Thermionic emission device

A thermionic emission device includes an insulating substrate, a patterned carbon nanotube film structure, a positive electrode and a negative electrode. The insulating substrate includes a surface. The surface includes an edge. The patterned carbon nanotube film structure is partially arranged on the surface of the insulating substrate. The patterned carbon nanotube film structure includes two strip-shaped arms joined at one end to form a tip portion protruded from the edge of the surface of the insulating substrate and suspended. The patterned carbon nanotube film structure includes a number of carbon nanotubes parallel to the surface of the insulating substrate. The patterned carbon nanotube film structure is connected between the positive electrode and the negative electrode in series.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims all benefits accruing under 35 U.S.C. §119 from China Patent Application No. 201210204601.5, filed on 2012/6/20 in the China Intellectual Property Office. This application is related to common-assigned applications entitled, “CARBON NANOTUBE BASED MICRO-TIP STRUCTURE AND METHOD FOR MAKING THE SAME” filed Aug. 23, 2012 Ser. No. 13/592,763; “CARBON NANOTUBE BASED MICRO-TIP STRUCTURE AND METHOD FOR MAKING THE SAME” filed Aug. 23, 2012 Ser. No. 13/592,751; “FIELD EMISSION ELECTRON SOURCE AND FIELD EMISSION DEVICE USING THE SAME” filed Aug. 23, 2012 Ser. No. 13/592,795; and “ATOMIC FORCE MICROSCOPE PROBE” filed Aug. 23, 2012 Ser. No. 13/592,852.

BACKGROUND

1. Technical Field

The present disclosure relates to a carbon nanotube based thermionic emission device.

2. Description of Related Art

Thermionic emission devices are widely applied in gas lasers, arc-welders, plasma-cutters, electron microscopes, x-ray generators, and the like. Thermionic emission devices are constructed by forming an electron emissive layer made of alkaline earth metal oxide on a base. The alkaline earth metal oxide includes BaO, SrO, CaO, or a mixture thereof. The base is made of an alloy including at least one of Ni, Mg, W, Al, and the like. When thermionic electron emission devices are heated to a temperature of about 800° C., electrons are emitted from the thermionic emission source. Since the electron emissive layer is formed on the surface of the base, an interface layer is formed between the base and the electron emissive layer, the electron emissive alkaline earth metal oxide easily splits off from the base. Further, thermionic electron emission devices are less stable because alkaline earth metal oxide tends to vaporize at high temperatures. Consequently, the lifespan of the thermionic emission device are low. Further, the response speed of the thermionic emission device to the heating is relatively low. It is difficult to rapidly emit electrons by using a current to heat the thermionic emission device, and rapidly stop the emission by cutting off the current. Thus, it is difficult to achieve a thermionic emission pulse current.

What is needed, therefore, is a thermionic emission device, which has stable and high electron emission efficiency, great mechanical durability, and is able to emit a thermionic emission pulse current.

DETAILED DESCRIPTION

Referring toFIG. 1, one embodiment of a thermionic emission device100includes an insulating substrate110, a patterned carbon nanotube film structure120, a positive electrode130, and a negative electrode140. The thermionic emission device100can emit a thermionic emission pulse current. The insulating substrate110includes a surface112. The surface112has an edge114. The patterned carbon nanotube film structure120is partially arranged on the surface112of the insulating substrate110. The patterned carbon nanotube film structure120includes two strip-shaped arms122. The two strip-shaped arms122can be narrow and long film shapes. The two strip-shaped arms122are joined at one end to form a tip portion124of the patterned carbon nanotube film structure120. An angle α between lengthwise directions of the two strip-shaped arms122can be smaller than 180°. The tip portion124of the patterned carbon nanotube film structure120protrudes from the edge114of the surface112of the insulating substrate110and is suspended. The patterned carbon nanotube film structure120includes a plurality of carbon nanotubes substantially parallel to the surface112of the insulating substrate110.

The patterned carbon nanotube film structure120is electrically connected between the positive electrode130and the negative electrode140in series. The positive electrode130, one of the strip-shaped arms122, the tip portion124, the other of the strip-shaped arms122, and the negative electrode140are connected together one by one in series. The positive electrode130and the negative electrode140apply a pulse voltage to the patterned carbon nanotube film structure120The patterned carbon nanotube film structure120is heated and cooled according to the pulse voltage, and emits a thermionic emission pulse current.

The thermionic emission device100has a relatively low turn-on voltage. The turn-on voltage is the smallest pulse voltage that is capable of emitting the thermionic emission pulse current. In one embodiment, the turn-on voltage is about 8 V to about 20 V.

The pulse voltage applied to the patterned carbon nanotube film structure120is higher than 8 V and lower than the broken voltage of the patterned carbon nanotube film structure120. The broken voltage is the highest voltage that can break down the patterned carbon nanotube film structure120. The value of the pulse voltage can be varied with time or constant. The patterned carbon nanotube film structure120can be heated by the pulse voltage and emit thermionic electron without applying of an additional electrical field.

The thermionic emission device100can include a pulse power source150electrically connected to the positive electrode130and the negative electrode140.

The thermionic emission device100can further include a vacuum sealing structure (not shown) to seal the insulating substrate110, the patterned carbon nanotube film structure120, the positive electrode130, and the negative electrode140in a vacuum in the sealing structure. In one embodiment, the vacuum degree in the vacuum sealing structure is about 2×10−5Pa.

The insulating substrate110can be a board or a sheet. A material of the insulating substrate110can be silicon, ceramic, glass, resin, or crystal. The insulating substrate110can also be a silicon substrate having a silicon oxide layer coated on the surface112. A thickness of the silicon oxide layer can be about 1 micron. An entire thickness of the insulating substrate110can be about 0.5 millimeters.

The patterned carbon nanotube film structure120can be a free-standing film shaped structure and can include a plurality of carbon nanotube films stacked together. Each carbon nanotube film may include a plurality of carbon nanotubes substantially aligned along the same direction. The carbon nanotube film can be an ordered and free-standing carbon nanotube film. In the plurality of stacked carbon nanotube films, at least one carbon nanotube film includes a plurality of carbon nanotubes aligned along a direction from the edge114to the tip portion124. In one embodiment, the carbon nanotubes in the at least one carbon nanotube film are aligned along a direction substantially perpendicular to the edge114of the insulating substrate110.

Referring toFIG. 2, the ordered and free-standing carbon nanotube film can be drawn from a carbon nanotube array. The carbon nanotube film can include or consist of a plurality of carbon nanotubes. In the carbon nanotube film, the overall aligned direction of a majority of carbon nanotubes is substantially aligned along the same direction parallel to a surface of the carbon nanotube film. A majority of the carbon nanotubes are substantially aligned along the same direction in the carbon nanotube film. 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 carbon nanotube film is capable of being free-standing structure. There may be a minority of carbon nanotubes in the carbon nanotube film that are randomly aligned. However, the number of the randomly aligned carbon nanotubes is very small and does not affect the overall oriented alignment of the majority of carbon nanotubes in the carbon nanotube film. 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 are not exactly aligned along the overall aligned direction, and can deviate from 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 carbon nanotube film. The carbon nanotube film may include 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 includes 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 carbon nanotube film. A thickness of the carbon nanotube film at the thickest location is about 0.5 nanometers to about 100 microns (e.g., in a range from 0.5 nanometers to about 10 microns).

The term “free-standing” includes, but not limited to, a carbon nanotube film that does not have to be supported by a substrate. For example, a free-standing carbon nanotube film can sustain the weight of itself when it is hoisted by a portion thereof without any significant damage to its structural integrity. So, if the free-standing carbon nanotube film is placed between two separate supporters, a portion of the free-standing carbon nanotube film, not in contact with the two supporters, can be suspended between the two supporters and yet maintain a film structural integrity. The free-standing carbon nanotube film is realized by the successive carbon nanotubes joined end to end by van der Waals attractive force.

Referring toFIG. 3, in the patterned carbon nanotube film structure120, the plurality of ordered carbon nanotube films are stacked together along at least two directions, such that the carbon nanotubes in the carbon nanotube films stacked along different directions are aligned along different directions. An angle β between the carbon nanotubes in the carbon nanotube films stacked along different directions can be in a range of 0°<β≦90°. The number of the carbon nanotube films in the patterned carbon nanotube film structure120is not limited, and can be determined by actual needs. In some embodiment, the patterned carbon nanotube film structure120can include 5 to 100 stacked carbon nanotube films. In one embodiment, the patterned carbon nanotube film structure120includes 50 stacked carbon nanotube films having the angle β of about 90° between the carbon nanotubes of adjacent carbon nanotube films. The patterned carbon nanotube film structure120is a stable free-standing film structure because the adjacent carbon nanotubes directly contacting each other are sufficiently joined by van der Waals attractive forces. In the patterned carbon nanotube film structure120, the adjacent carbon nanotubes are connected with each other, thus forming an electrically conductive network. The carbon nanotube film has an extremely thin thickness. Accordingly, the patterned carbon nanotube film structure120having the plurality carbon nanotube films stacked together can have a small thickness. In one embodiment, the thickness of the patterned carbon nanotube film structure120having 50 carbon nanotube films stacked together is in a range from about 50 nanometers to about 5 microns. The carbon nanotube film can have a uniform thickness. Accordingly, the patterned carbon nanotube film structure120having the plurality carbon nanotube films stacked together can have a uniform thickness, thus having a uniform electrical conductivity. The patterned carbon nanotube film structure120has a relatively small thickness, and accordingly, a relatively small heat capacity per unit area. The smaller the heat capacity per unit area, the smaller the turn-on voltage and the faster the response speed of the thermionic emission device100.

The patterned carbon nanotube film structure120is laid on the surface112of the insulating substrate110. Due to the free-standing property, a portion of the patterned carbon nanotube film structure120protruding from the edge114of the surface112can be suspended and remain parallel to the surface112of the insulating substrate110. That is, the carbon nanotubes in the suspended portion of the patterned carbon nanotube film structure120are still parallel to the surface112of the insulating substrate110.

Referring toFIG. 4andFIG. 5, in the patterned carbon nanotube film structure120, the two strip-shaped arms122are part of an integrated structure formed by patterning a carbon nanotube film structure. The integrated structure can have a V shape or a U shape to form the two strip-shaped arms122. Each strip-shaped arm122has a first end1220and a second end1222along the lengthwise direction. The two strip-shaped arms122are joined together at the first end1220to form a tip portion124. The angle α between the lengthwise directions of the two strip-shaped arms122can be smaller than 180°. In some embodiment, the angle α can be in a range from about 15° to about 120°. In one embodiment, the angle α is about 60°. The tip portion124can have a size smaller than 20 microns. In one embodiment, the tip portion124can only have a single protruding carbon nanotube having a diameter of about 0.5 nanometers. The patterned carbon nanotube film structure120can only protrude and suspend the tip portion124or can protrude and suspend the entire two strip-shaped arms122from the edge114of the surface112of the insulating substrate110. The shape of the two strip-shaped arms122are not limited but have an overall strip shape. In one embodiment, each strip-shaped arm122has a gradually decreased width from the second end1222to the first end1220. The resistance gradually increases from the second end1222to the first end1220, which may be useful for a thermal emission device or a thermal field emission device. In another embodiment, each strip-shaped arm122has a uniform width from the second end1222to the first end1220. The width of the strip-shaped arm122is not limited and can be in a range from about 10 microns to about 1 millimeter. The width at the first end1222can be in a range from about 10 microns to about 300 microns. The narrower the width of the strip-shaped arm122at the first end1220, the greater the resistance of the tip portion124and the easier to heat the tip portion124to emit thermionic electrons.

The strip-shaped arm122can have a uniform thickness or a blade shaped thickness that is thicker at the middle of the strip-shaped arm122compared to the thickness at the edge of the strip-shaped arm122. Referring toFIG. 5, the strip-shaped arm122has a light color at the middle and a gradually darkened color from the middle to the edge. This shows that the edge of the strip-shaped arm122is thinner than the middle of the strip-shaped arm122. The thickness of the strip-shaped arm122at the edge can be nanosize (e.g., smaller than 100 nanometers).

The two strip-shaped arms122can be reflection symmetrical about a symmetry axis passing through the tip portion124. In the patterned carbon nanotube film structure120, at least one carbon nanotube film can have the majority of carbon nanotubes aligned along the symmetry axis. In one embodiment, the edge114of the surface112is straight, and the symmetry axis can be perpendicular to the edge114. Referring toFIG. 6, in one embodiment, the symmetry axis is perpendicular to the edge114, half of the carbon nanotube films have the carbon nanotubes aligned along the symmetry axis and the other half of the carbon nanotube films have the carbon nanotubes aligned perpendicular to the symmetry axis. Referring toFIG. 7, in one embodiment, the carbon nanotubes at the tip portion124can have an open end (as pointed by the arrow in theFIG. 7), which may facilitate the electron emission from the open end in the thermionic emission device100.

The narrower width at the first end1220of the two strip-shaped arms122may facilitate the electron emission in the thermionic emission device. The patterned carbon nanotube film structure120can further include a cutout128at the joining portion of the two strip-shaped arms122. The cutout128can be a line shape. The lengthwise direction of the cutout128can be parallel to the surface112of the insulating substrate110, and extend along a direction from the edge114to the tip portion124. The width of the cutout128can be uniform or gradually decreasing from the edge114to the tip portion124. A distance exists from the end of the cutout128to the top of the tip portion124, where the two strip-shaped arms122join together. In one embodiment, the distance from the end of the cutout128to the top of the tip portion124is about 210 microns. The resistance at the tip portion124can be increased by defining the cutout128in the patterned carbon nanotube film structure120, which may improve the thermal emission performance or thermal field emission performance. In one embodiment, the two strip-shaped arms122are symmetrical about the symmetry axis, and the lengthwise direction of the cutout128can be along the symmetry axis. The patterned carbon nanotube film structure120can be seen as a bent conductive strip having the narrowest width and greatest resistance at the tip portion124. The bent conductive strip has the narrowest width and the largest resistance at the tip portion124to make the thermionic emission device100emit the thermionic electrons from the tip portion124.

Referring toFIG. 1, to facilitate the connection between the patterned carbon nanotube film structure120and an outer circuit and while supporting the suspended portion, the patterned carbon nanotube film structure120can further include two connecting portions126. The two connecting portions126are respectively connected to the two strip-shaped arms122and located on the surface112of the insulating substrate110. Similar to the two strip-shaped arms122, the two connecting portions126are also formed by patterning the carbon nanotube film structure. The two connecting portions126and the two strip-shaped arms122are part of the integrated structure. The shape of the connecting portion126is not limited. In one embodiment, the connecting portion126has a rectangular strip shape having the same width as the second end1222of the strip-shaped arm122. The positive electrode130and the negative electrode140can be mechanically and electrically connected to the two connecting portions126respectively, and connected to the two strip-shaped arms122through the two connecting portions126.

The tip portion124, the two connecting portions126, and the two strip-shaped arms122are portions of the integrated structure (i.e., the integrated patterned carbon nanotube film structure120). In one embodiment, the patterned carbon nanotube film structure120only includes the carbon nanotubes.

Referring toFIG. 8, another embodiment of the thermionic emission device is similar to the thermionic emission device100except that the patterned carbon nanotube film structure120includes a plurality of carbon nanotubes protruding from the tip portion124. The plurality of protruded carbon nanotubes extend radially from the tip portion124and are spaced from each other, to form a plurality of micro-tips at the tip portion124in a microscopic view. The plurality of protruded carbon nanotubes also belong to the patterned carbon nanotube film structure120, and are integrated to the two strip-shaped arms122. The plurality of protruded carbon nanotubes are joined to the carbon nanotubes in the two strip-shaped arms122by van der Waals attractive force.

Referring toFIG. 9, another embodiment of the thermionic emission device200includes an insulating substrate210and a plurality of patterned carbon nanotube film structures220. The insulating substrate210includes a surface212. The plurality of patterned carbon nanotube film structures220are spaced from each other and partially located on the surface212of the insulating substrate210. The insulating substrate210and the patterned carbon nanotube film structures220are similar to the above embodiment of the insulating substrate110and the patterned carbon nanotube film structure120, except that a strip-shaped recess216is defined on the surface212of the insulating substrate210. The edge214of the insulating substrate212having the portion of the patterned carbon nanotube film structures220protruding therefrom is an edge of the strip-shaped recess216. The plurality of patterned carbon nanotube film structures220at the tip portions224protrude from the same edge214of the insulating substrate210and suspend above the strip-shaped recess216. The strip-shaped recess214can be a blind groove or a through hole. In one embodiment, the strip-shaped recess216is a strip-shaped through hole.

The thermionic emission device200can further include a plurality of positive electrode conducting leads230and a plurality of negative electrode conducting leads240. The positive electrode conducting lead230is mechanically and electrically connected to one connecting portion226of the patterned carbon nanotube film structure220. The negative electrode conducting lead240is mechanically and electrically connected to the other connecting portion226of the patterned carbon nanotube film structure220. The plurality of patterned carbon nanotube film structures220and the plurality of recesses216can be arranged in an array with lines and columns. Each positive electrode conducting lead230corresponds to a column of patterned carbon nanotube film structures220, and is connected to one connecting portion226of each patterned carbon nanotube film structures220in the column. Each negative electrode conducting lead240corresponds to a line of patterned carbon nanotube film structures220, and is connected to the other connecting portion226of each patterned carbon nanotube film structures220in the line.

In one embodiment, the plurality of positive electrode conducting leads230and the plurality of negative electrode conducting leads240are all arranged on the surface212of the insulating substrate210. The plurality of positive electrode conducting leads230and parallel to and spaced from each other. The plurality of negative electrode conducting leads240and parallel to and spaced from each other. The plurality of positive electrode conducting leads230intercross with (e.g., perpendicular to) the plurality of negative electrode conducting leads240. The plurality of positive electrode conducting leads230are insulated from the plurality of negative electrode conducting leads240. In one embodiment, an insulating layer250can be further arranged between the plurality of positive electrode conducting leads230and the plurality of negative electrode conducting leads240, to space the plurality of positive electrode conducting leads230from the plurality of negative electrode conducting leads240at the intersection points. The adjacent two positive electrode conducting leads230and the adjacent two negative electrode conducting leads240can form a grid, each of which having a patterned carbon nanotube film structure220located therein.

The recesses216arranged in the array with columns and lines can be spaced from each other. The positive electrode conducting leads230can be arranged on the surface212between the two adjacent columns of the recesses216. The negative electrode conducting leads240can be arranged on the surface212between the two adjacent lines of the recesses216.

By using the positive electrode conducting leads230and the negative electrode conducting leads240, the thermionic emission device200can have an addressing function to address the specific patterned carbon nanotube film structure220.

Referring toFIG. 10, another embodiment of the thermionic emission devices300is similar to the above embodiment of array of thermionic emission devices200, except that in the thermionic emission devices300, the patterned carbon nanotube film structure320includes a plurality of strip-shaped arms322and a plurality of tip portions324formed by the plurality of strip-shaped arms322. The plurality of strip-shaped arms322are joined end to end to form a zigzag shaped structure having a plurality of tip portions324at two opposite directions of the patterned carbon nanotube film structure320. The insulating substrate310may only support the two strip-shaped arms322at the ends of the zigzag shaped structure, and the other strip-shaped arms322suspended therebetween. The two strip-shaped arms322at the ends of the zigzag shaped structure can be respectively connected to the positive electrode330and the negative electrode340. The plurality of strip-shaped arms322can have the same width and have the grooves at the tip portions324.

Referring toFIG. 11, another embodiment of the thermionic emission devices400is similar to the above embodiment of thermionic emission devices300, except that in the thermionic emission devices400, the plurality of strip-shaped arms422are joined end to end to form a serrated shaped structure having a plurality of tip portions424at two opposite directions of the patterned carbon nanotube film structure420. The insulating substrate410may only support the two strip-shaped arms422at the ends of the serrated shaped structure, and the other strip-shaped arms422suspended therebetween. The two strip-shaped arms422at the ends of the serrated shaped structure can be respectively connected to the positive electrode330and the negative electrode340. The patterned carbon nanotube film structure320may also define a strip-shaped through hole426extending along a direction from one tip portion424to the opposite tip portion424, and having a distance to both of the two opposite tip portions424. The strip-shaped through hole426can increase the resistance at the tip portion424.

Referring toFIG. 12andFIG. 13, one embodiment of a method for making the thermionic emission device includes steps of:

S1, providing a carbon nanotube film structure10and an insulting substrate20, wherein the insulating substrate20has a surface22, and at least one strip-shaped recess26is defined in the insulating substrate22at the surface22;

S2, covering the carbon nanotube film structure10on the surface22of the insulating substrate20, and having a suspended portion of the carbon nanotube film structure10suspended over on the at least one strip-shaped recess26;

S3, laser etching the suspended portion of the carbon nanotube film structure10, to define a first hollow pattern14in the suspended portion and form a patterned carbon nanotube film structure30according to the first hollow pattern14, wherein the patterned carbon nanotube film structure30includes two strip-shaped arms32joined at one end to form a tip portion34suspended above the strip-shaped recess26; and

S4, respectively connecting the two strip-shaped arms32to a positive electrode and a negative electrode.

In the step S1, the carbon nanotube film structure10can be formed by steps of:

S11, providing a plurality of carbon nanotube films;

S12, stacking the plurality of carbon nanotube films along different directions on a frame; and

S13, treating the plurality of carbon nanotube films on the frame by using an organic solvent, to form a carbon nanotube film structure10.

The carbon nanotube film can be drawn from a carbon nanotube array. The carbon nanotube film includes a plurality of carbon nanotubes joined end to end by van der Waals attractive force therebetween and aligned along substantially the same direction. The carbon nanotube films can be covered on the frame one by one to laminate together and suspend across the frame. The stacking directions of carbon nanotube films can be different, or the carbon nanotube films can be stacked along only several (e.g., two) directions. The carbon nanotube film structure is a free-standing structure supported only by the frame and suspended across the frame. In one embodiment, the frame has a square shape with each edge having a length of about 72 millimeters, and 50 carbon nanotube films are stacked on the frame. The carbon nanotube film is treated by applying the organic solvent to the drawn carbon nanotube film to soak the entire surface of the stacked carbon nanotube films and then removing the organic solvent by drying. In the step S13, the carbon nanotube films can be soaked by the organic solvent. The organic solvent can be a volatile solvent at room temperature such as an ethanol, methanol, acetone, dichloroethane, chloroform, or any appropriate mixture thereof. After being soaked by the organic solvent, the adjacent carbon nanotube films can be combined together by surface tension of the organic solvent when the organic solvent volatilizes until the stable carbon nanotube film structure10is achieved. The method for drawing and stacking the carbon nanotube films are taught in US patent publication number 2008/0248235A1.

In the step S1, the insulating substrate20can be etched or laser cut to form a plurality of strip-shaped recesses26at the surface22. In one embodiment, the strip-shaped recesses26are a plurality of strip-shaped through holes formed by using a reactive iron etching (RIE) method. In the step S1, a plurality of spaced strip-shaped recesses26can be formed on the insulating substrate20to prepare an array of thermionic emission devices or a batch of thermionic emission devices. The plurality of spaced strip-shaped recesses26can be parallel to each other in the lengthwise direction.

In the step S2, the previously formed carbon nanotube film structure10is laid on the surface22of the insulating substrate20. After the step S2of covering the carbon nanotube film structure10on the surface22of the insulating substrate20, an additional step of treating the carbon nanotube film structure10with the organic solvent similar to the step S13, can be processed. After the carbon nanotube film structure10is soaked by the organic solvent and the organic solvent is volatilized from the carbon nanotube film structure10, the carbon nanotube film structure10can be combined tightly with the surface22of the insulating substrate20, thus fixing the carbon nanotube film structure10on the insulating substrate20.

In the step S3, a laser device is provided to emit a laser beam. The carbon nanotube film structure10is irradiated by the laser beam which is focused on the surface of the carbon nanotube film structure10to burn the irradiated portions of the carbon nanotube film structure10. The laser beam scans the portions of the carbon nanotube film structure10to be etched out. The laser etching is carried out in an environment with oxygen, for example, in air. The carbon nanotubes in the irradiated portions absorb the laser beam energy, react with the oxygen in the air, and then decompose. Thus, the carbon nanotubes in the irradiated portions will be removed. The laser device can have a power of about 2 watts to about 50 watts. A scanning rate of the laser beam can be about 0.1 millimeter/second to about 10000 millimeter/second. A width of the laser beam can be about 1 micron to about 400 microns. In one embodiment, the laser device is an yttrium aluminum garnet laser device having a wavelength of about 1.06 microns, a power of about 3.6 watts, and a scanning rate of about 100 millimeter/second. Referring back toFIG. 5, due to the laser etching, the etched edge of the carbon nanotube film structure10formed by the laser etching has a blade-shaped thickness (i.e., the closer to the edge, the smaller the thickness). Thus, the two strip-shaped arms32and the tip portion34formed by the laser etching step can have the blade-shaped thickness. Referring back toFIG. 7, due to the laser etching, some of the outermost carbon nanotubes which may be partially etched by the laser beam have an open end at the etched edge.

Referring back toFIG. 6, in a microscopic view, the laser etching may form a relatively smooth top of the tip portion34. To facilitate the thermionic emission, an additional step of protruding the carbon nanotubes from the top of the tip portion34can be further processed. Specifically, in this additional step, some carbon nanotubes on the top of the tip portion34can be grabbed using a tool and pulled out from the top of the tip portion34. The tool can be an adhesive such as a glue rod or an adhesive tape. The adhesive can grab the carbon nanotubes by contacting the carbon nanotubes. The adhesive can be moved away from the tip portion34to draw the carbon nanotubes until the carbon nanotubes are protruding from the top of the tip portion34. The tool can be tweezers.

To form a batch of thermionic emission devices at one time or an array of thermionic emission devices300, a plurality of first hollow patterns14can define a plurality of patterned carbon nanotube film structures30arranged in an array, protruded from the edge of the surface22of the insulating substrate20and suspended above the strip-shaped recesses26.

To be used in a thermionic emission device, a cutout can be formed by the laser etching of the step S3on the patterned carbon nanotube film structure30to increase the resistance at the tip portion34. The cutout has a lengthwise direction parallel to the surface22of the insulating substrate20and extends along a direction from the edge of the surface22to the tip portion34.

The embodiment of the method for making the thermionic emission device can further include a step S4of patterning on-surface-portion of the carbon nanotube film structure10to form two connecting portions36respectively connected to the two strip-shaped arms32in a one to one manner. Applied to the insulating substrate20, the carbon nanotube film structure10can have two kinds of portions: the on-surface-portion and the suspended portion. The on-surface-portion is the portion of the carbon nanotube film structure10located on the surface22of the insulating substrate20.

The laser etching is performed on suspended portions of carbon nanotube film structure to prevent the heat absorption of the insulating substrate. Therefore, to easily pattern the on-surface-portion of the carbon nanotube film structure10, the step S4may be processed by different methods.

Referring toFIG. 14, a method (1) includes a step of previously laser etching the carbon nanotube film structure10before the step S2of covering the carbon nanotube film structure10on the insulating substrate20to define a second hollow pattern16and the two connecting portions36according to the second hollow pattern16. In this step, the carbon nanotube film structure10can be suspended across a frame during the laser etching. Therefore, the second hollow pattern16of the carbon nanotube film structure10can be easily laser etched. The two connecting portions36defined by the second hollow pattern16is located in the place according to where the two strip-shaped arms32will be formed in the following step S3. After the step S3, the finally achieved two strip-shaped arms32and the two connecting portions36are respectively connected. After the step S3, the first hollow pattern14and the second hollow pattern16cooperatively define the patterned carbon nanotube film structure30having the connecting portions36and the strip-shaped arms32. The other portion of the carbon nanotube film structure10is isolated from the patterned carbon nanotube film structure30by the first hollow pattern14and the second hollow pattern16. An additional step of removing the other portion of the carbon nanotube film structure10can be further included by the method (1), processed by simply peeling the other portion of the carbon nanotube film structure10from the surface22of the insulating substrate20.

Referring toFIG. 15, a method (2) includes steps of: previously forming a connecting portion etching groove28on the surface22of the insulating substrate20before the step S2of covering the carbon nanotube film structure10on the insulating substrate20; and laser etching the portion of the carbon nanotube film structure10suspended above the connecting portion etching groove28after the step S2. The connecting portion etching groove28can be a V-shaped groove with a relatively narrow width. The connecting portion etching groove28defines the outline of the two connecting portions36and reaches to the strip-shaped recess26. The portion of the carbon nanotube film structure10covering the connecting portion etching groove28is suspended above the connecting portion etching groove28, thus can be totally etched out by the laser etching. The patterned carbon nanotube film structure30having the two connecting portions36and the two strip-shaped arms32can be isolated from the other portions of the carbon nanotube film structure10by laser etching the portion of the carbon nanotube film structure10suspended above the connecting portion etching groove28. Similar to the method (1), the other portion of the carbon nanotube film structure10can be easily removed from the surface22of the insulating substrate20.

Referring toFIG. 16, a method (3) includes both the steps of previously etching the insulating substrate20and previously etching the carbon nanotube film structure10. The method (3) includes steps of:

S41, previously laser etching the carbon nanotube film structure10before the step S2of covering the carbon nanotube film structure10on the insulating substrate20to define a third hollow pattern18;

S42, according to the third hollow pattern18, previously forming an assisted etching groove24on the surface22of the insulating substrate20before the step S2, wherein the assisted etching groove24, the third hollow pattern18, and the strip-shaped recess26cooperatively define the outline of the two connecting portions36;

S43, after the step S2of covering the carbon nanotube film structure10on the insulating substrate20, laser etching the portion of the carbon nanotube film structure10suspended above the assisted etching groove24, to totally isolate the patterned carbon nanotube film structure30having the two connecting portions36and the two strip-shaped arms32from the other portions of the carbon nanotube film structure10; and

S44, removing the other portions of the carbon nanotube film structure10from the surface22of the insulating substrate20.

The third hollow pattern18of the method (3) and the second hollow pattern16of the method (1) of the carbon nanotube film structure10can be formed by the same method. The assisted etching groove24of the method (3) and the connecting portion etching groove28of the method (2) can be formed by the same method.

In one embodiment, the assisted etching groove24includes a plurality of line-shaped grooves spaced from each other and parallel to the lengthwise direction of the strip-shaped recesses26. Each strip-shaped recess26has one line-shaped groove located at one side thereof. The third hollow pattern18can be a plurality of groups of strip-shaped through holes. Each group of strip-shaped through holes can include three strip-shaped through holes parallel to and spaced from each other. The length direction of the strip-shaped through holes can be perpendicular to the line shaped grooves. The strip-shaped through holes can be located between one line-shaped groove and one strip-shaped recess. The two ends of the strip-shaped through holes along the length direction can respectively reach the line shaped groove and the strip-shaped recess, to define the outline of the connecting portions36and isolate the connecting portions36from the other portions of the carbon nanotube film structure10.

Referring toFIG. 17, a method for forming the assisted etching groove24on the surface22of the insulating substrate20can include steps of: providing a silicon substrate50; depositing a silicon nitride layer52on a surface of the silicon substrate50; patterning the silicon nitride layer52through a lithography method to expose the surface of the silicon substrate50that is to be etched out from the silicon nitride layer52; treating the silicon substrate50having the patterned silicon nitride layer52with a reacting solution (e.g., a KOH solution), to etch the exposed silicon substrate50and form the assisted etching groove24; and forming a silicon oxide film54on the surface of the silicon substrate50by using a plasma-enhanced chemical vapor deposition method. In one embodiment, the silicon oxide film54has a thickness of about 1 micron, and the assisted etching groove24has a V-shaped cross-section. After covering the assisted etching groove24with the carbon nanotube film structure10, the carbon nanotube film structure10is suspended across the assisted etching groove24, and can be etched out by laser etching.

To facilitate locating the carbon nanotube film structure10on the surface22of the insulating substrate20, an additional step S5of forming a plurality of location assisted lines40can be further processed. The location assisted lines40helps finding the location of the strip-shaped recesses26when covering the carbon nanotube film structure10on the insulating substrate20, to arrange the second hollow pattern16or the third hollow pattern18of the carbon nanotube film structure10at a right place according to the strip-shaped recesses26. The location assisted lines40can be perpendicular to the lengthwise direction of the strip-shaped recesses26. To form an array of carbon nanotube micro-tip structures, each of the strip-shaped recesses26can have a plurality of location assisted lines40formed on a side thereof. In one embodiment, the plurality of location assisted lines40at a side of one strip-shaped recess26are spaced an even distance.

Referring back toFIG. 13, the method for forming the thermionic emission device can further include an additional step S6of cutting the insulating substrate20to separate the plurality of patterned carbon nanotube film structures30to form a plurality of thermionic emission devices. In one embodiment, the insulating substrate20having the array of thermionic emission devices has a size of 25 millimeters×26.8 millimeters. After cutting, the insulating substrate20having the single thermionic emission device has a size of 3 millimeters×4 millimeters.

The above embodiments of thermionic emission devices and array of thermionic emission devices can all be formed by the above method. In some embodiments (e.g., shown inFIG. 10andFIG. 11), the carbon nanotube film structure can be covered on a strip-shaped recess having a wide width and laser etched to form the thermionic emission device having the plurality of tip portions.

Referring toFIG. 18, to test the joule heating performance, the two connecting portions of the thermionic emission device can be connected between the positive and negative electrodes of a direct current power source to electrically conduct the thermionic emission device in vacuum. In one embodiment, some other portions of the carbon nanotube film structure10can be kept in the step S44, to electrically connect the plurality of patterned carbon nanotube film structures30together. Specifically, the plurality of patterned carbon nanotube film structures30suspended above the same strip-shaped recess26are connected in series, and the plurality of patterned carbon nanotube film structures30suspended above different strip-shaped recesses26are connected in parallel. Referring toFIG. 19, when the plurality of patterned carbon nanotube film structures30are powered by the direct current power source, the plurality of tip portions34can be illuminated and emit visible lights. Referring toFIG. 20andFIG. 21, the thermionic emission device is heated during the electrifying. The heating temperature and the voltage and power of the direct current have a linear relationship. The thermionic emission device can be heated to about 1860 K at a voltage of about 13.5 V and a power of about 0.586 mW.

The thermionic emission performance of the thermionic emission device is tested. The thermionic emission device100is arranged in a vacuum having the tip portion124of the patterned carbon nanotube film structure120facing an anode and a photodiode. The anode is connected to an ammeter. The ammeter is used to test the pulse current emitted from the tip portion124at the anode. The photodiode is used to test the brightness of the thermionic emission device100during the thermionic emission. The pulse voltage applied to the thermionic emission device100is about 14 V having a pulse frequency of about 20 Hz.FIG. 22has three sub-figures. The top figure ofFIG. 22shows the waveform of the pulse voltage. The middle figure ofFIG. 22shows the voltage-time curve of the photodiode changed with the pulse voltage. The bottom figure ofFIG. 22shows the current-time curve of the ammeter changed with the pulse voltage. As shown inFIG. 22, the pulse voltage, the thermionic current, and the brightness of the thermionic emission device100correspond to each other. Corresponding to the pulse voltage, the thermionic emission and the brightness of the thermionic emission device100change rapidly. When the voltage applied to the thermionic emission device100changed from 14 V to 0 V, which causes the cooling of the patterned carbon nanotube film structure120, the response speeds of the thermionic emission and the brightness are both less than 1 millisecond. When the voltage applied to the thermionic emission device100changed from 0 V to 14 V, which causes the heating of the patterned carbon nanotube film structure120, the response speeds of the thermionic emission and the brightness are both less than 10 milliseconds. The response speed of the thermionic emission device100to voltage change is fast, and thus can emit a thermionic pulse current.

It is to be understood that the above description and the claims drawn to a method may include some indication in reference to certain steps. However, the indication used is only to be viewed for identification purposes and not as a suggestion as to an order for the steps.

Finally, it is to be understood that the above-described embodiments are intended to illustrate rather than limit the disclosure. Variations may be made to the embodiments without departing from the spirit of the disclosure as claimed. Elements associated with any of the above embodiments are envisioned to be associated with any other embodiments. The above-described embodiments illustrate the scope of the disclosure but do not restrict the scope of the disclosure.