Fabrication of flexible conductive films, with semiconductive material, formed with rubbing-in technology for elastic or deformable devices

Abstract

Rubber composites with regions doped with conductive material, e.g., carbon nanotubes, and patterned regions doped with both conductive material and semiconductive material, e.g., carbon nanotubes and polycrystalline silicon are created with rubbing-in technology. The composites provide for a deformable and elastic composite which maintains semiconductor operations under stress, and can be used for filtering, determining compressive force, and a variety of other applications.

Claims

1. A deformable, semiconductive, composite, comprising: a rubber substrate, a first conductive material comprised of carbon nanotubes, a second conductive material which may be the same or different from the first conductive material, and a semiconductive material comprised of polycrystalline silicon, wherein each of the first conductive material, the second conductive material, and the semiconductive material are embedded in a surface of the rubber substrate, wherein the composite is configured to comprise a first portion which is a rubber substrate which is not doped with the first conductive material, the second conductive material, or the semiconductive material, a second portion which comprises rubber doped with the first conductive material, and one or more third portions which comprises rubber doped with the second conductive material and the semiconductive material, wherein the rubber substrate is stretchable and deformable.

2. The deformable, semiconductive composition of claim 1 wherein the second conductive material is comprised of carbon nanotubes.

3. The deformable, semiconductive composite of claim 1 having a current-voltage nonlinearity coefficient beta of 1.25 to 1.75, an impedance of 90 to 150 MΩ/cm at 30 to 70° C. at 1 kHz, and an impedance at 1 kHz which decreases by 0.03% per μm of compressive displacement compared to an uncompressed impedance at 1 kHz.

4. A compressive displacement sensor comprising the deformable, semiconductive, composite of claim 1.

Description

DESCRIPTION OF THE DRAWINGS

(1) The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawings will be provided by the Office upon request and payment of the necessary fee.

(2) FIG. 1A is a schematic drawing illustrating the rubbing-in technology to form rubber-nanomaterials composite conductive/semiconductive films.

(3) FIG. 1B is a schematic drawing of rubbing-in technology which can be used within the practice of the invention.

(4) FIG. 2 are schematic diagrams respectively illustrating a side view and top view of a sample showing a rubber substrate having a top surface doped with electrically conductive carbon nanotubes (CNT), and a patterned region doped with both electrically conductive carbon nanotubes (CNT) and semiconductive polycrystalline silicon (p-Si). The exemplary device illustrated in FIG. 2 was used for the investigations of impedimetric (impedance) properties of built-in rubber CNT and p-Si based deformable composite described herein.

(5) FIG. 3 are X-ray diffraction patterns of original rubber, CNTs powder, silicon powder and rubber-CNTs+silicon composite.

(6) FIG. 4 are graphs respectively showing a comparison of the dependencies of the impedances of the CNT-p-Si-rubber composite according to the present invention (top graph) and p-Si crystal (bottom graph) on temperature at different frequencies.

(7) FIG. 5 is a graph showing the dependences of the impedances of the CNT-p-Si-rubber composite sample according to the present invention on humidity at different frequencies.

(8) FIG. 6 are graphs respectively showing impedance-voltage (I-V) characteristics of the CNT-p-Si-rubber composite of the present invention (top graph) and p-Si crystal (bottom graph) at room temperature conditions.

(9) FIG. 7 is graph showing a comparison of the β (V) relationships for a CNT-p-Si-rubber composite of the present invention and p-Si crystal.

(10) FIG. 8 is a graph showing he effect of the compressive displacement to the resistance of a CNT-p-Si-rubber composite according to the invention.

(11) FIG. 9 is a graph showing impedance-frequency relationship of CNT-p-Si-rubber composite according to the invention at temperature of 23° C.

(12) FIG. 10 is a graph showing an exemplary schematic diagraph of an equivalent circuit of the CNT-p-Si-rubber composite according to the invention.

DETAILED DESCRIPTION

(13) FIG. 1A illustrates the rubbing-in technology used to manufacture composites according to the invention. In a preferred embodiment, a polished metallic surface which holds the rubber substrate in extended (stretched) position. The base surface can be a metallic plate, but could also be any other material that can securely hold the rubber substrate and can withstand rubbing operations such as plastic, ceramic, glass, and wood. The rubber substrate which will be used to make the composites of the invention can be any kind of stretchable rubber including silicone, styrene-butadiene rubber, natural rubber, butyl rubber, nitrile rubber, neoprene rubber, ethylene, and viton. In some applications, a hard rubber, such as styrene-butadiene is preferred. Once the composite is created, the composite is intended to be stretchable, compressible, and deformable. Depending on the needs of the fabricator, this may affect the choice of material for the rubber substrate and the thickness of the material. In exemplary embodiments, the substrate may be 5-1000 μm thick; however, the rubber substrate may be thicker or thinner depending on the application. In addition, for good embedding of the conductive material and semiconductive material in the surface of the rubber substrate, the rubber substrate should be stretchable, for example, stretchable by, for example, 5-100% its length and width. In most applications, the rubber substrate will be stretched (put under strain) such that increases its length and/or width by 10-30%.

(14) In the rubbing-in technology, the conducting powder or conducting/semiconducting powder placed on a surface of the rubber substrate is rubbed into the surface of the substrate using a pressing/sliding unit that applies a downward force or load against the surface of the rubber substrate, and which moves in any of multiple directions across the surface of the rubber substrate while applying the downward force. FIG. 1A shows arrows going in the lengthwise direction and widthwise direction of the rubber substrate; however, the pressing sliding unit can move in any direction, including circularly, etc. The goal of the rubbing-in operation is to cause the conductive powder and/or conducting/semiconducting powder to penetrate into the enlarged pores of the rubber. Pre-stretching of the rubber substrate prior to rubbing-in the conductive powder or conductive/semiconducting powder enlarges the pores of the rubber and makes the surface more receptive to embedding the conductive material and semiconductive material. The rubbing-in operation forms a composite layer with the rubber which can be used for device fabrication of a wide variety of devices, and particularly semiconductive devices according to the present invention. The rubber-conducting/semiconducting powder composite layer can be formed on both surfaces of the rubber substrate simply by, for example, turning it over and repeating the rubbing-in operations.

(15) FIG. 1B illustrates an exemplary rubbing-in technology where a rounded metallic load is used for rubbing-in the conductive powder or conductive/semiconducting powder. The load may vary considerably depending on the rubber substrate, the conductive powder, the semiconductive powder, and other factors and may range from 1 gf/cm.sup.2 to 100 gf/cm.sup.2 (preferably ranging from 10 to 75 gf/cm.sup.2). For the samples described and tested herein the load for different rubbing-in processes ranged from 10 gf/cm.sup.2 to 20 gf/cm.sup.2. While FIG. 1B depicts a metallic load, the load may be any other hardened materials such as plastics and ceramics. The rounded shape allows the load to move easily in any direction. The rubbing-in will typically be performed under computer control, and rubbing can take place over the entire surface of the rubber substrate as shown in FIG. 1B or only in selected regions of the substrate.

(16) The conductive powder rubbed into the surface of the rubber can be any of a variety of materials including carbon nanotubes (CNT), graphene, and carbon powder. CNTs, as are shown in FIG. 1B are particularly preferred. Good results can be achieved with CNTs having a mean diameter of 5 to 50 nm and a mean length of 75 to 250 nm.

(17) The semiconductive powder rubbed into the surface of the rubber can be any of a variety of materials including an inorganic semiconductor such as polycrystalline silicon (p-Si), germanium (Ge), and gallium arsenide. Good results can be achieved with p-Si in particulate form with particles having a mean particle size of 7.5 to 15 μm.

(18) FIG. 2 shows an example of a patterned composite according to the invention, and particularly, a CNT and p-Si based deformable composite. The deformable rubber substrate 1 includes CNT embedded in the rubber 2 at the top surface. Centrally, the deformable rubber substrate 1 includes both CNT and p-Si embedded in the rubber 3 at the top surface. Terminals 4 and 5 can be connected to the device. While FIG. 2 shows a single region with semiconductor material therein, it should be understood that many different regions can be patterned to include the semiconductor material, e.g., a plurality of stripes between the terminals 4 and 5, a plurality of separated cylinders, etc. The patterning with semiconductor material will vary depending on the needs of the fabricator. FIG. 2 also shows, as an example, compressive forces 6 being exerted on the ends of the patterned composite. This might occur in the case of the composite of the invention being used in, for example, a compressive displacement sensor. However, it should also be recognized that the composite is designed to be stretched, deformed (e.g., twisted, bent over other devices), etc., without appreciable degradation in electrical performance.

(19) The patterned composite of FIG. 2 can be made by first rubbing-in a conductive powder (e.g., CNTs having a mean diameter of 5 to 50 nm and a mean length of 75 to 250 nm) over all or a part of a surface of a rubber substrate while the rubber substrate is under stress (i.e., stretched). Then, one or more portions of the region embedded with the conductive powder are removed to expose one or more portions of bare rubber. In FIG. 2, this can be accomplished by cutting away, or otherwise excising the embedded CNT powder and rubber in a central stripe across the surface of the rubber substrate. However, as discussed above, the pattern can vary considerably depending on the needs of the fabricator, and one or a plurality of portions of the embedded region at the surface of the rubber can be removed so as to expose one or more regions of the rubber substrate where the one or more portions were removed. Then, a powder of semiconductive material and conductive material and be rubbed into the surface of the rubber substrate at the exposed region. Preferably, the powder will have CNTs having a mean diameter of 5 to 50 nm and a mean length of 75 to 250 nm and p-Si in particulate form with particles having a mean particle size of 7.5 to 15 μm. The mixture of conductive material and semiconductive material assures that the semiconductor operates well with the doped conductive regions of the rubber substrate. Removal of the portions of the conductive material rubbed into the surface of the rubber substrate prior to rubbing in the inorganic semiconductor and conductive material, assures that the inorganic semiconductor is embedded in the surface of the rubber. The rubbing-in can be performed under the same or different conditions as was used for rubbing-in the conductive material.

(20) To demonstrate the utility of the invention a plurality of samples similar to those shown in FIG. 2 were constructed and tested under various conditions, and compared to, in some instances, the performance of p-Si.

(21) The deformable composite samples based on CNT and p-Si built-in rubber were fabricated using rubbing-in technology. The effect of humidity up to 90% RH, temperature up to 60° C. and frequency of applied voltage in the interval of 1-200 kHz on the impedance of the samples were investigated

(22) A p-Si wafer and CNTs (carbon nanotubes) powder were purchased from Sun Nanotek Co. LTD. The diameter of the multiwalled carbon nanotubes was 10 to 30 nm, while their length was in the range of 100 to 200 nm. The silicon power was prepared from by grinding the p-Si crystalline wafer with following specification: sizes 6-inch (15.6 cm×15.6 cm), p-type (Boron doped), thickness: 180 μm, resistivity: 1.8 ohm/cm, doping concentration: 8e.sup.15, textured both sides (random pyramid) and crystal orientation: <100>. The particle size of the p-Si powder was mostly in the range of 9 μm to 12 μm.

(23) Rubbing-in technology as described above was used to fabricate the rubber-CNT and rubber-CNT-p-Si conductive films. A polished metallic surface which holds the rubber substrate in an extended (stretched) position was prepared. To form the composite the conducting/semiconducting powder is poured on to the stretched rubber substrate. A round shaped polished metallic block is used to rub the powder on the rubber surface using a special mechanism which controls the frequency and direction of the block. By this process the powder penetrates into the enlarged pores of the rubber and form a composite layer with rubber, which can be used for device fabrication. The rubber-conducting/semiconducting power composite layer can be formed on both surfaces of the rubber substrate.

(24) Samples of various dimensions (length: 2 cm to 3 cm, width: 1.5 cm to 2 cm, thickness: 1 mm to 2 mm can be prepared on the fabricated set-up. The samples are strained normally about 15 to 20%. Depending upon the sample's requirements, a range of loads (10 gf/cm.sup.2 to 20 gf/cm.sup.2) may be applied using various rubbing blocks. The frequency of the rubbing blocks varies from 10 to 20 Hz. The duration of the sample preparation was 15-20 s on average.

(25) Exemplary deformable composite samples based on CNT and p-Si built-in rubber were fabricated. A 2 cm×3 cm rubber sheet of thickness of 1-2 mm was stretched up to 15-20% in two directions: length and width of the sample. While holding the sample in the strained position, carbon nanotubes (CNTs) powder was applied using the above-mentioned rubbing-in technology. The depth of the formed rubber-CNTs composite layer was 8-12 μm.

(26) In the middle of the sample, a gap of 2 cm in length and 0.5 cm in width was made by removing rubber-CNT layer down to the pure rubber surface. This gap was filled by the mixture of CNT-p-Si powders (50/50 wt. %) (this ratio can be varied considerably, e.g., from 80/20 wt % to 20/80 wt %). To incorporate the CNT-p-Si mixture in the rubber substrate, the same procedure (rubbing-in) was applied. All samples were fabricated at pressure of 40 g/cm.sup.2. Figure shows the schematic diagram of fabricated sample. In the prepared samples the rubber-CNTs-p-Si composite played the role of the sensitive material.

(27) A digital multi-meter DT-4253-Hioki was used for the measurement of current and voltages (I-V) characteristics. All experiments were conducted at room temperature conditions. The uniaxial compressive displacements or pressure were applied along the length of the rectangular prism sample by use of the micrometer mechanism or weights in special experimental arrangement. Processes of application of compressive forces on the rubber sample are shown in FIG. 2.

(28) X-ray diffraction of the samples were conducted with Philips PW1830 X-ray diffraction system in Bragg-Brentano (θ-2θ) scan mode using Cu-Kα radiation source at room temperature. Each sample was run three times and found repeatable results. The X-ray diffractograms of original rubber, CNTs powder, silicon powder and rubber-CNTs+silicon composite, are presented in FIG. 3.

(29) The XRD pattern of rubber shows notable high intensity Bragg's diffraction peaks at 2θ of 23.1°, 29.4° and 36°, respectively. These peaks and other peaks present in the rubber X-ray diffraction pattern are characteristics of polyvinyl chloride which correspond to high amount of structural arrangement (order) in its polymeric chain. In CNTs powder diffractogram, the broad and major peak of CNTs was observed around 26.2° (002) which is attributed to the hexagonal graphite structure, having high electrical conductivities. This peak is consistent with standard x-ray diffraction data (ICSD code: 031170). One additional peak is also observed at 38.4° as a result of a different functional group attached to the CNTs. A broad peak of CNTs consistent with other studies (see Lu Scientific Reports 5, 16584 (2015)10.1038/srep16584) was also observed at 2θ of 44.3° (101). A sharp peak of p-type silicon powder was observed at 2θ=68.9°. In rubber-CNTs+silicon composite diffractogram, the additional peak of CNTs and silicon powder was observed around 2θ of 26.2° and 68.9°.

(30) FIG. 4, in the top graph, shows the dependences of the impedances of the CNT-p-Si-rubber composite sample at different frequencies on the temperature. From the top graph of FIG. 4, it is seen that, from a practical perspective, impedances of the sample at 1 kHz, 10 kHz, 100 kHz and 200 kHz don't depend on temperature.

(31) FIG. 4 in the bottom graph, shows dependences of the impedances of the p-Si crystal sample at different frequencies on the temperature. It is seen that with the impedances a few decrease with increases in temperature and after that the impedances increase with increases in temperature. It was calculated that on average the impedances due to increases of the temperature from 22° C. to 64° C. increased 8% only in the case of p-Si crystal sample.

(32) Comparison of the results in the top and bottom graphs of FIG. 4 demonstrates that unlike the case of p-silicon crystal, formation of a CNT, p-Si and rubber composite according to the invention allows for fabrication of electronic material which can be potentially used for making of the resistors and other devices.

(33) FIG. 5 shows the dependences of the impedances of the CNT-p-Si-rubber composite sample at different frequencies based on the humidity. It was found that with an increase of the humidity from 47% to 90%, the impedances decreased on average only on 5%.

(34) FIG. 6, in the top graph, shows I-V characteristics of the CNT-p-Si-rubber composite sample at room temperature conditions. FIG. 6 in the bottom graph shows I-V characteristics of the p-Si crystal sample. It can be seen that I-V characteristics of CNT-p-Si-rubber composite sample and p-Si crystal sample are non-linear.

(35) The non-linearity coefficient (a) can be calculated by the following expression (see Khan World Applied Sciences Journal (2016) 24:1261-1268):
α=(log(I.sub.2/I.sub.1)/log(V.sub.2/V.sub.1)  (1)
It can be determined also by the following expression
β=dI V.sub.o/I.sub.odV  (2)
FIG. 7 shows β (V) relationships accordingly for the CNT-p-Si-rubber composite sample and the p-Si crystal sample. It is seen that CNT-p-Si-rubber composite sample has a non-linearity coefficient (β) that is in the range of (1.35-1.63) and is lower than of Si crystal sample by approximately 1.5 times. It can be considered that non-linearity of the I-V characteristics are, firstly, due to decrease of the resistance of the sample on heating by current passing through the sample and, secondly, due to the composition of the samples.

(36) The effect of the compressive displacement to the resistance was determined (See FIG. 8). Resistance decreased 1.26 times at increases of the displacement up to 100 μm.

(37) Sensitivity (S) of the displacement sensor can be found by the following expression:
S=dZ 100%/ZdD  (3)
where Z is impedance and D is displacement. It was found that S=0.03%/μm at frequency of 1 kHz. The deformable composite can be used for fabrication of displacement sensor or flexible resistor and impedance. FIG. 9 shows frequency dependences of the impedances at temperature of 23° C. The impedances are very much dependent of frequencies, and these are like the properties of the electronic filters (see Khan, ibid).

(38) By use of this composite sample in electronic circuits, in principle, it is possible to fabricate low or high pass filters. The impedance of the sample can be represented as parallel connection of the resistor and capacitor (FIG. 10), the contribution of the capacitance to the impedance is much with respect of resistance. The impedance (Z) of the circuit shown in FIG. 10 and is defined as:
Z=R/(1+jωRC)  (4)

(39) In summary, rubbing-in technology was used to fabricate the rubber-CNT-p-Si nanocomposite-based flexible conductive films. The effect of humidity up to 90%, temperature up to 60° C., frequency of applied voltage in the interval of (1-200) kHz, on the impedance of the samples were investigated. It was also revealed that the impedance of the rubber-CNTs-p-Si composite at 1 kHz was practically independent of temperature. At 10 kHz, 100 kHz and 200 kHz impedances of the composites decreased up to 9% when temperature increased from 23° C. up to 60 C. The temperature dependences of impedances of the p-Si crystal strips in average increased on 8-9% with the increase of the temperature and showed minimum at 25° C. As relative humidity was changed in the interval of (47-90) % the impedances of the composites decreased in the average on 5% of the values. On increasing displacement (compressive) up to 300 μm the impedance decreases from 4% to 9% depending on the frequencies: at lower frequencies the changes were higher than in higher frequencies. The impedances of the sample decreased sharply with increase of the frequency that potentially can be used for fabrication of the filters. The I-V characteristics of the CNT and p-Si composite samples and p-Si crystal samples as well were non-linear. The non-linearity coefficients of the I-V characteristics were equal to 3.1 and 2.5 for composite and p-Si crystal samples accordingly. The elastic composite based on rubber, CNT and p-Si can be used as a compressive displacement sensor and for the demodulation of the frequency-modulated signals. Testing showed the semiconductive composite had a current-voltage nonlinearity coefficient beta of 1.25 to 1.75, an impedance of 90 to 150 MΩ/cm at 30 to 70° C. at 1 kHz, and an impedance at 1 kHz which decreases by 0.03% per μm of compressive displacement compared to an uncompressed impedance at 1 kHz.