Dielectric,AlN/epoxy,and,SiC/epoxy,composites,with,enhanced,thermal,and,dynamic,mechanical,properties,at,low,temperatures

Zhengrong Zhou ,Rongjin Hung,＊ ,Huiming Liu ,Ylin Zho ,Zhicong Mio ,Zhixiong Wu ,Wnyin Zho,Chunjun Hung,Lifeng Li

a State Key Laboratory of Technologies in Space Cryogenic Propellants,Technical Institute of Physics and Chemistry,Chinese Academy of Sciences,Beijing,100190,China

b Center of Materials Science and Optoelectronics Engineering,University of Chinese Academy of Sciences,Beijing,100049,China

Keywords:Thermal properties Epoxy composites Low temperatures

ABSTRACT In a superconducting magnet,the thermal properties of the insulating material at low temperatures is an important technical index to ensure the normal operation of a superconducting coil.Herein,a low-temperature thermal conductivity testing device with a tiny uncertainty was developed to measure the composites.Furthermore,SiC/epoxy and AlN/epoxy composites were prepared and their properties were tested at low temperatures.The results showed that the composites have a better thermal conductivity at low temperatures than pure epoxy.In addition,the AlN/epoxy and SiC/epoxy composites have excellent dynamic mechanical properties and low coefficient of thermal expansion.Due to the fair thermal and dynamic mechanical properties,the composites will have potential application in the superconducting magnet.

In a large superconducting magnet,polymer-based insulating material with good thermal properties is an important guarantee for normal operation.It should be emphasized that the composites used in a superconducting magnet are desired to have high thermal conductivity at low temperatures.Materials with low thermal conductivity have been difficult to meet the heat transfer requirements of the magnet system.Furthermore,the thermal conductivity of polymer materials will decrease with the decrease of temperature.Therefore,it is imperative to research and develop practical composites with high thermal conductivity at low temperatures [1].

The thermal conductivity of composites is generally not fixed and will be affected by many factors such as the type of matrix and filler,the interfacial thermal resistance,and internal defects.Even the uniformity of the distribution of fillers inside the matrix,as well as the preparation and processing technology will affect the thermal conductivity of composites [2].Therefore,there is no accurate and widely applicable theoretical formula to calculate the thermal conductivity of composite materials[3].The testing devices are still the basic way to obtain thermal conductivity [4,5].Furthermore,their thermal conductivity values vary greatly for different materials or materials at different temperatures.As a result,there is currently no measuring device that can simultaneously satisfy such complex test conditions and high requirements.Therefore,an experimental system that could continuously measure the thermal conductivity of composites from 60 K to 300 K was built in this paper.Through standard sample measurement,it was proved that the device has a tiny uncertainty.

Because of its excellent insulating properties,chemical stability,mechanical properties,and easy processing,epoxy resin has incomparable and irreplaceable advantages in the insulating applications of superconducting equipment and magnet [6–8].However,epoxy is generally a poor conductor of heat,and its thermal conductivity is generally about 0.2 W/(m⋅K) at room temperature [9,10].The most common method to improve the heat transfer performance of epoxy is introducing thermal conductive filler into the epoxy matrix and composite demonstrates low cost as well as flexible design.As a commonly used packaging material,thermally conductive epoxy composites play an increasingly important role in manufacturing and high-tech fields such as appliances,microelectronics,superconducting devices,and aerospace[11–13].By filling epoxy with various fillers,the composites combine the superior chemical and physical properties of both epoxy and fillers[14–16].Many factors affect the thermal conductivity of composites,such as the type,shape,particle size,filling ratio,and dispersity of the filler in the epoxy matrix [17–20].At present,aluminum nitride (AlN)[21,22],alumina (Al2O3) [23],silicon carbide (SiC) [24,25],boron nitride (BN) [26–29] with high intrinsic thermal conductivity and good insulation performance are the focus of research as the filler.Moreover,the thermal conductivity of the inorganic fillers such as SiC and AlN will increase with the decrease of temperature in the temperature range of 100 K–300 K[30,31].In addition,the thermal conductivity,density,and heat capacity of SiC and AlN are similar.By comparing the thermal conductivity values of their composites,the accuracy of the test device is further verified.At the same time,the advantages and disadvantages of the two types of composite materials in various aspects can be compared.Therefore,SiC/epoxy and AlN/epoxy composites were prepared and their properties were tested in this paper.The measurement results showed that the SiC/epoxy and AlN/epoxy composites had a high thermal conductivity,i.e.0.6527 W/(m⋅K),0.6556 W/(m⋅K),respectively,at room temperature at 60 wt%.Compared with pure epoxy,the thermal conductivity of the AlN/epoxy or the SiC/epoxy composites with a ratio of 60 wt% is increased by about 300% at room temperature.And,the composites at low temperatures still exhibited better thermal properties than the pure epoxy.While the thermal conductivity of the composites was improved obviously,they had good dynamic mechanical properties(DMP),and a small coefficient of thermal expansion (CTE).The composites with fair thermal properties and electrical insulation properties can be used in the high-temperature superconducting coil.

2.1.Materials and preparation

Silicon carbide(SiC,99.9%)and aluminum nitride(AlN,99.9%)were purchased from Guangzhou NanoChem Technology Co.Ltd.,Guangzhou,China,and the average diameter of SiC and AlN particles was about 4 μm.The epoxy monomer (DGEBF) and the curing agent (DETD) were obtained from Zhangjiagang Yarui Chemical Co.Ltd.,Zhangjiagang,China.

The fabrication process of the SiC/epoxy and AlN/epoxy composites is illustrated schematically in Fig.1.Firstly,AlN and SiC fillers were dried at 80°C for 48 h.After that,the epoxy monomer was added to the beaker,followed by adding the curing agent.The mass ratio of DGEBF to DETD was 100 to 24.After mixing for 10 min,SiC and AlN were respectively filled into the epoxy matrix and stirred at a high speed at 60°C for 4 h.Meanwhile,because the dissolved air bubbles in the mixture would cause cracks,which would affect the performance of the sample,a vacuum rotary pump was used to degas the clarified homogeneous solution.Finally,the composites were cured at 80°C for 6 h and 150°C for 3 h to prepare the SiC/epoxy and AlN/epoxy composites with weight ratios of 20%,40%,and 60%,respectively.To facilitate the introduction of composites,different composites are marked.For example,the composites with 20% AlN filling are abbreviated as AlN-20/epoxy,and other composites are used in the same way.

2.2.Characterization

A digital camera(Canon 650D)was used to take optical photographs of the pure epoxy and the composites.The patterns of the AlN and SiC were characterized by the X-ray diffraction (XRD;D8 focus),Fourier transform infrared spectroscopy (Excalibur 3100),and Raman spectroscopy (inVia-Reflex).The microscopic images of the SiC,AlN,and composites were obtained from the Hitachi S-4800 field emission scanning electron microscope (SEM) and the JEM-2100F transmission electron microscopy(TEM).The DSC 404F3 instrument measured glass-transition temperatures(Tg)under the nitrogen atmosphere in the range of 30–200°C at a heating rate of 5°C/min.The DMA Q800 dynamic mechanical analyzer performed the dynamic mechanical properties at a frequency of 1 Hz,and the temperature range was from -100°C to 180°C with a heating rate of 2°C/min.The DMP was measured using a double cantilever beam with an amplitude of 20 μm.The sample length was about 60 mm,the thickness was about 2 mm,the width was about 12.5 mm.The strain gauge technique was used to measure the linear thermal expansion(△L/L)over the temperature range from-150°C to 50°C and the CTE was calculated as the slope of the △L/L-Temperature curve.The samples were 20 mm in diameter and coated with silver on both sides of the surface for electrical measurement.Dielectric permittivity (ε) and dielectric loss tangent (tan δ) was measured by an impedance analyzer(Agilent E4980A) in a frequency from 102to 106Hz.The mechanical measurements of the composites at room temperature were taken on a SANS/CMT4304 universal testing machine with a crosshead speed of 2 mm/min.

2.3.Experimental system and test method of thermal conductivity

To measure the thermal conductivity of the composites at low temperatures,an experimental system for the thermal conductivity of solid materials (ESTC) was built,as shown in Fig.2.Based on the onedimensional axial steady-state heat flow method,a system was constructed to continuously measure the thermal conductivity of solid materials in the range of 60–300 K with a micro pulse tube refrigerator as the cold source.The measuring method was the two-probe method(Fig.3a),suitable for measuring the sample with small thermal conductivity.In cartesian coordinates,when heat flows in and out of a sample in a specific direction(set as thez-axis,the positive direction of thez-axis in the direction indicated by the red translucent arrow in Fig.3c),the Fourier law is simplified as follows:

Where,Qis the heat flux(W/m2),which represents the heat(J)passing through a unit area per unit time.Ais the cross-sectional area(m2)of the specimen.Pis the power(W),which represents the heat flowing through the specimen section per unit of time.And in the actual calculation,Pis the power of the heat source.Tis the absolute temperature of the sample,andgradTis the temperature gradient (K/m).λ is the thermal conductivity(W/(m⋅K)),which is a parameter reflecting the heat transfer rate of the sample.Equation (1) is the principle of the one-dimensional axial steady-state heat flow method.

Fig.1.Fabrication process of the SiC/epoxy and AlN/epoxy composites.

Fig.2.Scheme of removable apparatus for the rapid measurement system.

Fig.3.(a)Layout method of temperature sensor and sample:the form of two probes.(b)Pressure curve in the cryogenic vacuum chamber after opening the vacuum pump.(c)Temperature curve of the cold head and hot head of the refrigerator,cold end,and hot end of the sample.(d)The measurement values and standard values of the 304L stainless steel standard sample.

Herein,the sample to be tested requires isotropy.Therefore,the thermal conductivity (λ) was calculated by the equation:whereQzis the heat flux,Lis the length of the sample,Ais the crosssectional area of the sample,and ΔTis the temperature difference.The measurement procedure is shown below.Firstly,the sample is placed in a cryogenic adiabatic vacuum chamber.After that,one end of the sample shall be in good thermal contact with the sample stage as a cold source,and the other end of the sample is connected to a thin copper sheet that acts as a heat source.The sample is connected to a thin copper sheet and a sample table utilizing silver glue.Then,the vacuum degree of the cryogenic vacuum chamber is reduced to less than 10-3Pa through a molecular pump unit,as shown in Fig.3b.When operating,a stable heat flow along the axial direction (the direction indicated by the red translucent arrow in Fig.2a)of the sample passed through the sample.Finally,while the heat transfer process reaches a steady state,the thermal conductivity of the sample can be calculated[32].

To guarantee that the heat flow can be transferred along the axial direction of the sample,and to reduce the heat leakage and ensure the accuracy of the measurement,the following measures were taken.Firstly,minimize the diameter of various leads to prevent heat transfer in other directions.Secondly,ensure that the vacuum degree of the test chamber is less than 10-3Pa to reduce convective heat transfer.Thirdly,gold plating on the shield around the sample to reduce radiation heat transfer.

It can be seen from the cooling curve (Fig.3c) that the lowest temperature of the sample could be stabilized at about 55 K.The temperature of the hot head of the refrigerator was stable at about 308 K during operation,and there would be no motor overheating.The heating resistor sheet fixed on the cold finger of the refrigerator acts as a compensation heat source.The temperature of the sample stage is controlled by the Proportional-Integral-Derivative (PID) temperature control element,including maintaining a constant temperature and regulating the temperature.The thermal conductivity of the stainless steel 304L from 60 K to 300 K was measured and compared with the standard data[33].It was found that the measured data were in good agreement with the reference data,and the repeatability of repeated measurement results was good,as shown in Fig.3d.There was very little deviation between the measured data and the standard data,which may result from the interface thermal resistance between sample and sample stage,heat loss and instrument error,etc.These results showed that the device has good repeatability and high precision.

3.1.Fabrication

The chemical constituents of the prepared AlN,SiC,and its composites were determined.Firstly,the crystalline structure of the AlN and SiC was revealed by the X-ray diffraction(XRD)spectra,as presented in Fig.4a.The XRD spectrum of the SiC shows the sharpest peak at 2θ=35.626°,which corresponds to the lattice plane of the Miller index[006],and thed-spacing between layers was 0.2518 nm.The XRD pattern also shows other sharp and strong peaks at 2θ=59.977°and 38.133°corresponding to the Miller indices [108] and [103] of the lattice planes,respectively.Then,the calculated lattice constant of the hexagonal closepacked (HCP) unit cell is 3.081 × 3.081 × 15.12 Å (90 × 90 × 120),which conforms to the standard value of 6H–SiC(P63mc,JCPDS card no.49–1428).Similarly,the sharpest peak in the XRD spectrum of the AlN appears at 2θ=33.216°,which corresponds to the lattice plane with the Miller index [100],and the interlayerd-spacing was 0.2695 nm.Other sharp and strong peaks at 2θ=37.916°and 36.040°correspond to the Miller indices[101]and[002]of the lattice plane,respectively.Then,it was calculated that the lattice constant of the HCP cell was 3.111 ×3.111× 4.979 Å (90 ×90 ×120) Å,which is in line with the standard value of AlN(P63mc,JCPDS card no.25–1133).The Raman spectroscopy of the samples was shown in Fig.4b.The peaks of the SiC appear at 787 and 968 cm-1,respectively,corresponding to the absorption bands of 6H–SiC at point G [34].787 and 968 cm-1represented the transversal optic and longitudinal optic mode Si–C vibrations,respectively [35].Also,a shoulder could be observed at 768 cm-1,which can be a defect[36,37].Similarly,the peaks of the AlN were observed at 611 and 657 cm-1corresponding to the phonon at the Γ point of AlN,which are related to the Polar optical mode (A1(TO)) and the Non-polar optical mode (E22),respectively [38].The Raman spectroscopy implies the monocrystal structure of SiC and AlN.Fig.4c and d exhibit the high-resolution X-ray photoelectron spectroscopy (XPS) spectra of the SiC and the AlN,respectively.AlN shows the existence of Al 2p,Al 2s,C 1s,N 1s,and O 1s.The N 1s and Ali 2p peaks of the XPS spectra of the AlN were identified at 397 eV and 74 eV(Fig.4c),respectively[39].SiC also shows the existence of Si 2p,Si 2s,C 1s,and O 1s.The C 1s and Si 2p peaks of the XPS spectra of the SiC were identified at 283 eV and 100 eV(Fig.4d),respectively[40,41].

Fig.4.(a)X-ray diffraction(XRD)spectra of the AlN and SiC.(b)Raman spectra of the AlN and SiC.High-resolution X-ray photoelectron spectroscopy(XPS)spectra of the SiC (c) and AlN (d).

To compare the performance of subsequent composites,SiC and AlN crystals with similar particle sizes and shapes were selected.The microstructure of the AlN and SiC particles was provided by the TEM and SEM,as shown in Fig.5.In Fig.5,the TEM image,the SEM image,the high-resolution TEM (HRTEM) image,and its corresponding selected area electron diffraction(SAED) pattern show the irregular shapes with～4 μm diameter(Fig.5d and k)and the single-crystal structure of the SiC and AlN.Fig.4c is a fast Fourier transform(FFT)diffraction pattern of the AlN particle,and the diffraction spot is attributed to the AlN phase with a lattice fringe spacing of 0.2695 nm between adjacent atomic layers(Fig.5b).The mapping of Fig.5e and f,and the energy dispersive spectrum(EDS)of Fig.5g illustrate that the particle(Fig.5a)consists mainly of N and Al.Similarly,Fig.5j is the FFT diffraction patterns,whose diffraction spots are ascribed to the 6H–SiC phase with a lattice fringe spacing of 0.2518 nm between the adjacent atomic layers (Fig.5i).The mapping of Fig.5l,n,and the EDS of Fig.5m illustrate that the particle(Fig.5h)consists mainly of C and Si.

Fig.6a and Fig.6h are optical images of the pure epoxy and composites.As can be seen from Fig.6a,the color of pure epoxy is bright yellow.but with the increasing content of the SiC,the color gradually changes from yellow to dark green.And,the color gradually changes from yellow to cream-colored with the increasing content of AlN,as shown in Fig.6h.Fig.6b,c,and Fig.6d are the SEM image and mapping of SiC/epoxy composites.Taking SiC-60/epoxy for example,SiC can be observed in the corresponding SEM images.The mapping of Fig.6c and d shows that the SiC was uniformly dispersed in the epoxy matrix.Fig.6e is the SEM image of the fractured surface of pure epoxy.The mapping of pure epoxy shows only the presence of C and O,as shown in Fig.6f and g.Similarly,Fig.6i,j,and Fig.6k are the SEM image and mapping of the AlN-20/epoxy composites.Fine AlN particles can be found in the matrix in Fig.6f.The mapping of Fig.6c and d shows that the AlN is uniformly dispersed in the matrix.In conclusion,the AlN/epoxy and SiC/epoxy composites were successfully prepared.And the fillers were well evenly distributed in the epoxy resin matrix,and the characteristics were crucial to effectively improve the thermal conductivity of composites,which will be discussed below.

3.2.Thermal conductivity of the AlN/epoxy and SiC/epoxy composites

The thermal conductivity of epoxy and its composites from room temperature to liquid nitrogen temperature was measured using the above experimental system of thermal conductivity,and the results are shown in Fig.7.Fig.7a shows thermal conductivities with different samples at room temperature.The thermal conductivity of the pure epoxy is only 0.218 W/(m⋅K)at room temperature.However,the thermal conductivity of epoxy composites steadily increased with the increase of the filler amount.The thermal conductivity of the SiC-20/epoxy composites was 0.354 W/(m⋅K),SiC-40/epoxy composites increased to 0.534 W/(m⋅K),and SiC-60/epoxy composites was further improved to 0.656 W/(m⋅K).Also,AlN/epoxy composites had high thermal conductivity values (0.389,0.572,0.653 W/(m⋅K)) at 20,40,60 wt% filler,respectively.The thermal conductivity of AlN/epoxy composites with a low filling ratio was slightly lower than that of SiC/epoxy composites with the same filling ratio,but the thermal conductivity of AlN/epoxy composites with 60 wt%was very close to that of SiC/epoxy composites.The two composites with the same filling ratio had similar thermal conductivity because of the similar intrinsic thermal conductivity of the fillers and the same blending mode.To further elucidate the extent of improvement,Table 1 summarizes previously reported thermal conductivity and thermal conductivity enhancement (TCE,the percent of thermal conductive increase of composites to the corresponding polymer matrix).The factor (TCE) could be regarded as a reference value to estimate thermal conductivity enhancement efficiency.As can be seen in Table 1,our composites show a fair TCE among the reported polymer composites with similar addition of fillers.This result indicates that AlN and SiC as highly efficient Thermo conductive fillers have a certain effect in terms of enhancing the heat-transfer capability of polymer composites.

Table 1 Thermal conductivity and Thermal conductivity enhancement (TCE) in various composites.

Meanwhile,the Agari model(a semi-empirical model)was used to fit the thermal conductivity of the composites [42].The model simulating the thermal conductivity of the composites obtained through numerical simulation and theoretical derivation is thus called the semi-empirical model,among which the classic model is the Agari model[42]:

Fig.5.TEM(a)and SEM(d)images of the AlN.(b)HRTEM image of AlN.(c)The fast Fourier transform(FFT)diffraction pattern of the AlN.The carbon(red)(e)and silicon(green)(f)mapping and its energy dispersive spectrum(EDS)results(g)of the AlN.TEM(h)and SEM(k)image of the SiC.(i)HRTEM image of SiC.(j)The FFT diffraction pattern of the SiC.The carbon (red) (l) and silicon (green) (n) mapping and its EDS results (m) of the SiC.

Fig.6.(a)Optical images of pure epoxy and SiC/epoxy composites.(b)SEM image of SiC/epoxy composites.The silicon(green)(c)and carbon(red)(d)mapping of SiC/epoxy composites. (e) SEM image of pure epoxy.The carbon (red) (f) and oxygen (yellow) (g) mapping of AlN/epoxy composites. (h) Optical images of pure Epoxy and AlN/epoxy composites. (i) SEM image of SiC/epoxy composites.The aluminum (red) (j) and nitrogen (solferino) (k) mapping of AlN/epoxy composites.

Fig.7.(a) Thermal conductivity of the AlN/epoxy and the SiC/epoxy composites at room temperature. (b) Thermal conductivity of different samples at low temperatures. (c) Thermal conductivity of pure epoxy and its composites from 60 to 300 K.

In Equation (2),C1andC2represent the contributing factors of particles to the formation of the thermal conduction chain inside the material,which are generally greater than 0 and less than 1.The model is suitable for multiphase systems and can be used to predict the thermal conductivity of composites filled with various types of particles without the restriction of temperature and filler ratio.The fitting formulas of the two composites at room temperature were respectively obtained,as shown in Fig.7a,whereKis the thermal conductivity of the composites andfis the mass filling amount.In the fitting formulas,the thermal conductivity of the AlN is assumed to be 100 W/(m⋅K),and the thermal conductivity of the SiC is assumed to be 120 W/(m⋅K).It can be seen from Fig.7a that the measured thermal conductivity of the composites was smaller than its calculated value when the filling amount was 60 wt%,which may be caused by the uneven dispersion of the filler.

Fig.7b presents the curve of thermal conductivity of the epoxy and its composites varying with temperature.Firstly,the thermal conductivity of the pure epoxy was compared with that of the reference,and it was found that the thermal conductivity of the epoxy measured around 270 K was in good agreement with that of the reference [43].The variation range of thermal conductivity of the pure epoxy with temperature is larger than that of the reference,which may be caused by the different types of epoxy,measurement methods,and corresponding measurement accuracy.The thermal conductivity of the composites tended to decrease with the decrease of temperature,as shown in Fig.7b,whereas the composites still maintained a high thermal conductivity at room temperatures.It is worth noting that the thermal conductivity of the composites with 60 wt%filler was 300%of that of the pure epoxy at room temperature,but the thermal conductivity of the composites was 500% of that of the pure epoxy at 150 K.This result shows that the composites have more outstanding heat transfer ability at low temperatures.Although the thermal conductivity of the pure epoxy decreased with the decrease of temperature,the thermal conductivity of two inorganic fillers,silicon carbide and aluminum nitride,increased with the decrease oftemperature in the temperature range of 100 K–300 K[30,31].This may be attributed to the thermal resistance caused by phonon scattering and the fact that the impurity scattering in inorganic filler decreased rapidly in this temperature region,whereas the thermal resistance caused by boundary scattering did not increase much.Therefore,with the decrease of temperature,the thermal conductivity of the composites decreased to a smaller extent than that of the pure epoxy.

The CTE of pure epoxy is much higher than that of copper [44,45].The CTE between the sample and the sample stage did not match,resulting in the sample falling off during the cooling down process.(Therefore,the thermal conductivity measurement values of the epoxy and composites with a low filling ratio below 150 K may be inaccurate.However,for the composites with a filling ratio of 60 wt%,the thermal conductivity showed a continuous decrease without a significant decrease at about 150 K,and the sample falling off did not occur during the sampling,as shown in Fig.7c.Because the CTE of the composites decreased with the increase of the filler (Fig.8d),the CTE of the composites with 60 wt% filler was close to that of the copper sample table(i.e.16.76×10-6K-1at 300 K)[44].Therefore,the above experimental system is relatively accurate for the measurement of thermal conductivity of the composites with a high filling ratio(above 60 wt%)in the range of 60 K–300 K.In Fig.7c,the thermal conductivity difference between SiC/Epoxy and AlN/Epoxy increased with decreasing temperature,possibly due to the even higher intrinsic thermal conductivity of SiC than AlN at low temperatures.

3.3.Other properties of the AlN/epoxy and SiC/epoxy composites

Fig.8.Storage modulus(a)and loss factor(b)of composites from-100 °C to 180 °C.(c)DSC heating curves of Pure epoxy and its composites.(d)The coefficient of thermal expansion curves of different samples at low temperatures.

The dynamic thermomechanical properties and CTE of pure epoxy and its composites at low temperatures were further measured,as shown in Fig.8a and b.The temperature dependence of loss factor and storage modulus can be used to characterize the viscoelastic behavior of polymer composites.Both the energy stored in the form of elastic energy and the energy dissipated in the process of strain in the composites are reflected by its dynamic characteristics[9].Fig.8a shows the effect of temperature on the storage modulus of the epoxy and its composites.As the temperature increased,epoxy and its composites changed from a glassy state to a highly elastic state,resulting in a rapid decrease in the storage modulus.Moreover,the storage modulus of the composite material greatly increased at low temperatures,which can be attributed to the fact that the molecular segments had less kinetic energy at lower temperatures.In particular,the storage modulus of the pure epoxy was 2984 MPa at room temperature,while it reached 6448 MPa at -100°C,which is much higher than that of the pure epoxy at room temperature.In the glass states and rubber states,the mass fraction of the inorganic filler in the epoxy matrix could affect the storage modulus dramatically.The storage modulus of the composites is higher than that of pure epoxy and increases with the increase of the filling ratio because SiC and AlN with high elastic modulus can prevent the movement of the epoxy molecular chain.For instance,the storage modulus of SiC-60/epoxy composites at-100°C reaches 15552 MPa,which is about 2.4 times that of pure epoxy.With the increase of AlN filling amount,the storage modulus of AlN/epoxy composite increases.Therefore,the uniform dispersion of the SiC and AlN leads to the higher storage modulus of the SiC/epoxy and AlN/epoxy composites.However,the storage modulus of the AlN/epoxy composites is different from that of the SiC/epoxy composites,which can be attributed to the fact that the DMP of composites varies with the dispersion,load transfer,and geometry[58].The storage modulus of the SiC/epoxy composites is higher than that of the AlN/epoxy composites at the filler ratio of 20 wt%and 60 wt%,which is due to the higher modulus of SiC than AlN.At a filling ratio of 40 wt%,the van der Waals force formed between the AlN particle surface and the epoxy resin gradually becomes apparent,which hinders the movement of the epoxy resin chain segments,so the storage modulus of AlN-40/epoxy is higher than that of SiC-40/epoxy.Furthermore,it was noted that the storage modulus of all the samples showed a peak around -95°C.At lower temperatures,the storage modulus decreases,which is worth exploring in the future.

Fig.8b shows that the loss factor (Tan(delta)) of the composite material changed with the change of the fillers.The loss factor is the loss modulus divided by the storage modulus.Firstly,pure epoxy resins and their composites exhibited a single peak,indicating that they were homogeneous systems.Secondly,the loss factor of the SiC/epoxy and the AlN/epoxy composites was lower than that of the pure epoxy,because the fillers hindered the cross-linking between epoxy resins,increased the flexibility of the molecular chain segments,reduced the friction losses inside the composites,and the weak interfacial strength between particles and epoxy resins had little resistance to the movement of the polymer chain segments.And,with the increase of the filling ratio,the Tan(delta)of the composites decreased first and then increased,since the large proportion of particles limited the molecular chain movement of epoxy resin,which increases the intramolecular friction movement,thus increasing the mechanical loss.Fig.8b also shows that Tan(delta) of epoxy and its composite varied with the increase of temperature.It is worth noting that the loss factor was basically 0 at low temperatures but fluctuated slightly at about -60°C,corresponding to a clear secondary transition [59].At about 80K,the loss factor gradually increased.The glass transition temperature(Tg)refers to the temperature corresponding to the transition from the glass state to the highly elastic state,which can be determined by the temperature corresponding to the peak position of the Tan(delta),and theTgis highly consistent with the measured results of the DSC (Fig.8c).After temperatures above 160°C,secondary peaks were caused by sample irregular rupture at excessive temperatures.As shown in Fig.8c,theTgchanged with the change of the filler ratio.For the SiC/epoxy composites,Tgfirstly decreased and then increased with the increase of the filler,while for the AlN/epoxy composites,Tgfirstly increased and then decreased with the increase of the filler.For instance,theTgof pure epoxy was 123.2°C,while it increased to 128.9°C for the AlN-20/epoxy composites.However,when the filling ratio of AlN increased to 40 wt%,Tgdecreased to 125.6°C.Particularly surprising was thatTgdecreased to 104.9°C when the filling ratio of AlN increased to 60 wt%.The obvious variation of the Tgin the composites indicates strong interfacial interactions between particles and epoxy.It can be proved that thein-situcombination of AlN particles and epoxy facilitates the formation of a stable network,resulting in a decrease in the free movement volume of chain segments when the filling ratio is 20 wt%.As the content of AlN increased from 40%to 60%,the Tgof the composite decreased from 125.6°C to 104.9°C.This is probably due to that the content of AlN is too much,which leads to a decrease in the cross-linking density of molecular chains of the epoxy resin.

The CTE of the pure epoxy and its composites as a function of temperature ranging from -150°C to 60°C is shown in Fig.8d,where the CTE values are reflected by the slope of the curves.Compared with pure epoxy,the CTE values of the composites decreased.The CTE of the composites decreased with the increase of the filling ratio.Moreover,the CTE of the SiC/epoxy composites was lower than that of AlN/epoxy composites,which is attributed to the smaller intrinsic thermal expansion coefficient of SiC than that of AlN.In particular,the CTE value of the SiC/epoxy composites reached 23.0×10-6K-1,which is close to the CTE of common metals[44,45].Among them,the SiC-60/epoxy composites had the highest thermal conductivity,storage modulus,and Tg,and the CTE closer to that of common metals,which are important for the application in the protection of the high-temperature superconducting coil.

The compression strength and bending strength of the composites were tested at room temperature (Fig.9a andb) to investigate the mechanical properties of the AlN/epoxy and SiC/epoxy composites.The composite exhibited declining mechanical properties with a dramatic increase with AlN or SiC when compared to pure epoxy resin.This is mainly attributed to two reasons,one is that the tiny inevitable pores between the filler and the matrix will become the starting point of cracks.As the filler increases,so does the porosity.When the material is subjected to external force,the stress on the pores is more concentrated,which decreases the toughness and strength of the material.Another is that it is easy to separate the filler from the matrix because of the weak bond between the filler and the matrix when the material is stressed.Moreover,the compression strength of the composites decreased gradually with the fillers increasing.Because fillers are aggregated in epoxy resin,which increases the number of defects in composites and weakens the interfacial strength between fillers and resin matrix.Nevertheless,the composites still have barely serviceable mechanical strength (The compression strengths are greater than 200 MPa and the bending strengths are greater than 50 MPa).Interestingly,unlike SiC/epoxy,the bending strength of AlN/epoxy increased with the increase of AlN,which may be due to two factors.One is that AlN has a certain bending ability with lower hardness and better toughness than SiC.Second,the van der Waals force between the epoxy resin and AlN increases with the increase of the fillers,and the formation of effective bonding between AlN particles through epoxy resin is conducive to enhancing the bending capacity of the composites.

The dielectric properties of insulating polymers are also an important evaluation index for superconducting equipment.As can be seen from Fig.9c,the introduction of fillers effectively increases the dielectric permittivity of the composites,and it increases with the increase of filling amount,due to the high intrinsic dielectric permittivity of SiC and AlN.In particular,the dielectric permittivity of SiC-60/epoxy was as high as 21.15 at a frequency of 1 MHz,more than five times that of pure epoxy.Moreover,compared with AlN,SiC has a smaller resistivity,which is beneficial to the polarization of composite materials.Therefore,SiC/epoxy has a much higher dielectric permittivity than AlN/epoxy.For the same reason,the dielectric loss tangent of SiC/epoxy increases greatly,as shown in Fig.9d.By contrast,AlN/epoxy has a lower dielectric loss tangent than pure epoxy due to its extremely low intrinsic dielectric loss tangent and high resistivity,and the dielectric loss tangent of SiC/epoxy further decreased with the increase of filler amount.To sum,the dielectric permittivity of the composites is improved significantly,and the dielectric loss tangent keeps a relatively low level,demonstrating excellent dielectric properties of the composites.

Fig.9.Variation of(a)compression strength and(b)bending strength of the AlN/epoxy and SiC/epoxy composites.(c)Dielectric permittivity and(d)dielectric loss tangent as a frequency function for the AlN/epoxy and SiC/epoxy composites.

First of all,an experimental system that measures the thermal conductivity of composites in a wide temperature range has been built,and the equipment has a tiny uncertainty.Secondly,AlN/epoxy and SiC/epoxy Composites with enhanced thermal conductivity have been successfully prepared.At low temperatures,the thermal conductivity of the composites is improved more obviously than that at room temperature.In addition,the composites also have enhanced dynamic mechanical properties,dielectric properties,a small thermal expansion coefficient,and maintain barely serviceable static mechanical properties.It is envisaged that this work offers an avenue to measure the thermal conductivity of the composites at low temperatures,and the composites with enhanced thermal conductivity at low temperatures as the packaging materials of superconducting equipment are developed in the present investigation.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by Strategic Priority Research Program of Chinese Academy of Sciences (No.XDB25040300),National Key Research and Development Program of China (No.2019YFA0704904),Key Research Program of Frontier Sciences of Chinese Academy of Sciences (No.QYZDB-SSW-JSC042),President"s International Fellowship Initiative(No.2019VEA0017),National Key Research and Development Program of China(NO.2017YFE0301403).

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