Improvement of the electrical resistivity of epoxy resin at elevated temperature by adding a positive temperature coefficient BaTiO3-based compound

Chenyuan TENG (滕陈源),Wenjun ZHOU (周文俊),Yuanxiang ZHOU (周远翔),3,Ling ZHANG (张灵),Yunxiao ZHANG(张云霄),Zhongliu ZHOU(周仲柳) and Wenpeng LI(李文鹏)

1 School of Electrical Engineering and Automation,Wuhan University,Wuhan 430072,People’s Republic of China

2 China State Key Laboratory of Power System and Generation Equipment,Department of Electrical Engineering,Tsinghua University,Beijing 100084,People’s Republic of China

3 School of Electrical Engineering,Xinjiang University,Urumqi 830047,People’s Republic of China

4 State Key Laboratory of Advanced Power Transmission Technology,Global Energy Interconnection Research Institute Corporation,Beijing 102211,People’s Republic of China

Abstract

Keywords: epoxy resin,positive temperature coefficient,barium titanate based compounds,resistivity-temperature characteristics,traps

1.Introduction

As the requirements of the safe operation of a high-voltage direct current (HVDC) project have been enhanced,dry-type bushings have potential development prospects due to their lower risk of dangerous explosion compared with oil impregnated paper bushings,such as SF6insulated bushing and resin impregnated paper bushing.Anhydride-cured epoxy resin is widely used as as an insulating material in these bushings due to its excellent insulating,mechanical,thermal,and other properties [1,2].In operation,there is a rise in temperature across the insulation caused by resistive power loss in the conductor and thermal dissipation within insulation[3,4].This rise in temperature phenomenon is more severe in HVDC equipment for its high-power operating characteristic.For example,the maximum oil side temperature of a valve side bushing in a ±800 kV converter transformer will reach 90.4°C [5].As a result,the operating temperature of insulating materials is always higher than the ambient temperature.It is known that the DC electrical resistivity of epoxy resin possesses a negative temperature coefficient (NTC)effect,namely its DC electrical resistivity decreases by orders magnitude as the temperature rises to its operational temperature[6].Such a characteristic will lead to thermal runaway followed by thermal breakdown at sufficiently high electric fields [7].In order to operate at a higher stress level,the insulation must have a higher electrical resistivity to minimize thermal losses and thus avoid breakdown [8].In addition,electrical field distribution is distorted and caused by gradient temperature distribution.The distorted electrical field will accelerate the aging of insulating materials.The reduction of electrical resistivity at an elevated temperature is the main cause.These complex phenomena make the design of insulation much more difficult compared with that under AC voltages.Therefore,weakening the changing of electrical resistivity of epoxy resin caused by temperature is essential to realize a higher voltage level and improve the running reliability of HVDC power equipment.

In recent years,regulation of the surface resistivity of insulating epoxy composites has become a research hotpot and has achieved great progress in surface charge regulation[9,10].However,less attention is paid to the enhancement of volume resistivity.Veena et al prepared SiO2-filled epoxy composites and found that properly added SiO2could increase the DC electrical resistivity of composites under the temperature range of 25°C–150°C [11].Cui et al coated SiO2on multi-walled carbon nanotubes to maintain the electrical resistivity of epoxy composites almost the same as the neat epoxy resin [12].Chen at al obtained a higher electrical resistivity epoxy composites by adding an α-alumina nanofiller under the temperature range of 30°C–80°C[13].Although many efforts have been made to achieve a higher electrical resistivity of epoxy,less attention is paid to improve the electrical resistivity-temperature characteristic of epoxy resin,for example,increasing electrical resistivity at an elevated temperature.

Unlike epoxy resin,donor-doped semiconducting barium titanate ceramics exhibit a positive temperature coefficient(PTC)characteristic[14],namely its electrical resistivity rises as the temperature exceeds its Curie temperature.Therefore,doping PTC materials into insulating materials may become a promising method for targeted regulation of electrical resistivity-temperature characteristic of composites.To the best of our knowledge,there is little research on regulation of the electrical resistivity of insulating materials using PTC materials.

In this article,BaTiO3-based compounds (BT60) with a Curie temperature of 60°C and average size of 1 μm were selected as fillers,whose surface was hydroxylated and then modified by (3-Aminopropyl) triethoxysilane (APTES).Epoxy composites with different loadings of BT60 fillers were prepared.The electrical resistivity-temperature characteristic of the composites was researched.DC breakdown strength,space charge distribution,and other characteristics were measured to describe the electrical performance of composites.

2.Experimental details

2.1.Sample preparation

The matrix,hardener,and accelerator used in this investigation were diglycidyl ether of bisphenol A epoxy (China National BlueStar (Group) Co.,Ltd),methylhexahydrophthalic anhydride (Puyang Huicheng Electronic Material Co.,Ltd),and N,N-dimethyl benzyl amine(Aladdin Reagent(Shanghai) Co.,Ltd),respectively.BT60 with a Curie temperature of 60°C and an average size of 1 μm was selected as the PTC material filler,and its surface was hydroxylated by H2O2(Beijing Chemical Works) and then modified with APTES (Aladdin Reagent (Shanghai)Co.,Ltd).

The grafting of -OH groups on the surface of the BT60 was performed to increase the grafting density and thus obtain a better dispersion of particles [15],following the procedure described in[16].BT60(15.00 g)and an aqueous solution of H2O2(80 ml,30 wt%) were mixed and ultrasonically irradiated for 30 min.Then the mixed solution was refluxed at 105°C for 6 h.After cooling,the mixture was centrifuged and dried in a vacuum oven at 80°C for 12 h.The hydroxylated BT60 is denoted as BT60-OH.

The surface modification of BT60-OH is as follows:16 g BT60-OH were dispersed in the mixed solution of 8 g APTES,190 ml of ethanol,and 10 ml of deionized water before being sonicated for 30 min at room temperature.Then the solution was heated to 80°C in an oil bath and was magnetically stirred for 6 h.After the reaction,the modified BT60-OH (denoted as BT60-OH-AP) was washed with ethanol three times to remove excessive APTES under centrifugal forces(4000 rpm,2 min),and dried in a vacuum oven at 80°C for 24 h.

BT60-OH-AP with different loadings (0 wt%,0.5 wt%and 2 wt% of epoxy) was added into epoxy,respectively,denoted as EP-0,EP-0.5,and EP-2 respectively.IKA overhead stirrers was used to mix the fillers and epoxy.The mixing was conducted from low speed (500 rpm) to high(2000 rpm) speed to enhance the dispersion of fillers in composite resin [17,18].Then,composite epoxy,hardener(88 wt% of epoxy) and accelerator (1 wt% of epoxy) mixture were likewise mixed following the process mentioned above.After mixing,the composite epoxy was quickly degassed for an hour to remove bubbles and immediately poured into the desired mold.All these samples were pre-cured at 80°C for 3 h,cured at 125°C for 6 h and post-cured at 145°C for 5 h.The samples had a thickness of 220 ± 20 μm.

Figure 1.Diagram of the DC electrical resistivity measurement under the temperature field.

2.2.Characterizations

The BT60-OH and BT60-OH-AP particles were dried and characterized by using thermogravimetric analysis(TGA)and Fourier transform infrared(FTIR).TGA was performed using TGA550 from 30°C–800°C at a heating rate of 20°C min-1under a nitrogen condition.The FTIR measurement ranged from 4000–600 cm-1,whose resolution was 2 cm-1.

Field emission scanning electron microscopy (FESEM)was performed using a JSM-6335 (JEOL Ltd,Tokyo,Japan)to observe the morphology of fractured faces of composites.Brittle fractured sections of the samples were obtained in liquid nitrogen.

The glass transition temperature was measured by differential scanning calorimetry(DSC),which was conducted on a DSC250 (TA Instrument,USA) at a ramp rate of 20°C min-1from 40°C–200°C under a nitrogen condition.

2.3.DC electrical resistivity

DC electrical resistivity was measured by the three-electrodes method according to IEC 62631 [19],as can be seen in figure 1.

A Keithley 6517B picoammeter was adopted to record the polarization current.The applied electric field was 20 kV mm-1.The poling current at 900 s was taken as quasisteady state conduction current.Four test environments with temperatures of 30°C,50°C,70°C,and 90°C were employed using an oven.Prior to the tests,samples and electrodes were kept at the desired test temperature for 2 h to ensure the uniformity and stability of the temperature distribution.Three samples were used to establish the reproducibility.The final figure of DC electrical resistivity for each group is the average of the three samples.

2.4.DC breakdown strength

DC breakdown strengths of neat epoxy and epoxy composites were performed according to IEC 60243 [20].The measurement system is shown in figure 2.The tests were conducted in transformer oil.Sphere (d = 20 mm) and plate (d = 25 mm)electrodes made of stainless steel were used.Measurements were conducted at 30°C controlled by an oven.Before the test,the oven was preheated for at least 2 h to ensure uniform temperature distribution.The rate of voltage increase was 1000 V s-1regulated by a computer.For each group,data validity was confirmed through 18 measurements.The results were presented using a Weibull distribution.

2.5.Space charge distribution

Space charge distribution was measured based on the pulsedelectro-acoustic (PEA) method.The details of the PEA measurements can be found in our previous paper [21].The polarization process was carried out under 50 kV mm-1for 30 min,and the depolarization process for 30 min with the measurement interval of 3 s.The temperatures of the space charge measurements were selected as 30°C and 70°C.

3.Results and discussion

3.1.Characterization

Successful modification of APTES was verified with FTIR and TGA,as shown in figure 3.It can be observed that a new band appeared at 882 cm-1in BT60-OH-AP after the treatment with APTES.In addition,the band at 1095 cm-1was reinforced.The former was assigned to the stretching vibrations of Si–O,while the other is ascribed to the stretching vibrations of the N–H bond of the amino group.Figure 3(b)presents the TGA curve of BT60-OH and BT60-OH-AP.Both groups have a similar TGA curve.However,the mass loss of BT60-OH-AP is more than that of BT60-OH,attributable to the decomposition of the organic portion from the APTES.These results reveal that the APTES is grafted to the surface of BT60-OH.

It should be noted that the underlying assumption for composites is that the fillers are well dispersed and are far away from each other.Figure 4 shows the fractured faces of neat epoxy and BT60-epoxy composites.Although the amount of BT60 accumulates in epoxy composites as the filler loading increases,it can be seen that the dispersion of BT60 has been well addressed and no obvious agglomerates are formed in this work.The size of BT60 was less than 1 μm in the epoxy resin matrix.

Figure 2.Diagram of the DC breakdown strength of the measurement system.

Figure 3.FTIR and TGA spectra of surface-modified BT60.

Figure 4.FESEM images of the fractured faces of the samples.

Figure 5.DSC curves of epoxy composites.

The DSC curves of the spectra of the epoxy composites are shown in figure 5.It can be calculated that the transition temperatures of EP-0,EP-0.5,and EP-2 are 115°C,109°C,and 105°C,respectively.Adding fillers will lower the transition temperature of epoxy composites,but the difference between EP-0.5 and EP-2 is less than the difference between EP-0 and EP-0.5.The compatibility of the interface between BT60 and epoxy will inevitably introduce tiny holes,which are exhibited as free volume and will affect the local crosslinking density of epoxy.Furthermore,lower crosslinking density will also increase free volume [22].According to the free-volume theory,free volume is contributed to the mobility of segments and thus influences the transition temperature[23].Therefore,the increased addition of BT60 causes a decrease in the transition temperature.

3.2.DC electrical resistivity

The insulation must have a higher electrical resistivity to avoid high thermal losses in a thermal breakdown process[8].In operation,the temperature distribution within the insulation is much higher than ambient temperature due to Joule heating.Therefore,a higher electrical resistivity at an increased temperature will result in a minimal increase in the temperature of the insulation and thus extend its service life.

The DC electrical resistivity of epoxy composites is calculated based on the results of DC conduction current according to IEC 62631.In order to illustrate the change in the electrical resistivity of epoxy composites compared with neat epoxy,the electrical resistivity ratio k is calculated and is shown in figure 6.

Figure 6.Ratio of the electrical resistivity of epoxy composites to neat epoxy resin.

where REP-nis the DC electrical resistivity of epoxy composites(n = 0.5,2),and REP-0is the DC electrical resistivity of neat epoxy.Insulated BaTiO3will become semiconductive and thus exhibit a PTC effect after being properly doped[24].As a type of PTC material,BT60 also has the same property.Therefore,the addition of BT60 can be regarded as impurities in a sense.Impurities are the main source of ionic conduction for insulating materials.So the addition of BT60 facilitates ionic conduction and thus leads to the decrease of the DC electrical resistivity of EP-0.5 and EP-2 by 2.57%and 3.35%at 30°C,respectively.

When the temperature rises to 50°C,the decline in the electrical resistivity of epoxy composites becomes smaller than that of neat epoxy.Although the volume expansion of epoxy at elevated temperature will result in the increase of the electrical resistivity of epoxy composites.However,the volume expansion is slight at 50°C.More importantly,the electrical resistivity of epoxy composites hardly exceeds the electrical resistivity of neat epoxy according to the volume expansion mechanism.So it can be inferred that the BT60 is activated at 50°C and influences the electrical resistivity of epoxy composites.Although the Curie temperature of BT60 is 60°C,the phase transition of BT60 is not a mutation process and shows a slow increase in the electrical resistivity below 60°C,contributing to the increase in the electrical resistivity of epoxy composites.So the electrical resistivity of EP-2 shows an increase of 13% compared with EP-0.

As the temperate exceeds the Curie temperature of BT60(60°C),the PTC effect of BT60 has a pronounced effect on the enhancement of the electrical resistivity of epoxy composites.The electrical resistivity of EP-0.5 and EP-2 is 1.33 ×1017Ω · cm and 1.46 × 1017Ω · cm at 70°C,respectively,which is 116.3% and 127.8% of neat epoxy.The electrical resistivity of EP-0.5 and EP-2 is 1.98 × 1016Ω · cm and 2.31 × 1016Ω · cm at 90°C,respectively,which is 133.6%and 155%of neat epoxy.It is shown that the PTC effect of BT60 is able to regulate the electrical resistivity-temperature dependence of epoxy composites.

Figure 7.Weibull distribution of the DC breakdown strength of epoxy composites.

Gradient temperature distribution exists in most DC power equipment,resulting in a gradient distribution of electrical resistivity and thus the electric field.This distorted electric field distribution becomes a threat to the safe operation of power equipment as well as power grids.The improvement of electrical resistivity at elevated temperature also means a smaller decline in electrical resistivity caused by a rise in temperature,achieving a more uniform electric field distribution within the insulation during the operation.

3.3.DC breakdown strength

Breakdown strength (BDS) is one of the most important properties of insulating materials.Functionalized composite materials should possess adequate electrical insulation performance to ensure the safe operation of power equipment.However,functional fillers sometimes tend to act as defects in matrix,leading to the distortion of the local electric field and thus decrease the breakdown strength of composites [25].Therefore,the DC breakdown strength of epoxy composites is assessed by the two-parameter Weibull distribution in this work,as shown in figure 7.

The results indicate that EP-0 has the lowest DC BDS with the scale indexes of 361.9 kV mm-1,and DC BDS of EP-0.5(374.6 kV mm-1)outperformed those of EP-2(364.6 kV mm-1)by 2.7%.Considering that the difference in BDS among three groups is small,it can indicate that the proper addition of BT60 will not affect the DC BDS of epoxy composites.

Leakage current will flow and the temperature will increase as a voltage is applied to the insulating materials.This rise in temperature will decrease the electrical resistivity,resulting in a continuous rise in temperature.The material that has a higher electrical resistivity at elevated temperature is able to mitigate the leakage current and thus has a lower rise in temperature.Therefore,EP-0.5 and EP-2 are able to maintain the DC breakdown strength with the addition of micro-size BT60 particles.

3.4.Space charge distribution

The space charge characteristic is able to reflect the carrier behavior of insulating materials,which is closely related to the conduction current.The space charge distribution in epoxy composites under 50 kV mm-1at 30°C and 70°C is presented in figure 8.Three groups have a similar phenomenon of space charge distribution at 30°C.In general,the main polarization of the space charge in these samples is negative.A homogeneous space charge accumulates near the cathode and a small amount of negative space charge moves to the anode and becomes a hetero-charge.

As the temperature reaches 70°C,the mobility of the carrier increases.However,the movement of the negative charge is faster than that of the positive charge,resulting in the negative space charge accumulating near the anode in EP-0 during the polarization process,while the space charge distributions of EP-0.5 and EP-2 are basically the same as those at 30°C.The heterogeneous space charge near the anode is suppressed and the amount of space charge in the bulk is less than that of neat epoxy.However,as the loading of BT60 increases,the amount of homogeneous space charge accumulated near the electrodes increases slightly.It is widely believed that heterogenous space charge will distort the internal electric field of the insulation,leading to the early failure of DC power equipment [26,27].Therefore,as the accumulation of a heterogenous space charge was mitigated in epoxy composites at high temperature,and the electric field distribution will be more uniform in operation.

3.5.Mechanism for the regulation of the electrical resistivity of BT60

Electrical conduction is essentially influenced by the generated charge carriers and their transport characteristics in a polymer.Generally,charge traps inside a polymer have significant influences on the mobility of charge carriers [28].Figure 9 shows the depth of the trap of epoxy composites after the removal of 50 kV mm-1at 30°C and 70°C.The depth of the trap was calculated based on the depolarizing process of a space charge test of insulating materials [29]:

where T is the temperature,k is the Boltzmann constant,R is the mean distance between localized states,e is the electron charge,μ is the mobility of space charge during the depolarization,ν = kT/h is the attempt frequency,and h is the Planck constant.

Although the surface of BT60 has been modified,an interface will form for the compatibility problem between the BT60 and epoxy resin.The decline in the transition temperature indicates the existence of such an interface.Many studies have pointed out that this interface always introduces traps [30,31].It can be seen that the trap characteristic of epoxy composites is different below and above the Curie temperature of BT60.Semiconductive BT60 lowers the depth of the trap of epoxy composites at 30°C.Shallower traps facilitate carrier migration [32],decreasing the electrical resistivity of epoxy composites.As the temperature rises from 30°C–70°C,BT60 becomes insulated and deepens the depth of the trap of epoxy composites.The depth of the trap of EP-0.5 and EP-2 exceeds that of the EP-0.Carriers trapped in deeper traps have difficulty escaping,inhibiting carrier migration and resulting in the higher electrical resistivity of epoxy composites.

Figure 8.Space charge distribution in epoxy composites under 50 kV mm-1.

Figure 9.Depth of the trap of epoxy composites after the removal of 50 kV mm-1 at 30°C (a) and 70°C (b).

As a type of PTC material,the DC electrical resistivity of BT60 will increase dramatically when the temperature exceeds 60°C.The Heywang model is the most accepted model to explain the PTC effect of BT60 [4].According to this model,there is a potential barrier along the grain boundary of BT60[33].This potential barrier can be written as:

where e is the electron charge,Nsis the density of the trapped electrons at the grain boundaries,ε0is the vacuum permittivity,ε is the relative permittivity of the grain-boundary region,and Ndis the charge carrier concentration.As the temperature exceeds the Curie point,ε will increase based on the Curie–Weiss law [34].The enhanced φ0makes the migration of electrons more difficult.It is known that changing the properties of the fillers will influence the properties of the composites.So well-dispersed BT60 introduces barriers into the matrix that suppress the movement of electrons in EP-0.5 and EP-2 compared with EP-0,as can be seen in figure 10.The higher loading of BT60 means more barriers to electron migration.This is accordance with the results of the depth of the trap shown in figure 9.In addition,it is known that the density of the surface states in the polymer is likely to be much higher than that of bulk trapping states [35].So deep traps mainly concentrate near the surface of a polymer.Therefore,as the loading of BT60 increases,the homogeneous space charge is captured and slightly increases near both electrodes,making it harder to form a heterogeneous accumulation like neat epoxy.

Figure 10.Electron hopping model in epoxy composites.

Figure 11.Residual charge in epoxy composites after the removal of 50 kV mm-1 at 30°C (a) and 70°C (b).

Electrical conduction is also governed by carrier injection from electrodes.As analyzed above,if the potential barrier is heightened by BT60,the carrier injection will be suppressed as well as the formation of the space charge in a material[36].Figure 11 shows the residual charge in epoxy composites after the removal of 50 kV mm-1.

It can be observed that the difference in the residual space charge between epoxy composites and neat epoxy is reduced as the temperature reaches 70°C from 30°C.This means that the residual space charge of epoxy composites has a steeper decline than that of neat epoxy.As the depth of the trap of epoxy composites is deeper than that of EP-0 at an elevated temperature,the mobility of the carrier is suppressed and thus the accumulation of space charge should increase.As a result,the phenomenon in figure 11 is mainly attributed to the heightened potential barrier caused by BT60.Therefore,the suppressed carrier injection and mobility in epoxy composites lead to a larger electrical resistivity than neat epoxy as the temperature is higher than the Curie point of BT60.

4.Conclusion

This paper selected PTC materials(BT60)as fillers to regulate the DC electrical resistivity of an epoxy composite at elevated temperature.The epoxy composites were characterized by SEM and DSC.The DC electrical resistivity of the epoxy composites was evaluated under a wide temperature range.The DC breakdown strength and space charge were also tested to obtain detailed information on the epoxy composites.

BT60 was hydroxylated and modified by APTES to obtain better dispersion.The SEM image shows that the dispersion of BT60 has been well addressed.The transition temperature of epoxy composites descended along with the increased addition of BT60.As the filler loading increases from 0–2 wt% of the matrix,the transition temperature decreases from 115°C–105°C.

The PTC effect of BT60 affects the DC electrical resistivity of epoxy composites positively at an elevated temperature.As the temperature rises to 90°C,the electrical resistivity of EP-2 increases by 55% compared with that of neat epoxy.More interestingly,the DC breakdown strength of the epoxy composites does not decrease after the addition of BT60 even with a loading of 2 wt%.Space charge distributions indicate that the addition of BT60 suppresses the heterogeneous space charge accumulated near the cathode and increases the depth of the trap at 70°C.This work provides a novel method to regulate the resistivity-temperature characteristics of polymeric insulating materials.

Acknowledgments

The research group sincerely appreciates the support from National Natural Science Foundation of China (No.51977186),the China Postdoctoral Science Foundation (No.2019M650029),the Young Elite Scientists Sponsorship Program by CAST (No.2018QNRC001),the National Key R&D Program of China (No.2017YFB0902704),the State Key Development Program of Basic Research of China (973 Program) (No.2014CB239501),and the Science and Technology Project of the State Grid Corp.of China (No.52110418001Y).

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