Abstract:
A method of forming a film is described. The method begins by forming a mixture including a polymer and a plurality of unordered nanomaterial. The film is dried and a plurality of pores is formed within the film. A sensitive film transducer capable of detecting changes in pressure and applied force can be made using this method.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to a film made of a polymer and nanomaterials. More particularly, this invention relates to the formation of pores in the film. Still more particularly, this invention relates to a pressure transducer made of the film that has a greater sensitivity because of pores in the film. 
       DESCRIPTION OF PRIOR ART 
       [0002]    There are many types of film sensors. These film sensors are commonly used in devices that measure acceleration, force, or pressure. However, commonly available film sensors typically have inherent problems such as thermal budget restrictions of the underlying substrates, mechanical integrity of the sensors on a flexible substrate, surface roughness of film substrates, signal routing, adhesion of metals to polyimide substrates, cross-talk between sensors, power supply requirements, high cost and unacceptable manufacturing requirements. 
         [0003]    Moreover, current flexible sensors often do not provide real-time feedback that can be used to adjust a stimulus, such as touch, force, acceleration and flow, or the operational characteristics of a device to compensate for the stimulus. Even if possible, the range of measurements that current sensors are able to measure lack the required sensitivity for certain applications, like health-care, automotive or consumer industries. Thus, currently available film sensors are not desirable for use in these industries. 
         [0004]    A particular problem with the prior art film sensors is that the films are rigid or semi-rigid. The rigidity of the films is often due to the fact that these film sensors are made of multiple layers. For example, commonly available force sensors are made using two layers of either polyester or polyimide. On each layer, a conductive material is applied, followed by a layer of pressure-sensitive ink. When the force sensor is unloaded, the resistance of the film is very high because the ink is separated by a gap between the layers. When a force is applied to the sensor, the resistance decreases as the distance between the inks on the layers decreases. As the sensor must be able revert to the original state after the application of a force, the choice of materials used to form the layers is restricted. This restriction in the type of material used for layers in turn limits the sensitivity of these types of sensors. 
         [0005]    Accordingly, there is a need in the art for a film that does not depend on such a two layer structure in order to provide a flexible, sensitive, non-intrusive, real-time, low cost, readily available feedback device to measure and assess acceleration, force and/or pressure. 
       SUMMARY OF INVENTION 
       [0006]    The above and other problems are solved and an advance in the art is made by a film comprising polymer, nanomaterials and pores; and a method in accordance with this film. A first advantage of a film in accordance with this invention is that the film contains pores that make the film more amenable to deformation. A second advantage of a film in accordance with this invention is that the conductivity of the film is improved. A third advantage of a film in accordance with this invention is that the use of the film improves detection or measurement at a lower pressure range. 
         [0007]    In accordance with embodiments of this invention, a method of forming a film having a polymer and nanomaterials is performed in the following manner. A polymer and unordered nanomaterials are mixed to form a mixture. The mixture is then dried in a process that will form pores within the film. In some embodiments of this invention, the nanomaterial is carbon nanotubes. In some of these embodiments, the carbon nanotubes are multi-walled carbon nanotubes. In others of these embodiments, the carbon nanotubes are single-walled carbon nanotubes. Other examples of nanomaterials that may be used include, but are not limited to, nanowires. In yet other embodiments of this invention, the polymer is electrically insulating and the nanomaterial is electrically conducting. 
         [0008]    In accordance with one of the embodiments of this invention, the mixture of polymer and nanomaterials further includes isopropyl alcohol. The isopropyl alcohol in the film is evaporated to form pores. These pores separate the carbon nanotubes in the film. In some embodiments of this invention, the drying process is aided by heating the film for a predetermined amount time at a predetermined temperature. In some particular embodiments, the film may be heated for between about 20 minutes to about 600 minutes and/ or at a temperature range of about 25 degrees Celsius to about 90 degrees Celsius. In yet some other embodiments in accordance with this invention, the film may be heated for approximately 20 minutes at a temperature of substantially 60 degrees Celsius. In still other embodiments in accordance with this invention, the film is heated under a specified pressure. In particular ones of these embodiments, the film may be heated under a pressure in a range from substantially 0.001 atm to substantially 1 atm. 
         [0009]    In some embodiments of this invention, the resulting film has a uniform thickness in the range of 100 nanometers to 1000 micrometers. In some other embodiments of this invention, the film is 0.5 mm thick. 
         [0010]    In some embodiments of this invention, the pores are each substantially 100 nanometers to 500 micrometers in size. The pores may be uniform or non-uniform in sizes. Typically, the density of the pores in the film is between 5 to 75 percent by volume. 
         [0011]    In some embodiments of this invention, characteristics of the film changes significantly in a pressure range of substantially 0.5 kPa to 100 kPa. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0012]    The above and other features and advantages of a film in accordance with this invention are described in the following Detailed Description and are shown in the following drawings: 
           [0013]      FIG. 1  illustrating cross sectional side view of an embodiment of a film in accordance with this invention without a force applied; 
           [0014]      FIG. 2  illustrating a magnified cross section side view of the embodiment shown in  FIG. 1 ; 
           [0015]      FIG. 3  illustrating a cross sectional side view of the embodiment shown in  FIG. 1  being deformed due to a force applied to the film; 
           [0016]      FIG. 4  illustrating a magnified cross section side view of the embodiment shown in  FIG. 3 ; 
           [0017]      FIG. 5  illustrating flowchart of an embodiment of a process for fabricating a film in accordance with this invention; and 
           [0018]      FIG. 6  illustrating experimental results from an embodiment of a pressure sensor compared in accordance with this invention compared to a prior art sensor. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    This invention relates to a film made of a polymer and nanomaterials. More particularly, this invention relates to the formation of pores in the film. Still more particularly, this invention relates to a pressure transducer made of the film that has a greater sensitivity because of pores in the film.  FIGS. 1-5  illustrate one particular embodiment of a pressure transducer in accordance with this invention, and  FIG. 6  illustrates the results in terms of sensitivity of such an embodiment. For clarity, a component shown in more than one figure has been given the same reference numeral in the different figures. 
         [0020]      FIG. 1  shows pressure sensor  100  including film  120  that is an exemplary embodiment of this invention. No force is applied to pressure sensor  100 . Film  120  is made of an electrically insulating polymer with electrically conducting carbon nanotubes  122 . Other examples of nanomaterials that may be used include, but are not limited to, nanowires. One example of an electrically insulating polymer is polydimethylsiloxane (PDMS). However, one skilled in the art will recognize that any other nanomaterials and any other electrically insulating polymer can also be used without departing from this invention. 
         [0021]    In the described embodiment, carbon nanotubes  122  are tubular forms of carbon and can have diameters from a few nanometers and lengths up to a few millimetres. The length-to-diameter ratios of carbon nanotubes  122  can be as large as 28,000,000:1 and are a hundred times stronger than steel at one-sixth of its weight. Carbon nanotubes  122  may be any one of many different forms, including, but not limited to, single-wall carbon nanotubes and multi-wall carbon nanotubes. 
         [0022]    An inset view  130  of the film  120  shows carbon nanotubes  122  dispersed throughout the film in a random unordered fashion. For purposes of this description, “unordered” means that the carbon nanotubes  122  are not arranged in any particular way and do not form any consistent or repetitive patterns. The concentration of carbon nanotubes  122  can be in the range of 4 to 12 mass percent of the polymer. The higher the concentration of carbon nanotubes  122  in film  120  will increase the sensitivity of pressure sensor  100 . Likewise, the lower the concentration of carbon nanotubes  122  in film  120  will decrease the sensitivity of pressure sensor  100 . In one of the preferred embodiments, the concentration of carbon nanotubes  122  is in the range of 4 to 8 mass percent of the polymer. This range of 4 to 8 mass percent gives an optimum performance for pressure sensor  100 . 
         [0023]    Pores  121  are formed within film  120 . Pores  121  are spaces or voids within film  120 . Pores  121  may contain air or gas. The exact contents of pores  121  are not important in accordance with this invention and are omitted for brevity. Carbon nanotubes  122  are physically separated from one another by pores  121  as is clearly shown in  FIG. 2 . The separation between carbon nanotubes  122  increases the resistivity of film  120 . Further, pores  121  lower the mechanical properties of film  120 , relative to a pore-free composite. Thus, film  120  is amenable to deformation. Hence, pores  121  improve detection and measurement capabilities of film  120 , particularly at lower pressure ranges. Further details regarding pores  121  are disclosed later in the specification. 
         [0024]    Contacts  105  and  110  are in contact with film  120 . These contacts can be made of metal, although any material that conducts electricity can be used. Sputtering is often used to lay these contacts on a film or surface. Other deposition techniques can also be used to lay metals or other electrically conducting materials on the film. These can be combined with masking of the film to achieve ideal shapes on the film. 
         [0025]    Ammeter  140  is representative of circuitry in pressure sensor  100  that measures the amount of current flowing through pressure sensor  100 . The exact configuration of the circuitry is not important in accordance with this invention and is omitted for brevity. Since no force is being applied to pressure sensor  100 , ammeter  140  records a low current due to film  120  having a high resistance. 
         [0026]      FIG. 3  shows film  120  as force  350  is applied to film  120 . Force  350  may be any type of force including a force resulting from pressure, physical impact, or any other action. As force  350  is applied, film  120  deforms. Inside film  120 , pores  121  are compacted by the force. The compaction of pores  121  allows film  120  to be more flexible and deform more, relative to a film without pores. Furthermore, as pores  121  are compacted, spacing between carbon nanotubes  122  decreases and, in turn, increases the conductivity of carbon nanotubes  122  and decreases the resistivity of film  120 . 
         [0027]      FIG. 4  shows an inset view of  230  which highlights the deformation of the pores  121 . Several carbon nanotubes  122  are shown in contact with each other. Thus, a percolating path between the contacts  105  and  110  is formed by carbon nanotubes  122 . The percolating path lowers the resistance of the film overall and such an effect can be charted and seen in  FIG. 6 . 
         [0028]    The film in accordance with this invention can be used as a pressure sensing devices. The film can also be used in a number of applications where there is a need to detect pressure or forces. Such applications may be applied in products, including, but not limited to, biomedical products and automation systems. One such application is the incorporation of film  120  into flexible mats to sense pressure distribution across the feet of a person for gait analysis. In some embodiments, film  120  may also be used as sensors to transduce pressure to electrical signals to control systems. One skilled in the art will recognize that the uses of film  120  described herein are meant to be illustrative purposes and film  120  may be used in a variety of applications that require a pressure sensor without departing from this invention. 
         [0029]      FIG. 5  shows an exemplary process  500  for fabricating film  120  in accordance with one embodiment of this invention. Process begins in step  502  by mixing in carbon nanotubes  122  into the polymer to form a mixture. The mixing can be done via manual stirring, magnetic stirring as well as ultrasonic agitation. The carbon nanotubes are commercially available carbon nanotubes. Film  120  is formed by a variety of coating processes such as spin coating, dip coating and screen printing in step  503 . Typically, film  120  is of a uniform thickness in the range of  100  nanometers to 1000 micrometers. For example, film  120  is proximately 0.5 mm in one preferred embodiment. 
         [0030]    After step  503 , film  120  undergoes drying, which can be aided by an oven in step  504 . In one embodiment, film  120  is heated by an oven for between about 20 minutes to about 600 minutes at a temperature range of approximately 25 degrees Celsius to approximately 90 degrees Celsius. In one preferred embodiment, film  120  is heated by an oven for 120 minutes at a temperature of 60 degrees Celsius. Further, film  120  may be heated under a pressure range of substantially 0.001 atm to substantially 1 atm. Pores  121  are formed at this stage by degradation of a gas forming agent. Alternatively, prior to the annealing in oven, the pores can be formed by placing the film through a degassing step. This step causes dissolved gases to coalesce to form pores. Typically, pores  121  formed have sizes which range from substantially 100 nanometers to substantially 500 micrometers. One skilled in the art will recognise that pores  121  may be uniform or non-uniform in sizes and the exact sizes may a design choice controlled by varying the process. In addition, the density of pores  121  in film  120  is between preferably, approximately 5 to approximately 75 percent by volume. The characteristics of film  120  changes significantly in pressure range of substantially 0.5 kPa to 100 kPa. 
         [0031]    The contacts are then added to form the pressure transducer in accordance to normal processes for adding such contacts in step  505 . 
         [0032]    The above is a description of one embodiment of a transducer in accordance with this invention. It is envisioned that those skilled in the art can and will design alternative embodiment of this invention that infringe on this invention as set forth in the followings claims.