Flexible Film Heaters Based On Silver Nanowires And Polydimethylsiloxane Composites

Transparent conductive films are commonly used in daily life.And they can be used in various fields including flexible solar cells,touch displays,electronic skins,nano-generators and smart windows.Simultaneously,the flexible conductive film is an important temperature regulator as a resistance heater.As a new application,the resistance heaters are expected to be wearable thermal therapy devices combined with human body.Wearable hyperthermia is widely used as an effective physical therapy to alleviate pain caused by joint damage and improve local blood circulation.In order to meet the special needs of the human body,we should pay more attention to the structure design of flexible heaters.First of all,considering that the heating performance will be affected by movement when the heater covering the joint of body,heaters should keep a good fit with the human body.Meanwhile,it requires a good mechanical tensile properties as well.In addition,the accumulation of sweat is not conducive to the health of the human body,so the breathability of the device is also one of the factors to be considered in the preparation of the film.In this paper,two kinds of flexible conductive films are designed for wearable heaters.Both the two structures combined with the advantages of flexibility of polydimethylsiloxane(PDMS)and the excellent conductivity of silver nanowires(Ag NWs).The sandwich structure conductive film prepared ensures the flexibility and efficient thermal conductivity.On the basis of flexibility,we prepared the 3D network structure of silver nanowires coated by polydimethylsiloxane(PDMS)to ensure the breathability of the device.The main research contents are as following:(1)The preparation and properties of sandwich structure flexible conductive film.In this chapter,a flexible conductive film with a sandwich structure was prepared by a simple spin coating method for the heaters.The silver nanowires are sandwiched between two layers of PDMS films.Such structure not only ensure that the sample has an excellent mechanical stretching properties but also protects the conductive silver nanowires from the influence of external environment.In this experiment,the surface resistance and transparency of the flexible conductive film were controlled by the amount of spin coating of the silver nanowire suspension.When the surface resistance of the film is reduced from 30.0Ω/sq to 5.4Ω/sq,the transparency is reduced from 70%to 46%at a wavelength of 600 nm.The transparency of the film is positively correlated with its resistance.In addition,the flexible film of the sandwich structure can reach a maximum stretch of 200%,and the device can almost return to the original state after1000 fatigue tests with a stretch of 50%.During the heating test in the experiment,the equilibrium temperature can reach 81℃at a constant voltage of 3.0 V.At the same time,in order to simulate the actual heating performance of the equipment,the conductive film of the sandwich structure was covered on a piece of pork in this experiment,and then measure the change of temperature at 1 mm under the skin,and it was found that the highest temperature could be raised by 12℃.In addition,the heating performance of the device was further fitting analyzed,we found that the energy conversion efficiency of the heater(the electrical energy converted to the surface energy of the device)decreased with the increase of the temperature of the device.The maximum conversion efficiency of the device can reach 60%under the condition of an external input voltage of 3 V.(2)A flexible breathable film heater based on a 3D conductive network.In the experiment,a flexible conductive film with a 3D conductive network structure was prepared by spin coating and chemical etching method with the foamed nickel as a template.The film can pass through various liquid vapor and has a permeability for deionized water of 7 mg/cm~2·h at 45℃.The flexible gas breathable conductive film can be heated from room temperature(23℃)to 67℃at a low voltage of 3.5 V,and its thermal resistance is 142.5℃·cm~2/W.The film heater was tested for mechanical flexibility,and the resistance of the heater remained substantially unchanged after 1000cycles of stretching at 10%elongation.At the same time,the sample was subjected to a bending test under heating,and the equilibrium temperature of the heater was maintained at a stable value during the bending process of 100 times.At the same time,in order to ensure the practical application of the film heater in the human body,the change of heating performance is also worth considering in the stretched state.The experimental results show that the steady-state temperature can still maintain 74%under the condition of 20%stretch.