CN106190051A - Graphene laminated film with thermal conductivity - Google Patents

Abstract

A graphene laminate film is deposited or compressed on a polyethylene terephthalate Polyester (PET) substrate to determine physical parameters that affect thermal conduction. Thermal conductivity measurements of graphene laminate films were performed using a photothermal raman technique and a set of samples having graphene laminate thicknesses of 8 to 44 microns. The thermal conductivity of the graphene laminated film is between 40W/m.DEG and 90W/m.DEG at room temperature. It has been surprisingly found that the average size of the graphene sheets is more important than the mass density of the graphene stack in terms of a defined parameter of thermal conduction. The thermal conductivity and the average size in the deposited and compressed laminate film can be increased substantially linearly. The maximum thermal conductivity of the plastic material coated on the graphene laminated film can be increased to x 600 grade distance, and the method has a substantial significance.

Description

Thermally Conductive Graphene Laminated Film

technical field

The present invention relates to a thermally conductive graphene stack.

Background technique

Graphene is known to have high thermal conductivity [XX]. The first measurements of suspended monolayer graphene showed that the graphene has a thermal conductivity K value that is equal to 2000 W/mK at room temperature (RT) [X] far exceeding that of bulk graphite. Despite similar phonon dispersion and lattice detuning, graphene has a higher K value than graphite basal planes. Such interesting facts can be conducted into two-dimensional (2-D) lattices [X] by specific long-wavelength phonons. Long-wavelength phonons in 2D graphene have a mean free path (MFP) of a certain length, and even though the thermal conduction can be diffused [X], it is limited by the size of the sample. Under different conditions, this means that the individual diffusion of this phonon Umklapp is not sufficient to restore the thermal equilibrium of the 2D lattice approximately in the 3D lattice [X]. The latter leads to the anomalous thermal conductivity of few-layer graphene (FLG) which must depend on the number of atomic planes in the sample [X]. The logarithmic divergent prediction of the intrinsic thermal conductivity of graphene with sample size [X] has recently been found to be experimentally confirmed [X]. Other interesting features of phonon thermal conductivity in graphene include the non-monotonic dependence of graphene ribbon width [X], strong isotopic, and point-defect heat dissipation effects [X].

The excellent thermal conductivity of graphene with graphene flexibility can be stimulated by various stacked materials under investigation, in which graphene and its derivatives can be used as fillers [X]. Liquid phase exfoliated (LPE) graphene can be produced inexpensively and in large quantities. Proper mixing of liquid phase exfoliated graphene and FLG sheets can make it an excellent filler in thermal interface materials (TIMs) [XX] and thermal phase change materials (PCMs) [XX]. The use of graphene as a filler is more promising than carbon nanotubes (CNTs) due to the thermal coupling of graphene to the substrate and the lower cost of liquid-phase exfoliated graphene. The loading portion of graphene in the stack is relatively low, ie up to 10% of thermal interface materials (TIMs) [X] and up to 20% of phase change materials [X]. It is generally believed that the graphene filler in this material does not form a percolation network, and its heat can be partially conducted through the graphene sheet and the substrate.

A different type of graphene substrate with thermal applications is graphene stack (GL). The chemically generated graphene and few-layer graphene in graphene sheets can be tightly packed in overlapping structures. Depositing graphene sheets in polyethylene terephthalate polyester (PET) substrates [X] is very common. After being "sprayed on" the graphene laminate can be rolled. In view of the increasing practical demand for thermally conductive coatings on various plastic materials, studies on the thermal conductivity of graphene sheets have also increased. The physical thermal conductivity in this material makes the graphene sheets appear in arbitrary overlapping regions, large size and thickness distribution and other disadvantages. The thermally conductive properties of graphene stacks (GL) and the known material parameters that limit thermal conduction contribute to the practical application of graphene stack thermal coatings.

Contents of the invention

The main purpose of the present invention is to provide a graphene laminate with thermal conductivity.

In order to achieve the above object, a kind of graphene laminated film provided by the present invention comprises:

A graphene stack, which is deposited or compressed on a polyethylene terephthalate polyester (PET) substrate, the graphene stack includes single-layer, double-layer, and stacks of several-layer graphene sheets department.

The PET substrate is made of plastic material and has a thermal conductivity ranging from 0.15 W/m·degree to 0.24 W/m·degree.

The thickness of the graphene stack was determined by a cross-sectional scanning electron microscope (SEM), which was in the range of 8 microns to 44 microns.

The mass density of the graphene stack is between 1.0 g/cm 3 and 1.9 g/cm 3 , and the resistance of the graphene stack is in the range of 1.8 ohm to 6.1 ohm.

Description of drawings

The subject matter of the present invention can be explained in more detail using preferred embodiments as follows:

Fig. 1 is the schematic diagram under the cross-sectional scanning electron microscope (SEM) of sample No. 1, which shows graphene sheets on a polyethylene terephthalate polyester (PET) substrate, which has a thickness The top layer of about 44 microns, the stacked part, and the lower layer of PET substrate with a thickness of about 100 microns, where the thickness of the graphene sheets under the image shown by the electron microscope (SEM) is in different sizes;

Fig. 2 is the image of No. 3 sample (a) and No. 4 sample (b) under the scanning electron microscope (SEM), which respectively shows an uncompressed and compressed stack surface; wherein, graphene stack Layers include monolayers, bilayers, and stacks of several-layer graphene sheets, which are arranged in arbitrary shapes and orientations; bright areas show vertically aligned graphene sheets, which are located at the edges of graphene sheets in uncompressed samples, while The number of compressed samples is significantly reduced;

Figure 3 is considering the large number of graphene sheets, so its average sheet size to its total value is in a state of convergence, where it is represented by the first, fourth and sixth sample numbers, and the number of sheets after it exceeds 100 Converged to final values 1.10, 1.18; and 0.96;

Fig. 4 is an optical image diagram of a fixed frame for many thin slice samples used in photothermal Raman measurement. The graphene stack (GL-on PET) on the polyethylene terephthalate polyester (PET) substrate in the test (in the shape of a strip) is passed through a fixed groove, and acts as a heat sink. The two aluminum sheets of the sheet are fixed to each other. The strip-shaped sample is heated with a Raman laser. The experimental device is an enlarged version originally used to measure the thermal conductivity of graphene;

Figure 5 shows the relationship between the Raman G peak shift and the temperature calibration measurement of the second and third samples, wherein the G peak position of the sample can be linearly decreased by increasing the temperature;

Fig. 6 shows that the Raman G peak shift of the first and fourth samples is related to the laser power, wherein, by increasing the laser power, the positions of the G peaks of the first and fourth samples decrease linearly, and the power is The OPHIR power meter is used to measure the top surface of the sample, and the thermal conductivity of the sample can be obtained through the slope of the curve;

Fig. 7 shows that the thermal conductivity of the compressed and uncompressed samples is related to the average size of the graphene laminations, wherein, when the average size of the graphene laminations of the compressed and uncompressed samples increases, the thermal conductivity increases linearly, And the thermal conductivity of the compressed sample is higher than that of the uncompressed sample.

detailed description

The method of the present invention for investigating graphene stack films allows for the precise investigation of the thermal conductivity of graphene stacks on polyethylene terephthalate polyester (PET) substrates using a set of deposited and Compressed samples with different mass densities, the thickness of the graphene sheets ranged from 8 μm to 44 μm, where the measurements were made using the photothermal Raman technique [X], which was originally introduced to the micrometer scale In the suspended graphene sample [X], the sample was extended and applied to a suspended film. Regarding the sample preparation and material characteristics of the thermal conductivity investigation method of the graphene laminate film, the graphene laminate is deposited on a PET substrate. The sample groups are represented by "uncompressed" and "compressed". The PET substrate is a plastic material that can be used to manufacture various containers. The thermal conductivity of the PET substrate is in the range of 0.15 W/m· degree to 0.24 watts/meter degree. The low K value of PET substrate will limit its application. The thickness of this graphene stack (abbreviation GL layer) is determined by a section scanning electron microscope (SEM), which is between Range of 8 microns to 44 microns. Due to the non-uniformity of thickness, an average value between several locations was used in this analysis. The mass density of the GL layer is between 1.0 g/cm3 and 1.9 g/cubic centimeter. The resistance of each GL layer is measured in the interval of 1.8 ohms to 6.1 ohms. Figure 1 is an image of a graphene stack on a PET substrate under a scanning electron microscope, wherein the PET substrate and GL layer can be It is easy to be distinguished. The graphene laminated film is made of single-layer graphene and few-layer graphene with different sizes and shapes. Quantitative analysis of thermal conductivity must be based on the average graphene sheet size, measured accurately Statistical data. But in the case of large size and shape change, this measurement is not easy to carry out. Figure 2 is the image of the graphene stack sample on the uncompressed and compressed PET substrate under the scanning electron microscope. We have in Scanning Electron Microscopy studies were performed to determine the average graphene sheet size D, which is defined as the average minimum and maximum diameter per sheet. Measurements were made on more than a few hundred graphene sheets in each sample so that the accumulation could be accurately calculated Sufficient statistics for the D value. Figure 3 shows the convergence of the average flake size D value for each sample. Here it can be seen that the average size changes after the 50th graphene sheet in the analysis run until a well-defined Values. Listing I provides the names of the samples and their corresponding sheet sizes.

Regarding the measurement of thermal conductivity, this thermal study was carried out using a non-contact photothermal Raman method [X]. This is a direct-state measurement technique that directly determines the thermal conductivity without calculating it from the thermal diffusion data. In this technology, the thermal conductivity of graphene [X] can be measured initially, and the micro-Raman spectrometer can be used as a thermometer to judge the temperature rise in the field. The Raman laser can also be used as a heater. The measurement process consists of two steps: a calibration measurement and a power-dependent Raman measurement. The micro-Raman spectroscopy (Renishaw In Via) was performed using a 488nm laser and a power level of 1 mW to 10 mW. The graphene laminate samples on the PET substrate were cut into strips (ie, 3 cm in length and 1 cm in width) and hung on a specially designed sample holder (see FIG. 4 ). A large amount of aluminum sheet-holder acts as an ideal heat sink, ensuring good thermal contact with the GL layer.

Calibration measurements were performed with a cold-heat cell (LINKAM THMS-600) with controllable sample temperature. The equipment used for this measurement can control temperatures between -196°C and 600°C with a temperature stability of less than 0.1°C. The low excitation power of the Raman laser (ˉ1 mW) can be used to avoid laser-induced heating in situ. Since low excitation power levels reduce the signal-to-noise (S/N) ratio, we increased the exposure time to 10 seconds in order to obtain an acceptable S/N ratio. This measurement is repeated 3 times, and the temperature of each time is provided in order to obtain the average temperature value. The position of the Raman G peak obtained in this calibration measurement can be obtained as a function of the temperature of the sample. Figure 5 shows the spectral position of the G peak as a function of temperature T for two graphene laminate samples on uncompressed PET substrates in the interval from 20°C to 200°C. It can be noticed that the approximate values of the detection temperature range and the slope of the line for two similar samples meet the conditions of a better laser. The slope determines that the temperature of the Raman G peak of these samples is G≈-1.910-2CM-1/K. It must be remembered that this G value depends not only on the sample properties, but also on the temperature range in which it is acquired. The second part of this measurement was to record the Raman G peak position (see Figure 4) of the graphene stack sample on the PET substrate as a function of increasing excitation laser power. The power on the sample surface was measured with a power meter (OPHIR) replacing the sample. The absorbed power is determined by replacing the graphene stack sample on the PET substrate with a power meter positioned in the sample holder fixing groove. The measurement results show that most of the power will be absorbed by the sample, and only a small part (below 1%) is not absorbed because the laser beam leaks from both sides of the sample. An increase in the absorbed laser power P can lead to localized heating of the shifted Raman G peak. Figure 6 shows the measured G peak shift for condensed and non-condensed samples as a function of absorbed power. It can be noticed that two samples with different microstructures (compressed and non-compressed) have different temperature rise results under the same heating power. The slopes Δω/ΔP of the graphene laminate samples Nos. 1 to 6 on the PET substrate were measured as: -0.2451, -0.2255, -0.1521, -0.1776, -0.1766-0.1739 and -1/mW . It can be known that the geometric shape of the sample and the temperature rise T=G-1 correspond to the absorbed power P, so the thermal conductivity K can be determined by solving the value of the heat diffusion equation. The details of the K acquisition process are detailed in the method text below.

The measured RT thermal conductivities of different uncondensed and condensed graphene laminate samples on PET substrates are shown in Fig. 7 and summarized in Table II. A few interesting points that must be noted are that the overall thermal conductivity of this graphene-on-PET substrate laminate sample is as high as K≈40W/mK to -90W/mK. Considering that PET and related plastic materials have K≈0.15 W/mK to -0.24 W/mK, PET coated with a graphene stack can increase thermal conductivity over two orders of magnitude (×170−×600). The measured data show that high thermal conductivity can be achieved in both compressed and uncompressed samples. It can be seen from Fig. 7 that the K value of the uncompressed and compressed samples has a linear relationship with the average graphene sheet size D. We did not find a direct relationship between this K value and the mass density of the sample. All K values and K-value dependencies on graphene sheet size show that the thermal conduction of the GL layer is limited by graphene sheets, rather than by the characteristics of single-layer graphene and few-layer graphene. Compression results in better alignment of the graphene sheets or closer proximity to each other, resulting in higher K values for each size of graphene sheet. This conclusion can be supported by the analysis of the upper view of the SEM image, but it is suggested to use less vertically oriented graphene sheets in the compressed sample. The misaligned graphene sheets are shown as bright white dots (see Figure 2).

We found that when higher K values were obtained, this value was of practical significance for graphene laminate samples on uncompressed and compressed PET substrates. This means that even "spraying" on top of the graphene coating, without rolling or other processing steps, still helps to improve the thermal conductivity of the plastic material. New applications of plastic materials, for example when packaging or cladding electronic components, require high thermal conductivity. The latter is related to increasing the heat dissipation density of current electrons and optoelectronics. In a sense, graphene stacks have potential thermal coating effects. Different thicknesses of the GL layer can be flexibly changed to increase thermal conductivity through the coating layer.

Therefore, a method to investigate the thermal conductivity of thin graphene stacks consists of the following steps:

A) Prepare the graphene laminated film, in which, the aqueous dispersion of the graphene nanosheets, Grat ink 101 is provided by Bluestone Global Technology, as the coating ink in this research, and the PET film generally on top as a substrate for research. Graphene ink is coated on PET with a laboratory-specific groove coater (Taiwan, Shining Energy Company, model SECM02), and dried at 80°C for 10 minutes to form a PET substrate Graphene stack samples on . A rolling machine (Taiwan, Shining Energy Company, model SERP02) can be applied to the condensed sample. The sheet resistance of the graphene laminated film is measured at different total 10 points by a 4-point probe (UK, Jandel Company, model RM3000).

B). Measure the density of the sample, wherein the graphene laminate sample on the PET substrate and the PET substrate are stamped by a disc-shaped stamper (diameter 12mm). In order to achieve the average weight of the graphene coating, a total of 10 groups of samples with different weights were measured by a 5-digit analyzer (Mettler-Toledo, USA, model XS 105DU). The density, coating thickness, die diameter and weight of the graphene stack can be calculated.

C). Obtaining the thermal conductivity, where the Fourier equation applied to the specific sample geometry can be used to obtain the thermal conductivity. Since the thickness of the GL layer is significantly larger (8 m to 44 μm), the heat diffusion equation can be applied to three-dimensional (3-D) structures. We use formulations and numerical solutions with appropriate boundary conditions in the COMSOL package. The laser point heat source is assumed to have a Gaussian distribution of power P(X, Y, Z), which is set in the sample by the following formula:

$\mathrm{<span\; class="notranslate">\; P</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; x</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; the\; y</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; z</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; t</span>}\mathrm{<span\; class="notranslate">\; o</span>}\mathrm{<span\; class="notranslate">\; t</span>}}}{\mathrm{<span\; class="notranslate">\; 0.5</span>}\sqrt{{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}\mathrm{<span\; class="notranslate">\; \&sigma;</span>}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 3</span>}}}}\mathrm{<span\; class="notranslate">\; exp</span>}<span\; class="notranslate">\; (</span><span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; x</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; the\; y</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; z</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{\mathrm{<span\; class="notranslate">\; 2</span>}{\mathrm{<span\; class="notranslate">\; \&sigma;</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; m</span>}\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

where P _tot is the total absorbed power of the sample, and σ is the standard deviation of the Gaussian distribution function defined by the laser spot size. The full width at half maximum (FWHM) is between 0.5 microns, which is the radius of the laser source. The graphene stack strip sample suspended on the PET substrate was fixed to both ends of the heat sink, which was at room temperature. All other boundary conditions are defined by the environment, that is, the temperature gradient across the boundary is set to zero:

$\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; no</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; k</span>}<span\; class="notranslate">\; \&dtri;</span>\mathrm{<span\; class="notranslate">\; T</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; m</span>}\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

The heat diffusion equation can be solved by an iterative procedure. Then, enter the total power and thermal conductivity into the equation, and determine the simulation results of the temperature distribution. The simulated temperature rise can be compared with the measured temperature at the laser point. The thermal conductivity can be adjusted up or down according to the comparison result. This task can be simplified by importing the slope parameter:

$\mathrm{<span\; class="notranslate">\; \&theta;</span>}<span\; class="notranslate">\; =</span>\frac{<span\; class="notranslate">\; \&part;</span>\mathrm{<span\; class="notranslate">\; \&omega;</span>}}{<span\; class="notranslate">\; \&part;</span>\mathrm{<span\; class="notranslate">\; P</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; \&chi;</span>}\frac{<span\; class="notranslate">\; \&part;</span>\mathrm{<span\; class="notranslate">\; T</span>}}{<span\; class="notranslate">\; \&part;</span>\mathrm{<span\; class="notranslate">\; P</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; m</span>}\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>$

The simulation results of K and the value of θ provide the actual value of the thermal conductivity K as a slope measurement.

Claims (1)

1. a Graphene stack membrane, it is characterised in that comprise:

One Graphene lamination, it is deposition or is compressed in polyethylene terephthalate polyester (PET) base material On, this Graphene lamination includes monolayer, double-deck, and the storehouse portion of number layer graphene sheet, wherein

This PET base material is made up of plastic material and has between 0.15 watt/ meter Du to 0.24 watt/meter The pyroconductivity of degree；

The thickness of this Graphene lamination be by one profile scanning formula ultramicroscope (SEM) judge, its be between The scope of 8 microns to 44 microns；

The mass density of this Graphene lamination between 1.0 grams/cc to 1.9 grams/cc, and The resistance of this Graphene lamination is in the interval of 1.8 ohm to 6.1 ohm.