Research progress on low dielectric constant modification of cellulose insulating paper for power transformers

Because of the increase in the transmission voltage levels,the demand for insulation reliability of power transformers has increasingly become critical.Cellulose insulating paper is the main insulating component of power transformers.To improve the insulation level of ultrahigh voltage transformers and reduce their weight and size,reducing the dielectric constant of oil-immersed cellulose insulating paper is highly desired.Cellulose is used to produce power-transformer insulating papers owing to its excellent electrical properties,renewability,biodegradability and abundance.The dielectric constant of a cellulose insulating paper can be effectively reduced by chemical or physical modification.This study presents an overview of the foreign and domestic research status of the use of modification technology to reduce the dielectric constant of cellulose insulating papers.All the mentioned methods are analyzed in this study.Finally,some recommendations for future modified cellulose insulating paper research and applications are proposed.This paper can provide a reference for further research on low dielectric constant cellulose insulating paper in the future.

Low dielectric constant and highly intrinsic thermal conductivity fluorine-containing epoxy resins with ordered liquid crystal structures

Epoxy resins with a high dielectric constant and low intrinsic thermal conductivity coefficient cannot meet the current application requirements of advanced electronic and electrical equipment.Therefore,novel fluorine-containing liquid crystal epoxy compounds(TFSAEy)with fluorinated groups,biphenyl units,and flexible alkyl chains are first synthesized via amidation and esterification reactions.Then,4,4′-diaminodiphenylmethane(DDM)is used as a curing agent to prepare the corresponding fluorine-containing liquid crystal epoxy resins.The obtained dielectric constant(ε)and dielectric loss(tanδ)values of TFSAEy/DDM at 1 MHz are 2.54 and 0.025,respectively,which are significantly lower than those of conventional epoxy resins(E-51/DDM,3.52 and 0.038).Additionally,the intrinsic thermal conductivity coefficient(λ)of TFSAEy/DDM is 0.36 W/(m⋅K),71.4%higher than that of E-51/DDM(0.21 W/(m⋅K)).Meanwhile,the corresponding elastic modulus,hardness,glass transition temperature,and heat resistance index of TFSAEy/DDM are 5.73 GPa,0.35 GPa,213.5◦C,and 188.7℃,respectively,all superior to those of E-51/DDM(3.68 GPa,0.27 GPa,107.2℃,and 174.8℃),presenting potential application in high-heating electronic component packaging and printed circuit boards.

Highly Transparent and Colorless Polyimide Film with Low Dielectric Constant by Introducing Meta-substituted Structure and Trifluoromethyl Groups

An effective design strategy for preparing highly transparent polyimide film with low dielectric constant is presented.The key to the strategy is to simultaneously introduce meta-substituted structure and trifluoromethyl in polymer chains.By using this design strategy,a highly transparent polyimide film with low-k was synthesized from 3,5-diaminobenzotrifluoride(m-TFPDA)and 4,4'-(hexafluoroisopropylidene)diphthalic anhydride(6FDA)through a two-step method.The obtained m-TFPDA/6FDA(CPI)film(~30 pm)possesses high optical transparency(λ_(cutoff)=334 nm,T_(450nm)=85.26%,Haze=0.31)and is close to colorless(L^(*)=96.03,a^(*)=-0.34,b^(*)=2.12,yellow index=3.96).The intrinsic k and dielectric loss value of the film are 2.27 and 0.0013 at 10 kHz,respectively.More importantly,such low dielectric performance could remain stable up to 280℃,and the film shows a low moisture rate(~0.51%),which helps to maintain the low-k property stability in different humid environments.Meanwhile,the film also shows good thermal stability and mechanical properties,with a glass transition temperature(T_(g))of 296℃and the 5 wt%decomposition temperature(T_(d,s%))of 522℃under N_(2).The tensile strength and tensile modulus of the film are 85.1 MPa and 1.96 GPa,respectively.In addition,the film is soluble in common solvents,which allows simple solution processing and low-cost,continuous roll-to-roll processes.This design strategy is beneficial to improving the transparency,lightening yellow color,lowering the dielectric constant and meanwhile maintaining the comprehensive properties of polyimide films,which is mainly due to the introduced meta-substituted and trifluoromethyl structures effectively inhibiting the transfer of charge transfer complex(CTC)effects and increasing the free volume of film.This design strategy could also be extended to other high-performance polymer systems.

Alumina/Polyimide Composite Porous Nanosolid:Dielectric Characteristics and Compressive Strength

Al2O3 porous nanosolid was prepared via solvothermal hot-press（SHP） method.The dielectric constant of Al2O3 porous nanosolid is as low as 2.34,while its compressive strength is very poor.In order to improve the compressive strength and maitain low dielectric constant,polyimide was introduced to prepare Al2O3 /polyimide composite porous nanosolid.Compared to Al2O3 porous nanosolid,Al2O3 /polyimide composite porous nanosolid possesses much higher compressive strength,which reaches its saturation value when the mass loading of polyimide is 7.75%.In addition,the in situ Fourier transformation infrared（FTIR） monitoring result reveals that Al2O3 /polyimide composite porous nanosolid is stable up to 400 °C.

Design and synthesis of liquid crystal copolyesters with high-frequency low dielectric loss and inherent flame retardancy

Ultra-low dielectric loss(Df)and low dielectric constant(Dk)materials are urgently required in highspeed and large-capacity transmission,in which the wholly aromatic liquid crystal polymer(LCP)has gained attention due to its excellent dielectric properties.However,the relationship between molecular structure and dielectric properties is still not clear.In this study,two copolyesters containing phenyl or naphthyl structures are synthesized,as well as the effects of benzene and naphthalene mesogens on dielectric properties are investigated.The synthesized copolyesters containing naphthalene structure have good comprehensive properties with high thermal stability(T_(5%)=479℃ and T_g=195-216℃),inherent flame retardance(LOI=33.0-35.0 and UL-94 V-0 level at 0.8 mm),low Dk(2.9-3.0@10 GHz)and low Df(0.0027-0.0047@10 GHz).Naphthalene mesogen can reduce the dielectric loss more significantly than benzene at high frequency by reducing the density and mobility of polarizable groups,which leads to the effectively limited dipole polarization in copolyesters.Consequently,we proposed a new strategy for designing low Dk and low Df materials.

Realization of Low Temperature Densification of Nickelate Ceramics for MLCC Application

We report an improved method for the preparation of highly dense nickelate ceramics at relatively low temperature. It is found that the introduction of appropriate additives during the ball-milling process facilitates the formation of nickelate phase through solid state reaction. Moreover, although high-purity nickelate powders can only be obtained by calcining the mixture of starting materials at temperature higher than 1100 ℃. The adoption of powders calcined at 1000 ℃, rather than those calcined at higher temperature, is conductive to the low-temperature densification of nickelate ceramics, which is attributed to the small and dispersive particles, and the solid state reaction of the residual starting materials during sintering. Compared with the conventional process, the improved method can reduce the sintering temperature of nickelate ceramics by about 100 ℃ and decrease the grain size of the obtained ceramics, and therefore makes nickelate meet the fabrication requirements of multi-layer ceramic capacitors（MLCC）.

Preparation of Low-dielectric Permittivity Polyimide Resins with High Surface Activity from Chemically Bonded Hyperbranched Polysiloxane

Thermosetting resin matrix is the key component of advaneed wave-tra nsparent composites,where low dielectric constant,excellent processability,high thermal stability,as well as good bonding ability are required for resins.Herein,we prepared a series of phenylethynyl terminated polyimide(PI)resins by grafting amine-functi onalized hyperbra nched polysiloxane(HBPSi)to PI chains during the in situ polymerization.The effects of HBPSi on the processability of oligomers,molecular packing,thermal stability,dielectric property and bonding ability to reinforce Kevlar fibers of the cured PI/HBPSi composite resins have been examined in detail.The dielectric constants of the cured composite resins were greatly reduced from 3.29 to 2.19 without compromising its processability and thermal stability.Meanwhile,the 10 wt%HBPSi-containing PI resin demonstrated better bonding ability to reinforce fibers with the in terfacial shear strength(IFSS)of 37.64 MPa,compared with that of neat PI-6 matrix(27.34 MPa),and better adhesion to metal with the lap shear strength of 10.48 MPa,50%higher than that of neat resin PI-6(6.98 MPa).These resultant PI/HBPSi composite resins exhibit excellent comprehensive properties,indicating their great potential as low-dielectric constant resin matrix in radar radome.

A Facile Strategy for Intrinsic Low-Dk and Low-Df Polyimides Enabled by Spirobifluorene Groups

Modified polyimides(MPIs)show great potential towards 5G communication applications,due to its excellent thermal stability,mechanical property and chemical stability as compared to most of polymers.Introducing fluoride groups or porous structure is favorable to ultra-low dielectric constant(D_(k))and dielectric loss(D_(f)).However,the cost of the fluorinated MPIs is high and their synthetic processes are complicated,and porous MPIs suffer poor mechanical properties.Also,increasing the fraction of free volume is a very effective way to lower D_(k)through introducing more ultra-low-D_(k)air component.However,most of this kind of MPIs lag far behind the fluorinated MPIs and the porous MPIs in terms of ultra-low D_(f),hindering the application of MPIs in high-speed communication devices.Thus,it is highly desirable to develop intrinsic ultra-low-D_(k)/D_(f)MPIs at high frequency with less fluoric groups and nonporous structure.Herein,we introduce a facile and effective strategy to lower D_(k)and D_(f)through introducing rigid and large sterically hindered aromatic groups into MPIs.On the one hand,their large steric hindrance effect leads to low D_(k)by increasing intrinsic free volume.On the other hand,the resulting highly stiff polymer chain and strong intermolecular interaction are favorable to reduce D_(f)by inhibiting dipole orientations.Based on this strategy,the spirobifluorene groups are preferred.The as-prepared MPIs show excellent dielectric performance with low D_(k)of 2.74–2.76 and low D_(f)of 0.00599 at 10 GHz,to some extent,exceeding the multiple fluorinated MPI with D_(k)/D_(f)of 2.67/0.00663 at 10 GHz.

Amelioration of sintering and multi-frequency dielectric properties of Mg_(3)B_(2)O_(6):A mechanism study of nickel substitution using DFT calculation

With the support of density functional theory(DFT)calculation,the amelioration of sintering and dielectric properties of the Mg_(3)B_(2)O_(6)(MBO)ceramic was realized through the substitution of magnesium with nickel.The TE-mode cylindrical cavity method was used to measure the dielectric properties at different frequencies.The thermo-mechanical analysis and simultaneous thermal analysis were used to characterize the chemical and mechanical properties.The phase composition was determined through the X-ray diffraction(XRD)and Raman spectrum.The microstructure was investigated using the scanning electron microscopy(SEM).Magnesium substitution with nickel(4 mol%)could ionize the B-0 bond of BO3,modify the vibration mode,improve the order degree,densify the microstmcture,decrease the intrinsic densification temperature,and ameliorate the dielectric properties of the MBO ceramics.The maximum values were achieved for the ceramics with 4 mol%nickel and sintered at 1175℃,that is,97.2%for relative density,72,600 GHz(10 GHz),75,600 GHz(11.4 GHz),and 92,200 GHz(15 GHz)for Q×f,7.1(10 GHz),7.01(11.4 GHz),and 6.91(15 GHz)for£r,and-56.3 ppm/℃ for if.