisted with each other. The resulting structure can be a planar structure if desired.

In other embodiments, the carbon nanotube structure can include other materials thus becoming a carbon nanotube composite. The carbon nanotube composite can include a carbon nanotube structure and a plurality of fillers dispersed therein. The filler can be comprised of a material, such as metal, ceramic, glass, carbon fiber or combinations thereof. Alternatively, the carbon nanotube composite can include a matrix and a plurality of carbon nanotubes dispersed therein. The matrix can be comprised of a material such as resin, metal, ceramic, glass, carbon fiber or combinations thereof. In one embodiment, a carbon nanotube structure is packaged in a resin matrix.

The color element218is made of thermochromatic material. The color of the thermochromatic material changes with the temperature and will come back to original color as the temperature comes back to the original temperature. The color change temperature of the thermochromatic material is below 200° C. in one embodiment. In one embodiment, the color change temperature of the thermochromatic material is in a range from about 40° C. to about 100° C. so that the thermochromatic device220can work in a room temperature environment and have a low working voltage. The thermochromatic material can be inorganic thermochromatic material, organic thermochromatic material, or liquid crystal thermochromatic material.

Inorganic thermochromatic materials include silver iodide, silver complex, silver double salt, copper iodide, copper complex, copper double salt, mercury iodide, mercury complex, mercury double salt, cobalt salt, nickel salt, methenamine compound, vanadium (III) oxide, chromate, or vanadate. Some inorganic thermochromatic materials and their colors, together with color change temperature, are shown in table 1 below.

The organic thermochromatic material includes color fixatives (electron donors), color developing agent (electron acceptors), and solvent. The perceived color of the organic thermochromatic material depends on the color fixatives. The color depth of the organic thermochromatic material depends on the color developing agent. The color change temperature of the organic thermochromatic material depends on the solvent used. The color fixatives can be triarylmethane dyes, fluorane dyes, thiodiphenylamine, spiropyran dyes, Schiff-base dyes, spiro compounds, bianthrone, or combination thereof. The triarylmethane dyes can be crystal violet lactone, or cresol red. The fluorane dyes can be thermochromic red or thermochromic green. The color developing agent includes organic color developing agents or inorganic color developing agents. The inorganic color developing agent can be acid clay, activated clay or kaolin, or magnesium aluminum silicate. The organic color developing agent can be bisphnol A, benzyl hydroxybenzoate, 4-hydroxycoumarin, n-hexanoic acid, caprylic acid, stearic acid, terephthalic acid, or Lewis acid. The solvent can be dodecanol, n-tetradecyl alcohol, hexadecanol, n-octadecyl alcohol, aliphatic ketones, ester, aether, amides, or carboxylic acid compound.

The liquid crystal thermochromatic material can be thermotropic liquid crystals. The thermotropic liquid crystals consist of organic molecules and exhibit a phase transition into the liquid crystal phase as temperature changes. The thermotropic liquid crystals can be divided into nematic liquid crystals, smectic liquid crystals, or cholesteric liquid crystals according to their optical properties. The cholesteric liquid crystals can be made of cholesterin.

The first electrode210and the second electrode212are spaced from each other and electrically connected with the heating element208. The first electrode210and the second electrode212can be located on the surface of the insulating substrate202, on a surface of the composite226, or on a supporting element. The first electrode210and the second electrode212are made of conductive material such as carbon nanotube, metal, alloy, indium tin oxides (ITO), antimony doped tin oxide (ATO), conductive polymer, or a conductive slurry. The first electrode210and the second electrode212can be conductive sheets with a thickness of about 0.5 nanometers to about 500 micrometers. In one embodiment, the first electrode210and the second electrode212are formed on the surface of the insulating substrate202by a printing process. The conductive slurry is composed of metal powder, glass powder, and binder. The metal powder can be silver powder, the glass powder has low melting point, and the binder can be terpineol or ethyl cellulose (EC). The conductive slurry may include from about 50% to about 90% (by weight) of the metal powder, from about 2% to about 10% (by weight) of the glass powder, and from about 8% to about 40% (by weight) of the binder.

During operation, a voltage is supplied to the first electrode210and the second electrode212. The temperature of the heating element208rises and the color element218is heated by the heating element208. When the color element218is heated to a color change temperature, the color of the color element218will change. For example, the color element218made of Ag2HgI4will change color from yellow to red when it is heated to a temperature of 42° C. Supplying a constant voltage, the temperature of the color element218will remain constant. Therefore, the thermochromatic device220will display a constant color. The color displayed by the thermochromatic device220can be suddenly changed through changing the voltage supplied to the heating element208. Because the color element218is made of thermochromatic material, the color element218will revert to original color as the color element218cools and reverts to original temperature.

Because the color element218and the heating element208form a composite226with at least one part free of contact with another element, any heat exchange or loss between the composite226and the surrounding environment is minimal as the heating element208heats the color element218. Thus, the color element218can be heated to the work temperature rapidly, and the thermochromatic device220has an improved heating response speed.

A test was performed on both the thermochromatic device220and the thermochromatic devices120of prior art. In the thermochromatic device220, the insulating substrate202was a PET film with a thickness of about 110 micrometers; the first electrode210and the second electrode212were formed on the insulating substrate202by printing conductive slurry; the heating element208included two stacked drawn carbon nanotube films; the color element218was a layer of Ag2HgI4with a thickness from about 10 micrometers to about 500 micrometers and deposited on the two stacked drawn carbon nanotube films by a sputtering or thermal deposition process; the composite226was suspended over the first electrode210and the second electrode212. As shown inFIG. 6, the angle between the aligned directions of the carbon nanotubes in two adjacent carbon nanotube films was about 90°, and a layer of Ag2HgI4was coated on a surface of the two stacked drawn carbon nanotube films. The thermochromatic devices120of prior art had a structure as shown inFIGS. 1-2. The thermochromatic devices120of prior art had the same experimental parameters as the thermochromatic device220.

The test results are shown in table 2 below. The heating response speed of the thermochromatic device220was improved significantly compared with the thermochromatic devices120of prior art. The heating and cooling response speed of the thermochromatic device220was faster than the heating response speed of the thermochromatic devices120of prior art. A heating response time of about 2.3 seconds was obtained, faster than heating response times of about 5.2 and 6.6 seconds for the thermochromatic devices120of prior art. A cooling response time of about 1.7 seconds was obtained, faster than heating response times of about 5 and 6.6 seconds for the thermochromatic devices120of prior art.

When the thermochromatic device220is used in the thermochromatic display apparatus by a consumer, the thickness of the insulating substrate202will be usually above 110 micrometers, or the insulating substrate202will be located on a plate or other supporting surface. In another test, the thermochromatic device220ofFIG. 1, the thermochromatic device120ofFIG. 13, and the thermochromatic device120ofFIG. 14were all located on a glass plate which had a thickness of about 1 millimeter. The heating response time of the thermochromatic devices120inFIGS. 13 and 14was greater than 10 seconds, but the heating response time of the thermochromatic device220ofFIG. 1was still about 2 seconds.

Referring toFIG. 7, a thermochromatic device320of a second embodiment includes an insulating substrate302, a color element318, a heating element308, a first electrode310, and a second electrode312. The color element318and heating element308form a composite326. At least one part of the composite326is free of physical contact with any other element.

The thermochromatic device320is similar to the thermochromatic device220described above except that a recess322is formed in a surface of the insulating substrate302, and the composite326is suspended over the recess322. The color element318is made of color-changeable material.

In one embodiment, the heating element308covers the recess322and extends to the surface of the insulating substrate302for support. That is, the periphery of the heating element308is located on the surface of the insulating substrate302, and the center portion of the composite326is suspended over the recess322. The color element318is located only on the unsupported center portion of the heating element308so that less of the surface of the composite326is in contact with the insulating substrate302. The first electrode310and the second electrode312are located on the portion of the heating element308that is on the insulating substrate302.

The color-changeable material transforms between the crystalline and amorphous stages when a heat write voltage impulse or a heat erase voltage impulse is supplied to the heating element308to heat the color-changeable material. The heat write impulse is a sudden voltage to the heating element308and causes the thermochromatic device320to display. The heat erase impulse is a sudden voltage to the heating element308and causes the material to revert to its original appearance. When the thermochromatic device320needs to display, a heat write impulse, intensive but of short duration, is supplied to heat the color element318. Because the temperature caused by the heat write impulse is very high, the color element318is instantly heated to a liquid state. Because the heating time caused by the heat write impulse is very short, the temperature of the color element318decreases almost instantly after the heat write impulse and the color element318reverts to an amorphous solid state from the liquid state. The color-changeable material of the color element318remains in an amorphous state, at a room temperature, without any outside energy being applied. Because the reflective properties of the crystalline color-changeable material and of the amorphous color-changeable material of the color element318are different, the color element318shows different brightnesses and the thermochromatic device320performs well as a display. In one embodiment, the brightness can be detected by the eye. When the display needs to be blanked out, a heat erase impulse, of less intensity and longer duration, can be supplied. This process is an annealing process. After annealing, the color-changeable material of the color element318changes to the original crystalline state from amorphous state, and the display is blanked out. The color-changeable material of the color element318remains in a crystalline state, at a room temperature, without any outside energy being applied. Because of this, the thermochromatic device320can perform as a bistable display. A bistable display means that a display does not require any outside energy to maintain either a displaying state or a non-displaying state.

In one embodiment, the temperature at which the color-changeable material of the color element318transforms between the crystalline and amorphous states is in a range from about 40° C. to about 600° C. The color-changeable material, having a temperature-governed phase change point above 40° C. allows the thermochromatic device320to work in a room temperature situation. The color-changeable material having a temperature-governed phase change point below 600° C. allows the thermochromatic device320to work at a low working voltage. In addition, the color-changeable material having a temperature-governed phase change point below 600° C. avoids oxidation of the heating element208of the carbon nanotube, and allows a long lifespan. The period of time of the phase change for the color-changeable material to transform between the crystalline and amorphous states is as short as possible so that the thermochromatic device320has a fast response speed. In one embodiment, the phase change time is shorter than 40 milliseconds.

The color-changeable material can be a chalcogenide compound such as sulfur-based compound, tellurium-based compound, selenium-based compound, or tellurium-selenium-based compound. The sulfur-based compound can be germanium-sulfur, arsenic-sulfur, or indium-sulfur. The tellurium-based compound can be germanium-tellurium, arsenic-tellurium, antimony-tellurium, or indium-tellurium. The selenium-based compound can be germanium-selenium, arsenic-selenium, antimony-selenium, or indium-selenium. Furthermore, additives improve the phase change speed of the color-changeable material. The additive can be copper, silver, gold, palladium, nickel, cobalt, or combinations thereof. The phase change time of the chalcogenide compound from crystalline to amorphous states is in a range from about several nanoseconds to about hundreds of nanoseconds. The phase change time of the chalcogenide compound from amorphous to crystalline states is in a range from about 0.5 microseconds to about 1 millisecond. In addition, other crystalline materials such as semiconductors, semiconductor compounds, metal compounds, or polymers having a phase change temperature above 40° C., will make the phase change time even shorter than 40 milliseconds, and different reflectivities of the crystalline and amorphous states can be used to achieve the color-changing function in the color element318.

During operation, an impulse voltage is supplied to the first electrode310and the second electrode312. The temperature of the heating element308rises and a burst of heat is supplied to the color element318from the heating element308. When the impulse voltage is short and high, a short and intensive burst of heat is supplied by the heating element308. When the impulse voltage is long and low, a longer and weaker heating is supplied by the heating element308. When the thermochromatic device320needs to display, a short and intensive burst of heat is supplied to the color element318. For example, the temperature of the burst of heat is in a range from about 900° C. to about 1000° C. and the duration of the heat write impulse voltage is in a range from about 50 nanoseconds to about 200 nanoseconds. Because the temperature caused by the heat write impulse is very high, the color element318is instantly heated to a liquid state. Because the heating time of the heat write impulse is very short, the temperature of the color element318decreases almost instantly after the burst of heat and the liquid state color element318becomes an amorphous solid. Because the reflectivity of the amorphous color-changeable material is different from that of the original crystalline color-changeable material, the color element318can show different brightnesses and the thermochromatic device320can display. The color-changeable material remains in amorphous state, at room temperature, without any outside energy being applied. When the thermochromatic device320needs to be blanked, a longer and weaker heat erase impulse can be supplied to the color element318. For example, the temperature of the longer and weaker heat application is in a range from about 500° C. to about 600° C. and the duration of the heat erase impulse voltage is in a range from about 1 microsecond to about 1 millisecond. The process is an annealing process. After annealing the color-changeable material, it changes to the original crystalline state from the amorphous state, and the display is blanked. The color-changeable material can keep in a crystalline state, at room temperature, without any outside energy being applied. Because of this, a bistable display is created. The bistable display means that energy is only consumed during the process of writing and blanking, there is no energy consumption after these processes. Thus, the thermochromatic device320saves energy.

In one embodiment, the insulating substrate302is a PET film with a thickness of about 500 micrometers. The first electrode310and the second electrode312are formed by printing conductive slurry. The recess322is formed by an impressing process. The heating element308includes two layers of drawn carbon nanotube films. The color element318is a layer of germanium-selenium compound with a thickness from about 10 micrometers to about 500 micrometers. The color element318can be formed on the drawn carbon nanotube films by a sputtering or thermal deposition process.

Referring toFIG. 8, a thermochromatic device420of a third embodiment includes an insulating substrate402, a background color layer428, a color element418, a heating element408, a first electrode410, and a second electrode412. The color element418and heating element408form a composite426. At least one part of the composite426is free of physical contact with any other element.

The thermochromatic device420is similar to the thermochromatic device220described above except that a background color layer428located on the surface of the insulating substrate402is included. The color element418is made of a material which can transform between a transparent state and a nontransparent state at a phase change temperature.

When the color element418is transparent, the thermochromatic device420reveals the color of the background color layer428. When the color element418is nontransparent, the thermochromatic device420masks the background color layer428and presents a blank appearance. The phase change temperature of the color element418is below 200° C. In one embodiment, the phase change temperature of the color element418is in a range from about 40° C. to about 100° C. so that the thermochromatic device420can work in a room temperature environment using a low working voltage. The present disclosure provides three groups of possible materials for the color element418, respectively named first color element418, second color element418and third color element418.

The background color layer428can be a layer of any material which can show a single color, or white or black. The color of the background color layer428will not change at a temperature below 200° C. The thickness of the background color layer428is in a range from about 1 micrometer to about 100 micrometers. The background color layer428can be formed by printing, spraying, coating, or sputtering.

The material of the first color element418is a mixture of polymer and fatty acid. The working principle of the first color element418is described as follows. The crystals of the material of the first color element418are in a dispersed state within a certain temperature range. The crystal size of the material of the first color element418performs a reversible change as the temperature changes. Different crystal sizes of the material of the first color element418cause different light transmissivities, so the material of the first color element418transforms between the transparent state and the nontransparent state. In one embodiment, the polymer and fatty acid mixture of the first color element418can be a mixture of vinylidene chloride acrylonitrile copolymer and eicosanoids, a mixture of butadiene styrene copolymer and stearic acid, or a mixture of vinyl chloride vinyl acetate copolymer and stearic acid.

The color element418, made of the mixture of vinylidene chloride acrylonitrile copolymer and eicosanoids, is white and nontransparent at room temperature. When the color element418is heated to about 74° C. from room temperature by suddenly applying a heat write impulse, it becomes transparent and colorless. Thus, the thermochromatic device420reveals any color of the background color layer428. When the color element418is heated to about 63° C. from room temperature by suddenly applying a heat erase impulse, it becomes white and nontransparent again. Thus, the color of the thermochromatic device420is masked. Because the heat impulse is short and the color element418will cool down to room temperature rapidly, either the transparent state or the nontransparent state can exist at room temperature. Thus, performance as a bistable display can be achieved. The bistable display means that a display does not require any outside energy to maintain either a displaying state or a non-displaying state.

In one embodiment, the mixture of butadiene styrene copolymer and stearic acid can be made by dissolving a butadiene styrene copolymer and a stearic acid in a mixture solution of THF and toluene. The mixture of butadiene styrene copolymer and stearic acid changes from nontransparent to transparent at a temperature of about 57° C. The mixture of butadiene styrene copolymer and stearic acid changes from transparent to nontransparent at a temperature of above 70° C.

The second color element418is a mixture of at least two polymers which are changeable between a compatible-elements state and an incompatible-elements state. The working principle of the second color element418is described as follows. The second color element418has a critical compatibility temperature above 40° C. When the temperature is below the critical compatibility temperature, the second color element418is colorless and transparent due to compatibility of the different polymer components or elements. When the temperature is above the critical compatibility temperature, the second color element418is nontransparent due to incompatibility between different polymer components. When the nontransparent second color element418cools down to between about 40° C. to about 10° C. from a temperature above the critical compatibility temperature, the nontransparent state persists, without any outside energy. When the second color element418, in a nontransparent state, is heated to a temperature which is below the critical compatibility temperature but above 40° C., the nontransparent second color element418color element418becomes transparent. Because the transparent and the nontransparent states can persist within certain temperature limits, without any outside energy, any color displayed by the thermochromatic device420remains. Thus performance as a bistable display can be achieved. In one embodiment, the material of the second color element418is a mixture of vinylidene fluoride hexafluoroacetone copolymer and low molecular weight poly methyl methacrylate with a mass ratio of about 1:3. The polymerization degree of the poly methyl methacrylate is about 60%.

The third color element418is a polymer material which is phase-changeable between crystalline and amorphous states. The working principle of the third color element418is described as follows. Because the light transmissivity of the crystalline third color element418and the light transmissivity of the amorphous third color element418are different, the color element418transforms between transparent state and nontransparent state when a heat write impulse or a heat erase impulse is supplied. The heat write impulse allows the thermochromatic device420to display colors. The heat erase impulse blanks the display. In one embodiment, the third color element418is a polymer (1,4-thiophenol-1,4-divinylbenzene). The amorphous polymer (1,4-thiophenol-1,4-divinylbenzene) is transparent with a light transmissivity of 91%. The crystalline polymer (1,4-thiophenol-1,4-divinylbenzene) is nontransparent with a light transmissivity of less than 1%. The color element418is made of polymer (1,4-thiophenol-1,4-divinylbenzene) with a thickness from about 0.1 micrometers to about 0.5 micrometers and changes from a nontransparent crystalline state to a transparent amorphous state in about 1 second to about 2 seconds at a temperature of about 170° C., and changes from a transparent amorphous state to a nontransparent crystalline state in about 20 minutes to about 30 minutes at a temperature from about 70° C. to about 80° C.

When the thermochromatic device420needs to display colors, a short and intensive heat write impulse voltage heats the color element418. Because the temperature caused by the heat write impulse voltage is very high, the color element418is instantly heated to a liquid state. Because the heating time of the heat write impulse is very short, the temperature of the color element418then decreases almost instantly and the liquid state color element418becomes a transparent amorphous solid. Thus, the thermochromatic device420reveals the color of the background color layer428. The third color element418remains in a transparent amorphous state, at a room temperature, without any outside energy. When the thermochromatic device420needs to blank the display after the thermochromatic device420has cooled to room temperature, a longer and weaker heat erase impulse is applied to heat the color element418. The process is an annealing process. After annealing, the third color element418of the color element418is changed to the original nontransparent crystalline state from a transparent amorphous state. Thus, the thermochromatic device420masks any color of the background color layer428, and the display is blanked. The third color element418remains in a nontransparent crystalline state, at room temperature, without any outside energy. Because of this, the color originally displayed by the thermochromatic device420remains. Thus, performance as a bistable display can again be achieved.

In one embodiment, the insulating substrate402is a PET film with a thickness of about 300 micrometers. The first electrode410and the second electrode412are formed by printing conductive slurry. The heating element408includes a single layer of drawn carbon nanotube film. The color element418is a layer of polymer (1,4-thiophenol-1,4-divinylbenzene) with a thickness from about 10 micrometers to about 400 micrometers. The color element418can be formed on the suspended portion of the drawn carbon nanotube film by spraying, printing, sputtering or thermal deposition process.

Referring toFIG. 9, a thermochromatic device520of a fourth embodiment includes an insulating substrate502, a color element518, a heating element508, a first electrode510, and a second electrode512. The color element518and the heating element508form a composite526. At least one part of the composite526is free of physical contact with any other element.

The thermochromatic device520is similar to the thermochromatic device220described above except that the composite526includes a carbon nanotube wire used as the heating element508and a plurality of thermochromatic material powders dispersed in the carbon nanotube wire and used as the color element518.

In one embodiment, the carbon nanotube wire includes a plurality of carbon nanotubes combined by van der Waals attractive force. The thermochromatic material powders are located on or between the plurality of carbon nanotubes. The composite526can be made by depositing the thermochromatic material on the carbon nanotube film, and then curling or twisting the carbon nanotube film with the thermochromatic material thereon to form the composite526.

As shown inFIG. 10, the carbon nanotube wire includes a plurality of carbon nanotubes helically oriented around an axial direction of the carbon nanotube wire and a plurality of thermochromatic material powders fixed to the carbon nanotube wire. Thus, the thermochromatic material powders of the color element518are firmly fixed to the carbon nanotube wire of the heating element508and are not easily removed or dislodged.

The disclosure further provides a thermochromatic display apparatus using the thermochromatic device described in above embodiments. The thermochromatic display apparatus includes a plurality of thermochromatic devices arranged to form a pixel matrix, a driving circuit capable of controlling the plurality of thermochromatic devices and a number of lead wires electrically connecting the driving circuit and the number of thermochromatic devices. The plurality of thermochromatic devices can use one common insulating substrate and be controlled by an addressing circuit. The thermochromatic display apparatus using the thermochromatic device220of the first embodiment is used below to illustrate the thermochromatic display apparatus of this embodiment of the disclosure.

Referring toFIGS. 11 and 12, a thermochromatic display apparatus20includes an insulating substrate202, a number of substantially parallel first electrode down-leads204, a number of substantially parallel second electrode down-leads206, and a number of thermochromatic devices220. The first and second electrode down-leads204,206are located on the insulating substrate202. The first electrode down-leads204are generally set at an angle to the second electrode down-leads206, forming a grid. A cell214is defined by each two substantially adjacent first electrode down-leads204and each two substantially adjacent second electrode down-leads206of the grid. One of the thermochromatic devices220is located in each cell214. Each thermochromatic device220corresponds to one pixel of the thermochromatic display apparatus20.

The insulating substrate202supports the first electrode down-leads204, the second electrode down-leads206, and the thermochromatic devices220. The shape, size, and thickness of the insulating substrate202can be chosen according to need. In one embodiment, the insulating substrate202is a square PET substrate with a thickness of 1 millimeter and an edge length of 48 millimeters. The plurality of thermochromatic devices220uses a common insulating substrate202.

The first electrode down-leads204are equidistantly apart. A distance between adjacent two first electrode down-leads204ranges from about 50 micrometers to about 2 centimeters. The second electrode down-leads206are equidistantly apart. A distance between adjacent two second electrode down-leads206ranges from about 50 micrometers to about 2 centimeters. A suitable orientation of the first electrode down-leads204and the second electrode down-leads206is that they are set at an angle with respect to each other. The angle of divergence ranges from about 10 degrees to about 90 degrees. In one embodiment, the angle is 90 degrees, and the cell214is square in shape.

The first electrode down-leads204and the second electrode down-leads206are made of conductive material such as metal or conductive slurry. In one embodiment, the first electrode down-leads204and the second electrode down-leads206are formed by applying conductive slurry on the insulating substrate202using a screen printing process. The conductive slurry is composed of metal powder, glass powder, and binder. The metal powder can be silver powder, the glass powder having low melting point, and the binder can be terpineol or ethyl cellulose (EC). The conductive slurry includes about 50% to about 90% (by weight) of the metal powder, about 2% to about 10% (by weight) of the glass powder, and about 8% to about 40% (by weight) of the binder. In one embodiment, each of the first electrode down-leads204and the second electrode down-leads206are formed with a width in a range from about 30 micrometers to about 100 micrometers and with a thickness in a range from about 10 micrometers to about 50 micrometers. However, the dimensions of each of the first electrode down-leads204and the second electrode down-leads206must vary to correspond with dimensions of each cell214.

The first electrodes210of the thermochromatic devices220are arranged in a row of the cells214and are electrically connected to the first electrode down-lead204. The second electrodes212of the thermochromatic devices220are arranged in a column of the cells214and are electrically connected to the second electrode down-lead206.

Each of the first electrodes210has a length in a range from about 20 micrometers to about 15 millimeters, a width in a range from about 30 micrometers to 10 millimeters and a thickness in a range from about 10 micrometers to about 500 micrometers. Each of the second electrodes212has a length in a range from about 20 micrometers to about 15 millimeters, a width in a range from about 30 micrometers to about 10 millimeters and a thickness in a range from about 10 micrometers to about 500 micrometers. In one embodiment, the first electrode210has a length in a range from about 100 micrometers to about 700 micrometers, a width in a range from about 50 micrometers to about 500 micrometers and a thickness in a range from about 20 micrometers to about 100 micrometers. The second electrode212has a length in a range from about 100 micrometers to about 700 micrometers, a width in a range from about 50 micrometers to about 500 micrometers and a thickness in a range from about 20 micrometers to about 100 micrometers.

The first electrodes210and the second electrode212can be made of metal or conductive slurry. In one embodiment, the first electrode210and the second electrode212are formed by screen printing the conductive slurry on the insulating substrate202. As mentioned above, the conductive slurry forming the first electrode210and the second electrode212is the same material as that used to form the electrode down-leads204,206.

The thermochromatic display apparatus20includes a plurality of insulators216sandwiched between the first electrode down-leads204and the second electrode down-leads206, to avoid short-circuiting. The insulators216are located at every intersection of the first electrode down-leads204and the second electrode down-leads206and provide electrical insulation. In one embodiment, the insulator216is a dielectric insulator.

In one embodiment, 16×16 (16 rows stacked one above another, and 16 thermochromatic devices220on each row) thermochromatic devices220are arranged on a square PET insulating substrate202with an edge length of 48 millimeters. Each heating element208is a single drawn carbon nanotube film with a length of 300 micrometers and a width of 100 micrometers. The single drawn carbon nanotube film is fixed on the surface of the insulating substrate202with an adhesive. The ends of the heating element208are located between the insulating substrate202and the electrodes210and212. The carbon nanotubes of the heating element208extend from the first electrode210to the second electrode212.

The thermochromatic display apparatus20includes a heat-resistant material222located around each thermochromatic device220. The heat-resistant material222can be in a space between the thermochromatic device220and the electrode down-leads204,206in the cell214. The thermochromatic devices220in adjacent cells214are heat-insulated and thus will not thermally interfere with each other. The heat-resistant material222can be aluminum oxide (Al2O3) or organic material such as PET, PC, PE, or PI. In one embodiment, the heat-resistant material222is PET with a thickness the same as the thickness of the electrode down-leads204,206. The heat-resistant material222can be formed by printing, chemical vapor deposition (CVD) or a physical vapor deposition (PVD) process. In one embodiment, the heat-resistant material222in each cell214extends around and is spaced from the thermochromatic device220so that the composite226is prevented from being in contact with the heat-resistant material222and the heating response speed of the thermochromatic device220is thus further improved.

The thermochromatic display apparatus20includes a protecting layer224located on the insulating substrate202to cover all the electrode down-leads204,206, and the thermochromatic devices220. The protecting layer224is an insulating and transparent layer that can be made of aluminum oxide (Al2O3), silicon dioxide (SiO2), or organic material such as PET, PC, PE, or PI. The thickness of the protecting layer224can be selected according to need. In one embodiment, the protecting layer224is a PET sheet with a thickness in a range from about 0.5 millimeter to about 2 millimeters. The protecting layer224prevents the thermochromatic display apparatus20from being damaged and polluted. In one embodiment, the protecting layer224is spaced from the thermochromatic device220so that the composite226is prevented from being in contact with the protecting layer224and the heating response speed of the thermochromatic device220is thus further improved. The protecting layer224and the insulating substrate202are sealed.

In use, the thermochromatic display apparatus20includes a driving circuit (not shown) to drive the thermochromatic display apparatus20to display an image. The driving circuit controls the thermochromatic devices220through the electrode down-leads204,206to display moving images. The color change speed of the pixel units of the thermochromatic display apparatus20is fast enough because at least one part of the composite226is free of physical contact with any other element and thus heat-isolated. The thermochromatic display apparatus20can be used in a field of advertisement billboards, newspapers, or electronic books.

It is to be understood that the above-described embodiments are intended to illustrate rather than limit the disclosure. Any elements described in accordance with any embodiments can be used in addition or as substitutes in other embodiments. Embodiments can also be used together. Variations may be made to the embodiments without departing from the spirit of the disclosure. The above-described embodiments illustrate the scope of the disclosure but do not restrict the scope of the disclosure.