The present invention relates to a temperature sensing device. In particular the invention relates to a negative temperature coefficient (NTC) thermistor based on printed nanocomposite films.
Thermistors, i.e. temperature sensitive resistors, are successfully used as temperature sensors relying on the large temperature dependence of the resistivity of the resistor. Traditionally, these devices are made of transition-metal oxide (MnO2, CoO, NiO, etc.) with the process of ceramic technology (sintering of powders at high temperature, 900° C.). With the resistivity decreasing by increasing temperature (negative temperature coefficient, NTC), NTC thermistors show a wide range of opportunities in industrial and consumer applications, such as compensation of thermal effects in electronic circuits and thermal management in high-power electronic systems.
It is therefore an object of the present invention to provide a temperature sensitive conductive thin film and a method of producing the same. This invention is about the fabrication of screen printable thermistor based on composite silicon-carbon nanoparticles (NPs).
Accordingly, the present invention, in one aspect, provides a conductive thin film comprising a binder and a composite of silicon crystals and carbon particles, wherein the carbon particles are in the range of 1%-10% by weight percentage of said composite.
In an exemplary embodiment, the carbon particles are in the range of 5%-10% by weight percentage of the Si—C composite.
In another exemplary embodiment, the respective size of the silicon crystal and carbon particle is in the range of 1 nanometer to 100 micrometers, or 80-300 nanometers, or 50-200 nanometers, 40-60 nanometers.
In a further exemplary embodiment, the silicon crystals are selected from doped silicon or nondoped silicon, and the carbon particles are selected from the group consisting of carbon blacks, graphite flakes and graphene nanoplatelets.
In a further exemplary embodiment, the film is useful for producing a negative temperature coefficient thermistor.
In another aspect, the present invention provides a negative temperature coefficient thermistor. This thermistor contains a substrate with a conductive thin film disposed thereon and, at least a pair of electrodes contacting said thin film for connections with external electronic circuits.
In yet another aspect, the present invention provides a method of producing a conductive thin film. This method comprises the steps of a) mixing carbon particles with silicon crystals to obtain a Si—C composite; b) mixing said Si—C composite with a binder and a thinner to obtain a temperature sensitive ink; c) printing said ink on a substrate to form said conductive thin film. In this method, the carbon particles are in the range of 1%-10% by weight percentage of the Si—C composite.
Compared to traditional NTC by metal oxide, Si—C nanocomposites NTC shows many advantages of low cost, full printability, low fabrication temperatures and higher sensitivity.
a) shows TEM images of Si NPs;
a) shows SEM image of screen printed Si—C nanocomposite film;
a) shows resistance versus temperature dependence for different carbon particles content;
As used herein and in the claims, “comprising” means including the following elements but not excluding others.
Carbon particles refer to the either amorphous or crystalline carbon particles.
Si NPs are single-crystal, non-doped, and about 70 nm size. In
About 1.3 g of commercial polymer binder, e.g. acrylic polymer binder was dissolved into 5.5 ml of ethylene glycol (EG). Then carbon NPs were added to silicon NPs so that 5 g Si—C nanocomposite powders contained 5% weight of carbon NPs. Eventually, the whole mixtures were homogenized in a planetary mixer (Thinky AR-100) for two minutes and a Si—C nanocomposite paste was obtained for screen printing. The temperature sensor is fabricated on flexible polyethylene terephthalate (PET) substrate. Two electrodes with distance of 1 mm were printed using DuPont 5064H silver conductor material and subsequently cured under ambient conditions. Afterwards, Si—C nanocomposite paste was printed with area of 15 mm×15 mm and made a continuous film covering above two Ag electrodes (as shown in
Under the scanning electron microscopy (SEM), Si—C nanocomposite films were highly dense and no pores were observed in
With different percentages of carbon particles in these Si—C nanocomposite films, it can be observed NTC thermistor properties with different resistivity of printed films. The resistance R of printed films was investigated in terms of the temperature dependence and is plotted in
In a third example, a fully printable NTC thermistor was produced according to the design in
In a fourth example, a fully printable NTC thermistor was produced, also according to the design in
In a fifth example, a fully printable NTC thermistor was produced, also according to the design in
In a sixth example, a printed structure was produced for Hall measurement, according to the design in
In a seventh example, a printed temperature sensor was integrated with active RFID modules, according to the schematic design in
In this invention, high-crystal-quality silicon NPs are mixed with highly conductive carbon NPs, and then an acrylic screen printing polymer binder is used to form Si—C nanocomposite paste. To meet the rheological requirements for screen printing, analytical grade ethylene glycol (EG) is used as a thinner. As a result, printed Si—C nanocomposite thermistors show very high temperature sensitivity close to intrinsic Si bulk material. And the resistance of these thermistors is reduced to 10-100 kΩ near room temperature, which is compulsory to integrate with low-cost readout circuits. This surprising phenomenon may benefit from high-crystal-quality Si NPs surrounded by highly conductive Carbon NPs. Electrons tended to tunnel from Si to C and then high conductivity of carbon materials enhanced electrical transport in printed Si—C nanocomposite films. The resulted resistivity of this Si—C nanocomposite film is smaller than 50 Ω-cm, which is much better than reported resistivity of Si NPs films, >10 kΩ-cm [Robert Lechner, et al, J. Appl. Phys. 104, 053701 (2008)].
The invention provides a method of forming an ink, the ink configured to form a highly conductive Si—C nanocomposite film. The method includes producing nanocomposites with Si NPs homogeneously mixed with carbon NPs. The method also includes formulate Si—C nanocomposites with acrylic polymer solutions resulting in a homogeneous Si NPs, C NPs and polymer blend. This means mixtures of Si/C NPs are homogeneously dispersed in polymer matrix and the rheology of these mixtures must meet requirements for screen printing inks.
Printed Si—C nanocomposite films in this invention show both high temperature sensitivity and high conductivity for mass production of NTC thermistors. Because the carbon nanoparticles are closely surrounding silicon, electrons can easily tunnel from silicon into carbon and carbon clusters enhance the hopping process in printed Si—C nanocomposite films. Not only can the method in this invention efficiently reduce the resistivity of printed Si NPs films, but also provide high temperature coefficients thermistors with quite high volume production and low cost in ambient environment.
The exemplary embodiments of the present invention are thus fully described. Although the description referred to particular embodiments, it will be clear to one skilled in the art that the present invention may be practiced with variation of these specific details. Hence this invention should not be construed as limited to the embodiments set forth herein.
For example, the binder may include, but not limited to acrylic polymer, epoxy, silicone (polyorganosiloxanes), polyurethanes, polyimides, silanes, germanes, carboxylates, thiolates, alkoxies, alkanes, alkenes, alkynes, diketonates, etc. The thinner is selected from the group consisting of ethylene glycol, polyethylene glycol, hydrocarbons, alcohols, ethers, organic acids, esters, aromatics, amines, as well as water, and mixtures thereof etc. It is conventional for a skilled person to select different types of thinners to serve as a solvent for different binders to meet rheological requirements.
The weight of Si—C composite may account for 50-90% in the paste, preferably 60-90%, more preferably 80-90%.
A substrate on which the ink is printed to form conductive thin film is conventional in the art. For example, substrate may include, but not limited to polyethylene terephthalate, paper, plastics, fabric, glass, ceramics, concretes, wood, etc.
A conductive thin film refers to the conductive film having a thickness of 100 nanometer to 100 micrometers, preferably 1-100 micrometers, more preferably 5-10 micrometers.
An electrode refers to any electrical conductor, including electrodes, metallic contacts, etc.
Carbon particles may have high electrical conductivity, preferably at least 100 S/cm.
For the printing of Si—C composites, some types of printing methods can be used, such as offset printing, flexography, gravure printing, and screen printing. In particular for screen printing, mesh numbers of printing screens can be in range of 100-500. The best reproducibility is obtained for screens with mesh no. 200-300.
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the invention, the preferred methods and materials are now described. All publications mentioned herein are incorporated herein by reference to describe and disclose specific information for which the reference was cited in connection with.
All references cited above and in the following description are incorporated by reference herein. The practice of the invention is exemplified in the following non-limiting examples. The scope of the invention is defined solely by the appended claims, which are in no way limited by the content or scope of the examples.
This application claims benefit under 35 U.S.C. §119(e) of U.S. Provisional Application having Ser. No. 61/967,124 filed on 11 Mar. 2014, which is hereby incorporated by reference herein in its entirety.
Number | Date | Country | |
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61967124 | Mar 2014 | US |