Patent Publication Number: US-9843869-B2

Title: Thermoacoustic device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a division application of U.S. patent application Ser. No. 14/609,600, filed on Jan. 30, 2015, entitled “THERMOACOUSTIC DEVICE AND METHOD FOR MAKING THE SAME,” which all benefits accruing under 35 U.S.C. §119 from China Patent Application No. 201410346736.4, filed on Jul. 21, 2014 in the China Intellectual Property Office, the contents of which are hereby incorporated by reference. 
    
    
     FIELD 
     The subject matter herein generally relates to thermoacoustic devices and methods for making the same. 
     BACKGROUND 
     Thermoacoustic device is based on thermoacoustic effect having a conversion of heat into acoustic signals and distinct from the mechanism of conventional loudspeakers, in which the pressure waves are created by the mechanical movement of the diaphragm. When signals are supplied to a thermoacoustic element of the device, heat is produced in the thermoacoustic element according to the variations of the signal and/or signal strength. The heat propagates into surrounding medium. The heating of the medium causes thermal expansion and produces pressure waves in the surrounding medium, resulting in sound wave generation. Such an acoustic effect induced by temperature waves is commonly called “the thermoacoustic effect”. Xiao et al. discloses an thermoacoustic device with simpler structure and smaller size, working without the magnet in an article of “Flexible, Stretchable, Transparent Carbon Nanotube Thin Film Loudspeakers”, Xiao et al., Nano Letters, Vol. 8 (12), 4539-4545 (2008). The thermoacoustic device has a carbon nanotube film as the thermoacoustic element. The carbon nanotube film used in the thermoacoustic device has a large specific surface area, and extremely small heat capacity per unit area that make the sound wave generator emit sound audible to humans. Accordingly, the thermoacoustic device adopted the carbon nanotube film has a potential to be actually used instead of the loudspeakers in prior art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       Implementations of the present technology will now be described, by way of example only, with reference to the attached figures, wherein: 
         FIG. 1  is a schematic side view of an embodiment of a thermoacoustic device. 
         FIG. 2  shows a scanning electron microscope (SEM) image of a carbon nanotube film drawn from the carbon nanotube array. 
         FIG. 3  shows a schematic structure view of one embodiment of carbon nanotubes joined end-to-end. 
         FIG. 4  is a flow chart of an embodiment of a method for making the thermoacoustic device. 
         FIG. 5  is a schematic side view of an embodiment of the method for making the thermoacoustic device. 
         FIG. 6  is a schematic top view of an embodiment of the method for making the thermoacoustic device. 
         FIG. 7  is a schematic side view of an embodiment of a method for transferring the carbon nanotube array. 
         FIG. 8  is a schematic structural view of another embodiment of the method for transferring the carbon nanotube array. 
         FIG. 9  is a schematic structural view of yet another embodiment of the method for transferring the carbon nanotube array. 
     
    
    
     DETAILED DESCRIPTION 
     The disclosure is illustrated by way of example and not by way of limitation in the figures of the accompanying drawings in which like references indicate similar elements. It should be noted that references to “another,” “an,” or “one” embodiment in this disclosure are not necessarily to the same embodiment, and such references mean “at least one.” 
     It will be appreciated that for simplicity and clarity of illustration, where appropriate, reference numerals have been repeated among the different figures to indicate corresponding or analogous elements. In addition, numerous specific details are set forth in order to provide a thorough understanding of the embodiments described herein. However, it will be understood by those of ordinary skill in the art that the embodiments described herein can be practiced without these specific details. In other instances, methods, procedures and components have not been described in detail so as not to obscure the related relevant feature being described. Also, the description is not to be considered as limiting the scope of the embodiments described herein. The drawings are not necessarily to scale and the proportions of certain parts have been exaggerated to better illustrate details and features of the present disclosure. 
     Several definitions that apply throughout this disclosure will now be presented. 
     The term “contact” is defined as a direct and physical contact. The term “substantially” is defined to be essentially conforming to the particular dimension, shape, or other description that is described, such that the component need not be exactly conforming to the description. The term “comprising,” when utilized, means “including, but not necessarily limited to”; it specifically indicates open-ended inclusion or membership in the so-described combination, group, series, and the like. 
     Referring to  FIG. 1 , the present disclosure is described in relation to a thermoacoustic device  100 . The thermoacoustic device  100  comprises a substrate  30  (e.g., a base), a first electrode  90 , a second electrode  92 , at least two supporting members  80 , and a first carbon nanotube film  40 . The first electrode  90  and the second electrode  92  are located on a same surface  302  of the substrate  30  and spaced from each other. The at least two supporting members  80  are spaced from each other and respectively located on surfaces of the first electrode  90  and the second electrode  92 . The first carbon nanotube film  40  are located on surfaces of the at least two supporting members  80  and supported by the at least two supporting members  80 . A portion of the first carbon nanotube film  40  between the two supporting members  80  are suspended above the substrate  30 . The supporting members  80  are electrically conductive and electrically connecting the first carbon nanotube film  40  with the first and second electrodes  90 ,  92 . The supporting members  80  comprise a plurality of carbon nanotubes that are wall by wall and parallel with each other. The plurality of carbon nanotubes are substantially perpendicular to the surface  302  of the substrate  30 . 
     The material of the substrate  30  can be at least one of soft, elastic, and rigid solid substrate, such as metal, glass, crystal, ceramic, silicon, silicon dioxide, plastic, and resin, such as polymethyl methacrylate, poly(dimethylsiloxane) (PDMS) and polyethylene terephthalate. In one embodiment, the substrate  30  is electrically insulating. In another embodiment, an insulating layer is located between the substrate  30  and the first and second electrodes  90 ,  92  to electrically insulate the first and second electrodes  90 ,  92  from the substrate  30 . 
     The first and second electrodes  90 ,  92  are made of conducting material, such as metal, conducting polymer, conducting binder, metallic carbon nanotubes, and tin indium oxide (ITO). The first and second electrodes  90 ,  92  are respectively connected with the first carbon nanotube film  40  to input electrical signals to the carbon nanotube film  40 . Shapes of the first and second electrodes  90 ,  92  are not limited. In one embodiment, the first and second electrodes  90 ,  92  are strip shaped layers spaced and parallel to each other. The lengths of the first and second electrodes  90 ,  92  can be larger than or equal to the width of the carbon nanotube film  40 . The thicknesses of the first and second electrodes  90 ,  92  can be in a range from about 1 micron to about 1 millimeter. The widths of the first and second electrodes  90 ,  92  can be in a range from about 5 microns to about 1 millimeter. 
     The thermoacoustic device  100  can comprise a plurality of first electrodes  90  and a plurality of second electrodes  92  alternatively arranged and spaced from each other. One first electrode  90  is located between each two adjacent second electrodes  92 , and one second electrode  92  is located between each two adjacent first electrodes  90 . 
     The at least two supporting members  80  have shapes substantially according to the shapes of the first and second electrodes  90 ,  92 . In one embodiment, the supporting members  80  are strip shaped members spaced and parallel to each other. The lengths of the supporting members  80  can be larger than or equal to the width of the carbon nanotube film  40 . The heights of the supporting members  80  can be in a range from about 10 microns to about 5 millimeters. The supporting members  80  are formed by patterning a carbon nanotube array and comprise a plurality of carbon nanotubes combined with van der Waals attractive forces. The heights of the supporting members  80  are the height of the carbon nanotube array (i.e., the lengths of the carbon nanotubes in the carbon nanotube array). The widths of the supporting members  80  can be as small as several microns, such as in a range from about 5 microns to about 1 millimeter. Due to the excellent conductivity of the carbon nanotubes that are substantially perpendicular to the surfaces of the carbon nanotube film  40  and the first and second electrodes  90 ,  92 , an excellent electrical connection between the carbon nanotube film  40  and the first and second electrodes  90 ,  92  can be formed through the supporting members  80 . An amount of the supporting members  80  is the total amount of the first and second electrodes  90 ,  92 . Each of the first and second electrodes  90 ,  92  has a supporting member  80  located thereon. Thus, the functions of the supporting members  80  are suspending the first carbon nanotube film  40  and conducting every first and second electrodes  90 ,  92  with the first carbon nanotube film  40 . The supporting member  80  that is in contact with the first electrode  90  is not in contact with the second electrode  92  or the supporting member  80  that is in contact with the second electrode  92 , but spaced from the second electrode  92  or the supporting member  80  that is in contact with the second electrode  92 . The first carbon nanotube film  40  cannot be short circuited between the first electrode  90  and the second electrode  92 . 
     The first carbon nanotube film  40  comprises a plurality of carbon nanotubes joined end to end and is a macroscopic assembly of carbon nanotubes. The first carbon nanotube film  40  is free-standing, located on surfaces of the supporting members  80 , and supported by the supporting members  80 . The first carbon nanotube film  40  located between the two adjacent supporting members  80  is suspended. In use, the electrical signals are conducted from the first electrode  90  to the first carbon nanotube film  40  through one supporting member  80  and then conducted from the first carbon nanotube film  40  through another supporting member  80  to the second electrode  92 . The carbon nanotubes in the first carbon nanotube film  40  are substantially parallel to the surface of the first carbon nanotube film  40  and substantially aligned along the same direction. The width direction of the first carbon nanotube film  40  is substantially perpendicular to the aligned direction of the carbon nanotubes. In one embodiment, the carbon nanotubes are substantially aligned along a direction from the first electrode  90  to the second electrode  92 . 
     The first carbon nanotube film  40  is a thermoacoustic element that is capable of converting the electrical signals into heat signals to produce sounds by heating surrounding medium, such as ambient air. The first carbon nanotube film  40  has a very small heat capacity per unit area (e.g., less than 2×10 −4  J/cm 2 *K) to rapidly increase and decrease temperature thereof with the frequency of the electrical signals. The first carbon nanotube film  40  has a small thickness (e.g., from about 0.5 nanometers to about 500 microns) and a large specific surface area (e.g., above 30 m 2 /g), to propagate heat into surrounding medium, heats the surrounding medium at the frequency. The heating of the surrounding medium causes thermal expansion and produces pressure waves in the surrounding medium, resulting in sound wave generation. 
     The first carbon nanotube film  40  can be drawn from a carbon nanotube array that is capable of having a film drawn therefrom and comprises or consists a plurality of successive and aligned carbon nanotubes joined end-to-end by van der Waals attractive force therebetween. Referring to  FIG. 2  and  FIG. 3 , the overall aligned direction of a majority of the carbon nanotubes is substantially aligned along the same direction parallel to a surface of the first carbon nanotube film  40 . A majority of the carbon nanotubes are substantially aligned along the same direction in the first carbon nanotube film  40 . Along the aligned direction of the majority of carbon nanotubes, each carbon nanotube is joined to adjacent carbon nanotubes end to end by van der Waals attractive force therebetween, whereby the first carbon nanotube film  40  is capable of being free-standing structure. There may be a minority of carbon nanotubes in the first carbon nanotube film  40  that are randomly aligned. However, the number of the randomly aligned carbon nanotubes is very small, in comparison, and does not affect the overall oriented alignment of the majority of carbon nanotubes in the first carbon nanotube film  40 . Some of the majority of the carbon nanotubes in the carbon nanotube film that are substantially aligned along the same direction may not be exactly straight, and can be curved at a certain degree, or not exactly aligned along the overall aligned direction by a certain degree. Therefore, partial contacts can exist between the juxtaposed carbon nanotubes in the majority of the carbon nanotubes aligned along the same direction in the first carbon nanotube film  40 . The first carbon nanotube film  40  can comprise a plurality of successive and oriented carbon nanotube segments. The plurality of carbon nanotube segments are joined end to end by van der Waals attractive force. Each carbon nanotube segment comprises a plurality of carbon nanotubes substantially parallel to each other, and the plurality of paralleled carbon nanotubes are in contact with each other and combined by van der Waals attractive force therebetween. The carbon nanotube segment has a desired length, thickness, uniformity, and shape. There can be clearances between adjacent and juxtaposed carbon nanotubes in the first carbon nanotube film  40 . In one embodiment, the first carbon nanotube film  40  has a specific surface area ranged from 200 m 2 /g to 2600 m 2 /g. The specific surface area of the first carbon nanotube film  40  is tested by a Brunauer-Emmet-Teller (BET) method. In one embodiment, the first carbon nanotube film  40  has a specific weight of about 0.05 g/m 2 . The first carbon nanotube film  40  is a free-standing structure. The term “free-standing” comprises, but is not limited to, a structure that does not have to be supported by a substrate and can sustain the weight of it when it is hoisted by a portion thereof without any significant damage to its structural integrity. The suspended part of the first carbon nanotube film  40  will have more sufficient contact with the surrounding medium (e.g., air) to have heat exchange with the surrounding medium from both sides of the first carbon nanotube film  40 . 
     Referring to  FIG. 4  to  FIG. 6 , the present disclosure is described in relation to a method for making the thermoacoustic device  100 . 
     In block S 1 , the substrate  30  of the thermoacoustic device  100  is provided. The substrate  30  has a surface  302 . 
     In block S 2 , the first and second electrodes  90 ,  92  are formed on the surface  302  of the substrate  30 . The first and second electrodes  90 ,  92  can be formed by coating, printing, depositing, etching, or plating method. 
     In block S 3 , a carbon nanotube array  10  is transferred onto the substrate  30  and covers the first electrode  90  and the second electrode  92 . The carbon nanotube array  10  has a second surface  104  adjacent to the substrate  30  and a first surface  102  away from the substrate  30 . The carbon nanotube array  10  has an ability to have a second carbon nanotube film  42  drawn therefrom. The second carbon nanotube film  42  comprises a plurality of carbon nanotubes joined end to end. 
     In block S 4 , the first surface  102  of the carbon nanotube array  10  is laser etched to divide the carbon nanotube array  10  into two areas which are a preserving area  12  and a removing area  14 . The preserving area  12  is the area of the carbon nanotube array  10  that covers the first electrode  90  and the second electrode  92 . The removing area  14  is the area the carbon nanotube array  10  other than that covers the first electrode  90  and the second electrode  92 . 
     In block S 5 , a second carbon nanotube film  42  is drawn from the removing area  14 , thus removing the carbon nanotubes in the removing area  14  and leaving carbon nanotubes in the preserving area to form the supporting members  80  on the first and second electrodes  90 ,  92 . 
     In block S 6 , the first carbon nanotube film  40  is placed on the supporting members  80  and suspended between the supporting members  80 . 
     The second carbon nanotube film  42  can be a free-standing structure including a plurality of carbon nanotubes joined end-to-end by van der Waals attractive force therebetween. The second carbon nanotube film  42  and the first carbon nanotube film  40  can have a same structure and can be drawn from different carbon nanotube arrays. 
     Transferring of Carbon Nanotube Array 
     Referring to  FIG. 7 , in block S 3 , the carbon nanotube array  10  is originally grown/formed on a growing substrate  20  and is transferred to the substrate  30 . 
     First, the growing substrate  20 , having the carbon nanotube array  10  grown thereon is provided. The first surface  102  of the carbon nanotube array  10  is on the growing substrate  20 . The second surface  104  of the carbon nanotube array  10  is away from the growing substrate  20 . The carbon nanotube array  10  is grown to have a state/shape/form that is capable of having a second carbon nanotube film  42  drawn therefrom. The carbon nanotube array  10  is transferred from growing substrate  20  to the substrate  30  and the state/shape/form of the carbon nanotube array  10 , before, during, and after the transfer onto the substrate  30 , is still capable of having the second carbon nanotube film  42  drawn therefrom. 
     The carbon nanotube array  10  is grown on the growing substrate  20  by a chemical vapor deposition (CVD) method. The carbon nanotube array  10  comprises a plurality of carbon nanotubes oriented substantially perpendicular to a growing surface of the growing substrate  20 . The carbon nanotubes in the carbon nanotube array  10  are closely bonded together side-by-side by van der Waals attractive forces. By controlling growing conditions, the carbon nanotube array  10  can be essentially free of impurities such as carbonaceous or residual catalyst particles. Accordingly, the carbon nanotubes in the carbon nanotube array  10  are closely contacting each other, and a relatively large van der Waals attractive force exists between adjacent carbon nanotubes. The van der Waals attractive force is so large that when drawing a carbon nanotube segment (e.g., a few carbon nanotubes arranged side-by-side), adjacent carbon nanotube segments can be drawn out end-to-end from the carbon nanotube array  10  due to the van der Waals attractive forces between the carbon nanotubes. The carbon nanotubes are continuously drawn to form a free-standing and macroscopic second carbon nanotube film  42 . The carbon nanotube array  10 , that can have the second carbon nanotube film  42  drawn therefrom, can be a super aligned carbon nanotube array. A material of the growing substrate  20  can be P-type silicon, N-type silicon, or other materials that are suitable for growing the super aligned carbon nanotube array. 
     In the present disclosure, the growing of the carbon nanotube array  10  and the drawing of the second carbon nanotube film  42  are processed on different structures (i.e., the growing substrate  20  and the substrate  30 ). The substrate  30  for drawing the second carbon nanotube film  42  can be made of low-price materials, and the growing substrate  20  can be recycled quickly. Thus, production of the second carbon nanotube film  42  can be optimized. 
     The substrate  30  has the surface  302  to accept the carbon nanotube array  10  thereon. The surface  302  of the substrate  30  can be flat when the carbon nanotube array  10  is grown on a flat growing surface  202  of the growing substrate  20 . During transferring of the carbon nanotube array  10  from the growing substrate  20  to the substrate  30 , the state of the carbon nanotube array  10  is still capable of drawing the second carbon nanotube film  42  from the carbon nanotube array  10  on the substrate  30 . The carbon nanotube array  10  transferred to the substrate  30  is still a super aligned carbon nanotube array. The carbon nanotubes of the carbon nanotube array  10  are substantially perpendicular to the surface of the substrate  30 . 
     The carbon nanotube array  10  is arranged upside down on the surface  302  of the substrate  30 . The carbon nanotubes are grown from the growing surface  202  of the growing substrate  20  to form the carbon nanotube array  10 . The carbon nanotube comprises a bottom end adjacent or contacting the growing substrate  20  and a top end away from the growing substrate  20 . The bottom ends of the carbon nanotubes form the first surface  102  of the carbon nanotube array  10 , and the top ends of the carbon nanotubes form the second surface  104  of the carbon nanotube array  10 . After the carbon nanotube array  10  is transferred to the substrate  30 , the second surface  104  of the carbon nanotube array  10  is now adjacent to or contacting the substrate  30 , and the first surface  102  of the carbon nanotube array  10  is now away from the substrate  30 . 
     In one embodiment, the carbon nanotube array  10  is transferred by:
         contacting the surface  302  of the substrate  30  to the second surface  104  of the carbon nanotube array  10 ; and   separating the substrate  30  from the growing substrate  20 , thereby separating the first surface  102  of the carbon nanotube array  10  from the growing substrate  20  to transfer the carbon nanotube array  10  from the growing substrate  20  to the substrate  30 .       

     The carbon nanotube array  10  can be transferred from the growing substrate  20  to the substrate  30  at room temperature (e.g., 10° C. to 40° C.). 
     The surface  302  of the substrate  30  and the second surface  104  of the carbon nanotube array  10  can be bonded only by van der Waals attractive forces, and a bonding force (F BC ) between the carbon nanotube array  10  and the substrate  30  is smaller than the van der Waals attractive forces (F CC ) between the carbon nanotubes in the carbon nanotube array  10 . Meanwhile, the bonding force F BC  is larger than the bonding force (F AC ) between the carbon nanotube array  10  and the growing substrate  20 , to separate the carbon nanotube array  10  from the growing substrate  20 . Therefore, F AC &lt;F BC &lt;F CC  must be satisfied. 
     To satisfy F AC &lt;F BC &lt;F CC , the substrate  30  can have a suitable surface energy and a suitable interface energy can exist between the substrate  30  and the carbon nanotube array  10 . Thus, the substrate  30  can generate enough bonding force (e.g., van der Waals attractive force) with the carbon nanotube array  10  simply by contacting the carbon nanotube array  10 . A suitable material of the substrate  30  must have a sufficient bonding force F BC  (e.g., van der Waals attractive force) with the second surface  104  of the carbon nanotube array  10  to overcome the bonding force F AC  between the carbon nanotube array  10  from the growing substrate  20 . The surface  302  of the substrate  30  can be substantially flat. In one embodiment, the material of the substrate  30  is poly(dimethylsiloxane) (PDMS). 
     The substrate  30  can adhere to the carbon nanotube array  10  without another substance (e.g., an adhesive binder) and only by van der Waals attractive forces. Although the adhesive binder can have a bonding force with the carbon nanotube array  10  greater than the bonding force between the carbon nanotube array  10  and the growing substrate  20 , because the van der Waals attractive force between the carbon nanotubes in the carbon nanotube array  10  is small, the bonding force provided by the adhesive binder may be too great (i.e., greater than the bonding force F CC  between the carbon nanotubes in the carbon nanotube array  10 ). In this situation, the second carbon nanotube film  42  cannot be drawn from the transferred carbon nanotube array  10 . During the transferring, the substrate  30  can always be in a solid state. 
     In one embodiment, to satisfy F AC &lt;F BC &lt;F CC , the substrate  30  can increase the surface area of the surface  302  by using the microstructures  304 , thus increasing the F BC . The substrate  30  can have the surface  302  with a plurality of microstructures  304  located thereon. The microstructure  304  can have a point shape and/or a long and narrow shape, and can be protrusions and/or recesses. The cross section of the microstructures  304  can be semicircular, rectangular, conical, and/or stepped. The microstructures  304  can be hemi-spheres, convex or concave columns, pyramids, pyramids without tips, and any combination thereof. In one embodiment, the microstructures  304  can be parallel and spaced grooves. In another embodiment, the microstructures  304  can be uniformly spaced hemispherical protrusions. The plurality of microstructures  304  are uniformly distributed on the surface  302  of the substrate  30 . In one embodiment, the surface  302  having the microstructures  304  located thereon has a surface area of 30% to 120% more than a smooth surface of equivalent area. The surface  302  sufficiently contacts the second surface  104  of the carbon nanotube array  10 . Thus, the material of the substrate  30  is not limited to PDMS and can be other conventional substrate materials such as soft, elastic, and rigid solid materials. 
     The height of the protrusion and the depth of the recess of the microstructures  304  can be 0.5% to 10% of the height of the carbon nanotube array  10 . In one embodiment, the height of the protrusion and the depth of the recess can be in a range from about 5 microns to about 50 microns. The surface  302  needs an overall flatness to sufficiently contact the second surface  104  of the carbon nanotube array  10 . The microstructures  304  can be formed on the surface  302  by laser etching, chemical etching, or lithography. 
     The microstructures  304  make the surface  302  of the substrate  30  relatively rough. When the recessed portion of the surface  302  is in contact with the second surface  104  of the carbon nanotube array  10 , the protruded portion of the surface  302  may slightly curve the carbon nanotubes contacting the protruded portion. However, the microstructures  304  are small, so the curve is small, and when the substrate  30  and the growing substrate  20  are separated, the carbon nanotubes can elastically restore to a substantially straight shape and the carbon nanotube array  10  can restore to its original height. Thus, the state of the carbon nanotube array  10  is still capable of having the second carbon nanotube film  42  drawn from the carbon nanotube array  10 . 
     To ensure almost all the top ends of the carbon nanotubes in the carbon nanotube array  10  have sufficient contact with the surface of the substrate  30 , the substrate  30  and the growing substrate  20  can be brought close enough to each other. A distance from the surface  302  of the substrate  30  to the surface  202  of the growing substrate  20  can be less than or equal to the height of the carbon nanotube array  10  to apply a pressing force (f) to the carbon nanotube array  10 . The pressing force f cannot be too large to ensure the state of the carbon nanotube array  10  is still capable of drawing the second carbon nanotube film  42  when transferred to the substrate  30 . The pressing force is not to press the carbon nanotubes down or vary the length direction of the carbon nanotubes in the carbon nanotube array  10 , otherwise the state of the carbon nanotube array  10  could change. Thus, the distance between the surface  302  of the substrate  30  and the surface  202  of the growing substrate  20  cannot be too small and should be larger than an extreme value. The extreme value is a value that causes the state of the carbon nanotube array  10  to be unable to draw the second carbon nanotube film  42 . 
     However, the pressing force is difficult to control, and the height of the carbon nanotube array  10  is often in tens of microns to hundreds of microns. If the pressing force is too large, the carbon nanotubes in the array  10  may be pressed down. In one embodiment, a spacing element  22  is provided. The substrate  30  is spaced from the growing substrate  20  by the spacing element  22 . The spacing element  22  is used to limit the distance between the surface  302  of the substrate  30  and the surface  202  of the growing substrate  20 . The height of the spacing element  22  located between the substrate  30  and the growing substrate  20  is smaller than or equal to the height of the carbon nanotube array  10  and larger than the extreme value. A height distance (z) between the spacing element  22  and the carbon nanotube array  10  can exist. The spacing element  22  is a solid member. In one embodiment, the spacing element  22  is rigid. By controlling the height of the spacing element  22 , the distance between the substrate  30  and the growing substrate  20  can be precisely controlled. The height (m) of the spacing element  22  can be 0.9 times to 1 time of the height (n) of the carbon nanotube array  10  (i.e., m=0.9 n to n). 
     During the pressing of the carbon nanotube array  10 , the carbon nanotubes in the carbon nanotube array  10  are still substantially perpendicular to the growing surface of the growing substrate  20 . When the height (m) is smaller than the height (n), the carbon nanotubes in the carbon nanotube array  10  can be pressed to be curved slightly. However, the curve is small and when the substrate  30  and the growing substrate  20  are separated, the carbon nanotubes can restore the straight shape and the carbon nanotube array  10  can restore the original height. Thus, the state of the carbon nanotube array  10  is still kept to be capable of having the second carbon nanotube film  42  drawn from the carbon nanotube array  10 . 
     In one embodiment, the spacing element  22  is arranged on the growing substrate  20 . In another embodiment, the spacing element  22  is arranged on the substrate  30 . In yet another embodiment, the spacing element  22  can be a part of the growing substrate  20  or the substrate  30 . A shape of the spacing element  22  is not limited and can be a block, a piece, a column, or a ball. There can be a plurality of spacing elements  22  uniformly arranged around the carbon nanotube array  10 . The spacing element  22  can be a round circle around the carbon nanotube array  10 . In another embodiment, the spacing elements  22  are a plurality of round columns uniformly arranged around the carbon nanotube array  10 . The spacing element  22  can be used with or without the microstructures  304 . 
     During the separating of the substrate  30  away from the growing substrate  20 , a majority of the carbon nanotubes in the carbon nanotube array  10  can be detached from the growing substrate  20  at the same time by moving either the substrate  30 , the growing substrate  20 , or both, away from each other along a direction substantially perpendicular to the growing surface of the growing substrate  20 . The carbon nanotubes of the carbon nanotube array  10  are detached from the growing substrate  20  along the growing direction of the carbon nanotubes. The two substrates both moves along the direction perpendicular to the growing surface of the growing substrate  20  and depart from each other. 
     Referring to  FIG. 8 , in another embodiment, the carbon nanotube array  10  is transferred by:
         placing the substrate  30  on the second surface  104  of the carbon nanotube array  10  and sandwiching liquid medium  60  between the substrate  30  and the carbon nanotube array  10 ;   solidifying the liquid medium  60  between the substrate  30  and the carbon nanotube array  10  into solid medium  60 ′;   separating the substrate  30  from the growing substrate  20 , thereby separating the first surface  102  of the carbon nanotube array  10  from the growing substrate  20 ; and   removing the solid medium  60 ′ between the substrate  30  and the carbon nanotube array  10 .       

     The liquid medium  60  can be in a shape of fine droplets, mist, or film. The liquid medium  60  can spread on the entire second surface  104 . The liquid medium  60  can be water and/or organic solvents with small molecular weights that are volatile at room temperature or easily evaporated by heating. The organic solvent can be selected from ethanol, methanol, and acetone. The liquid medium  60  has a poor wettability for carbon nanotubes. Thus, when a small amount of the liquid medium  60  is on the second surface  104  of the carbon nanotube array  10 , it cannot infiltrate inside the carbon nanotube array  10  and will not affect the state of the carbon nanotube array  10 . A diameter of the liquid droplet and a thickness of the liquid film can be in a range from about 10 nanometers to about 300 microns. The substrate  30  and the second surface  104  of the carbon nanotube array  10  are both in contact with the liquid medium  60 . 
     During the placing the substrate  30  on the second surface  104 , the substrate  30  may apply a pressing force as small as possible to the carbon nanotube array  10 . The pressing force can satisfy 0&lt;f&lt;2N/cm 2 . The pressing force does not press the carbon nanotubes down or vary the length direction of the carbon nanotubes in the carbon nanotube array  10 . The carbon nanotubes in the carbon nanotube array  10  between the substrate  30  and the growing substrate  20  are always substantially perpendicular to the growing surface of the growing substrate  20 . 
     In one embodiment, the liquid medium  60  is formed on the second surface  104  of the carbon nanotube array  10 . The liquid medium  60  can be formed into fine droplets or a mist in the air and drop or collect onto the second surface  104  of the carbon nanotube array  10 . The substrate  30  and the carbon nanotube array  10  on the growing substrate  20  are brought together such that the surface of the substrate  30  and the liquid medium  60  on the second surface  104  are contacting each other. 
     In another embodiment, the liquid medium  60  is formed on the surface of the substrate  30 . The liquid medium  60  can be formed into fine droplets or a mist in the air and drop or collect onto the surface of the substrate  30 . The substrate  30  and the carbon nanotube array  10  on the growing substrate  20  are brought together such that the second surface  104  of the carbon nanotube array  10  and the liquid medium  60  on the surface of the substrate  30  are contacting each other. 
     During the solidifying of the liquid medium  60 , the temperature of the liquid medium  60  can be decreased to below the freezing point of the liquid medium  60 . After the liquid medium  60  is solidified, the substrate  30  and the carbon nanotube array  10  can be firmly bonded together by the solid medium  60 ′ therebetween. In one embodiment, water is frozen into ice below 0° C. 
     In one embodiment, the laminate of the growing substrate  20 , the carbon nanotube array  10 , the liquid medium  60 , and the substrate  30  can be arranged in an area, such as be put into a freezer  70 , with a temperature below the freezing point to freeze the liquid medium  60 . 
     Referring to  FIG. 9 , in another embodiment, when the liquid medium  60  is formed on the second surface  104  of the carbon nanotube array  10 , a temperature of the substrate  30  can be decreased to below the freezing point before contacting the substrate  30  with the liquid medium  60 . For example, the substrate  30  can be kept in the area, such as the freezer  70 , for a period of time until the substrate  30  reaches a temperature below the freezing point. Thus, when the substrate  30  contacts the liquid medium  60  on the second surface  104  of the carbon nanotube array  10 , the liquid medium  60  can be directly frozen into solid medium  60 ′. 
     During the separating of the substrate  30  from the growing substrate  20 , due to the bonding between the carbon nanotube array  10  and the substrate  30  by the solid medium  60 ′, the separating of the two substrates can separate the carbon nanotube array  10  from the growing substrate  20 . During the separating, a majority of the carbon nanotubes in the carbon nanotube array  10  can be detached from the growing substrate  20  at the same time by cutting means, or moving either the substrate  30  or the growing substrate  20 , or both, away from each other along a direction substantially perpendicular to the growing surface of the growing substrate  20 . The carbon nanotubes of the carbon nanotube array  10  are detached from the growing substrate  20  along the growing direction of the carbon nanotubes. When both the substrate  30  and the growing substrate  20  separate, the two substrates both moves along the direction perpendicular to the growing surface of the growing substrate  20  and depart from each other. 
     During the removing of the solid medium  60 ′, the solid medium  60 ′ can be heated and melt into liquid medium, and dried between the substrate  30  and the carbon nanotube array  10 . In another embodiment, the heating can directly sublimate the solid medium  60 ′. The removal of the solid medium  60 ′ does not affect the state of the carbon nanotube array  10 . Due to the thickness of the solid medium  60 ′ being small, after the removal of the solid medium  60 ′, the second surface  104  of the carbon nanotube array  10  can be in contact with the surface of the substrate  30  and bonded by van der Waals attractive forces. 
     For drawing the second carbon nanotube film  42 , the bonding force between the carbon nanotube array  10  and the substrate  30  should be small. The bonding force is increased by the solid medium  60 ′ to separate the carbon nanotube array  10  from the growing substrate  20  and decreased by removing the solid medium  60 ′ before drawing the second carbon nanotube film  42 . Thus, the material of the substrate  30  is not limited to PDMS and can be soft, elastic, and rigid solid materials. 
     Pattering of Carbon Nanotube Array 
     Referring back to  FIG. 4  to  FIG. 6 , in the block S 4 , the laser etches the carbon nanotube array  10  to form one or more etching grooves  106  on the first surface  102 . Laser beam scans on the first surface  102  and the scanned carbon nanotubes absorb the laser energy to increase the temperature thereof. The heated carbon nanotubes react with the oxygen gas in air and are burnt. Thus, the scanning of the laser beam removes some carbon nanotubes to forms the etching groove  106  on the first surface  102  of the carbon nanotube array  10 . The scanning route of the laser beam can be controlled accurately by a computer, and a complicated and fine pattern of the etching grooves  106  can be formed on the first surface  102  of the carbon nanotube array  10 . A power of the laser beam ranges from about 20 watts to about 50 watts and a moving speed of the laser beam ranges from about 0.1 millimeters per second (mm/s) to about 10000 mm/s. A width of the laser beam can be in a range from about 1 micron to about 400 microns. 
     The etching groove  106  can have a depth that is smaller than or equal to a height of the carbon nanotube array  10 . In one embodiment, the depth of the etching groove  106  can be in a range from about 0.5 microns to about 10 microns. The etching groove  106  can have a width larger than or equal to 1 micron. The width and depth of the etching groove  106  is suitable for separating the carbon nanotubes in the preserving area  12  and the removing area  14 . The carbon nanotubes are combined with each other by enough van der Waals attractive force to have the second carbon nanotube film  42  drawn therefrom. Thus, even the carbon nanotubes in the etching groove  106  are just shortened by the etching, the van der Waals attractive force can be decreased. Thus, during the drawing of the second carbon nanotube film  42  from the removing area  14 , the carbon nanotubes in the preserving area  12  will not be drawn out with those in the removing area  14 . 
     The etching groove  106  can have a line shape to divide the first surface  102  of the carbon nanotube array  10  into the preserving area  12  and the removing area  14 . In one embodiment, the etching groove  106  forms two closed areas and the preserving area  12  and the removing area  14  are completely separated from each other by the etching groove  106 . The preserving area  12  and the removing area  14  are divided according to the locations of the first electrode  90  and the second electrode  92 . 
     In block S 5 , the second carbon nanotube film  42  is drawn from the removing area  14  thus removing the carbon nanotubes from the removing area. 
     Block S 5  can comprise: 
     selecting a carbon nanotube segment having a predetermined width from removing area  14  by using a drawing tool; and 
     drawing a plurality of carbon nanotube segments joined end to end by van der Waals attractive force by moving the drawing tool  50 , thereby forming a continuous second carbon nanotube film  42 . 
     The drawing tool can be adhesive tape, pliers, tweezers, or other tool allowing multiple carbon nanotubes to be gripped and pulled simultaneously. 
     The carbon nanotube segment comprises a single carbon nanotube or a plurality of carbon nanotubes substantially parallel to each other. The drawing tool such as adhesive tape can be used for selecting and drawing the carbon nanotube segment. The adhesive tape may contact with the carbon nanotubes in the carbon nanotube array to select the carbon nanotube segment. The drawing tool can select a large width of carbon nanotube segments to form the carbon nanotube film, or a small width of the carbon nanotube segments to form the carbon nanotube wire. 
     An angle between a drawing direction of the carbon nanotube segments and the growing direction of the carbon nanotubes in the carbon nanotube array  10  can be larger than 0 degrees (e.g., 30° to 90°). 
     In the block S 5 , when drawing to the edge of the removing area  14 , due to the etching groove  106 , the second carbon nanotube film  42  will naturally separate from the carbon nanotube array  10 . The carbon nanotubes in the preserving area  12  are thus left on the substrate  30  to form the supporting members  80 . 
     In block S 5 , the second carbon nanotube film  42  is drawn from the carbon nanotube array  10  that was transferred to the substrate  30 , not from the carbon nanotube array  10  located on the growing substrate  20 . The second carbon nanotube film  42  can be drawn from the carbon nanotube array  10  upside down on the surface  302  of the substrate  30  (e.g., drawn from the first surface  102  of the carbon nanotube array  10 ). 
     Block S 5  is different from the separating of the carbon nanotube array  10  as a whole from the growing substrate  20 . The carbon nanotube array  10  separated from the growing substrate  20  still in the array shape. The purpose of block S 5  is to draw out carbon nanotubes one by one or segment by segment to form a carbon nanotube film or wire from the carbon nanotube array  10  on the substrate  30 . 
     In Block S 6 , the first carbon nanotube film  40  is placed on the supporting members  80  and the carbon nanotubes in the first carbon nanotube film  40  are substantially aligned along a direction from one supporting member  80  to the other supporting member  80 . 
     Depending on the embodiment, certain blocks/steps of the methods described may be removed, others may be added, and the sequence of blocks may be altered. It is also to be understood that the description and the claims drawn to a method may comprise some indication in reference to certain blocks/steps. However, the indication used is only to be viewed for identification purposes and not as a suggestion as to an order for the blocks/steps. 
     The embodiments shown and described above are only examples. Even though numerous characteristics and advantages of the present technology have been set forth in the foregoing description, together with details of the structure and function of the present disclosure, the disclosure is illustrative only, and changes may be made in the detail, especially in matters of shape, size and arrangement of the parts within the principles of the present disclosure up to, and including the full extent established by the broad general meaning of the terms used in the claims. It will therefore be appreciated that the embodiments described above may be modified within the scope of the claims.