Reversible dielectric polymers with switchable conduction and insulation for electrostatic protection

Domain: neuroscience
Published: 2026-02-12
Editorial outcome: major_revision

Abstract
The pursuit of power density and integration in advanced electronic devices poses a risk of electrostatic discharge to traditional packaging materials. Albeit optimizing the insulation performance of packaging materials seems to be an effective strategy, long-term charge accumulation will seriously threaten the reliability of devices. Here, we fabricate an adaptive field grading material that can transition between insulating and conductive states, enabling the rapid dissipation of charges during electrostatic discharge events. A silicon carbide-derived nanofiber mat with a “step-by-step” double Schottky barrier was synthesized using electrospinning. The coordination between the insulating epoxy-based matrix and the oriented semiconductor fiber mat achieves desirable electric field grading performance at ultra-low content (0.3 vol%), and the switching electric field of the insulation-conductive transition can be precisely customized through the nanofiber mat content. Given the diversity between insulating polymers and metal oxides, these findings have prominent guiding significance for electrostatic protection of numerous electronic devices.

Introduction
Polymer dielectrics are characteristic of high-voltage endurance and ease of processing, establishing them as critically important materials in modern customer electronics, 5G-6G high-frequency communications substrate, as well as high-power devices, e.g., spacecraft and high-voltage converter1–3. However, conventional insulating materials face huge challenges with the enhancement of power density and high integration, e.g., the accumulated charge from long-term operation and the electrostatic pulses from external factors will seriously threaten the safe operation of power electronic devices4,5. Doping highly insulating nanostructures, such as silicon dioxide, aluminum oxide, and boron nitride nanosheets, into the encapsulated polymer can significantly improve the breakdown threshold of the material6–9. Mainstream advanced electronic devices, such as Metal Oxide Semiconductor Field Effect Transistor (MOSFET) and flexible wearable electronic skin, have low insulation breakdown requirements for insulating polymers and only withstand high electric field strength during electrostatic pulses10–12. In this case, field-grading materials (FGMs)12–14, which maintain high insulation performance at low electric fields and transition to a conductive state to dissipate energy at high electric fields, can more effectively ensure equipment safety compared to traditional insulation materials.

Inorganic semiconductors, including silicon carbide and zinc oxide microvaristor, have been confirmed in previous reports to possess the ability to endow insulating polymers with nonlinear electrical conductivity performances based on the “back-to-back” double Schottky barrier15,16. Unfortunately, the nonlinear coefficients of FGMs based on the above theory are difficult to fine control, thereby cannot meet the diverse application requirements. More importantly, forming long distance conductive paths in the polymer matrix requires relatively high loading amounts (over 30 vol%), which not only poses greater challenges to processing difficulty, but also affects the stability of the field grading behavior due to the uncontrollable distribution of fillers17–20.

Herein, we propose an effective strategy to synthesize nanofiber mats with electric field grading performances by large-scale coaxial electrospinning. Completely different from traditional “back-to-back” double Schottky barriers, this nanostructure has a “step-by-step” double Schottky barrier that can be independently controlled the barriers height of the two side by designing the material composition, thereby achieving adjustable nonlinear coefficients to cope with diverse application scenarios. Furthermore, the nanofiber mat can be easily assembled into multiple alignment modes, enabling the optimal nanocomposite to have insulation-conductive transition ability under a recorded low loading (0.3 vol%). Particularly, we have verified the reliability of this FGM in practical applications through light emitting diode (LED) and ESD immunity testing.

Results
Structure and physical mechanism of the SiC-GaW
Particle-particle contact has been the origin of nonlinear J-E behavior in SiC-filled FGMs (Fig. 1a), in which the macroscopic conduction pathway forms due to the successive contact interface16. Since the spontaneous formation of thin interfacial oxide layer surrounded SiC will lead to surface charging and band bending, each contact point between adjacent SiC is similar to a traditional “back-to-back” Schottky barrier inside ZnO microvaristors14. Figure 1b shows the SiC energy band bending towards the thin oxide layer and thus a barrier height of Φ forms21,22. Prior to the application of a bias voltage, the potential barrier heights on both sides are identical, making it difficult for carriers to overcome the barrier for transportation. Once a bias voltage is applied on the barrier, the energy band structure on both sides will exhibit varying evolution trends (Fig. 1c). The barrier height on the forward side will decrease, whereas increasing on the reverse side. Consequently, the carrier transportation process is tuned by the voltage-dominated Schottky barriers, thereby exhibiting electric field grading behavior.

However, for FGMs incorporating SiC fillers, the nonlinear coefficient which is highly dependent on the interface between contacted SiC fillers and characterized by a relatively low barrier tends to result in an undesirable and unstable field grading performance14,23. Drawing inspiration from ZnO microvaristors, we consequently fabricate SiC nanofibers with high Schottky barrier height inside the nanofiber. The 1D nanofibers, after self-assembly and fabrication into FGMs, can preserve its theoretically high nonlinear coefficient along the electric field direction. Figure 1d, e shows that the polymer-derived nanofibers possess high aspect ratio, which determines the potential for high-field conductive pathways (Supplementary Fig. S1). The doped Ga and W elements are distributed uniformly within the SiC nanofiber24,25 (Supplementary Fig. S2). The phase structure is verified through XRD, with the results demonstrating various diffraction peaks corresponding to SiC, Ga2O3 and WO3, associated with distinct lattice planes (Fig. 1f). The lattice mismatch caused by different crystal lattices in contact introduces abundant localized states and a redistribution of interfacial charges, thereby forming an internal electric field that acts as a potential barrier inside the nanofibers15,20–22. In particular, the XRD results of SiC nanofibers doped with various contents of Ga2O3 and WO3 suggest the formation of stable potential barriers (Supplementary Fig. S3). The measured interplanar spacings of 0.25 nm, 0.19 nm, and 0.22 nm correspond to the (111) plane of SiC, (024) plane of Ga2O3, and (222) plane of WO3, respectively, as indexed in the high-resolution TEM image of the nanofiber, demonstrating well-defined crystalline transitions within the nanofiber structure (Supplementary Fig. S4).

Our aim for incorporating WO3 and Ga2O3 into SiC nanofibers is to construct a Schottky barrier inside the nanofiber and enhance the barrier height, thus increasing the nonlinear coefficient and significantly improving field grading properties. Due to Ga2O3 possessing both higher conduction band energy and Fermi level compared to SiC, while WO3 exhibits lower conduction band energy and Fermi level relative to SiC (Fig. 1g)25,26, the formation of heterojunctions with SiC sandwiched between Ga2O3 and WO3 induces electron transfer from the high-Fermi-level Ga2O3 side through SiC to the low-Fermi-level WO3 side. These transferred electrons become trapped at the SiC-Ga2O3 and SiC-WO3 interface states, achieving Fermi level equilibration across the heterostructure. Ultimately, a built-in electric field at the interface formed and corresponding band bending occurred, as the double Schottky barrier schematically presented in Fig. 1h. It is crucial to note that this band structure is fundamentally different from the traditional “back-to-back” double-Schottky barrier, which we have designted as “step-by-step” double-Schottky barrier. This barrier not only exhibits a separately tuned bias voltage V1, V2, V3 and V4, but also possesses notably enhanced barrier height ΦGS + ΦSW which will result in a larger nonlinear coefficient responsible for the electric field-grading performance (Fig. 1i, j; Supplementary Fig. S5). Besides, the layer-by-layer structured materials were also produced by roll-to-roll sputtering system to further confirm the proposed “step-by-step” double Schottky barrier model. The cross-section SEM image and corresponding element mapping results demonstrate distinct layer-by-layer structure and the general applicability of the synthetic methodology across diverse material systems (Supplementary Fig. S6).

The voltage-modulated barrier height process of the “step-by-step” double-Schottky barrier can be further demonstrated by DFT calculations (Supplementary S2; Supplementary Data 1). The optimized structure model of SiC-Ga2O3-WO3 (denoted as SiC-GaW) hybrid system is presented in Fig. 2a, in which the n-type Schottky barrier heights are indicated between the conduction band minimum and the Fermi level27. The overall band structure and density of states (DOS) of the SiC-GaW configuration without bias voltage indicate that the Fermi level of this hybrid system locates in the band gaps of the respective semiconductors, resulting in the spontaneous formation of Schottky barriers (ΦSB) as electrons transit between Ga2O3, SiC, and WO3 (Fig. 2b). It is evident that the DOS near the conduction band is primarily determined by the states of Ga, W and O atoms, with Si and C atoms contributing less to the DOS. A strong and sharp peak can be clearly observed in the DOS above the Fermi level, indicating that the changes in the conduction band and the corresponding barrier height are mainly regulated by the doping Ga2O3 and WO3. It should be noted that due to the momentum mismatch between the valence band maximum and conduction band minimum in k-space for indirect semiconductors, electronic transitions necessitate simultaneous changes in energy and crystal momentum. This process requires phonon (lattice vibration) assistance to satisfy momentum conservation, resulting in a low transition probability. Therefore, it suppresses the intrinsic thermal generation of electron-hole pairs, leading to a very low concentration of free charge carriers under normal conditions. This inherently contributes to the excellent insulating properties observed at low electric fields. At high electric field, while the valence band can excite electrons to the conduction band, the key functional behavior of FGMs is primarily governed by the engineered interface states and potential barriers (“step-by-step” double Schottky barrier) within the SiC-GaW hybrid system. Doping and composite structures introduce a high density of defect levels and interface states, enabling adaptive field grading performance through trapping and release of charge carriers under applied electric fields20.

Calculations derived from Fig. 2b indicate that, at zero bias, the overall ΦSB for the SiC-GaW heterojunction is 0.94 eV. It is much more closed to experimental results compared to the PBE functional that has a strong self-interaction error that tends to delocalize electrons, resulting in an underestimated band gap by 30% or more28–30. In this work, we employed DFT+U method to correct the density delocalization errors (DDE) issues so that accurate structure and properties can be precisely predicted. In the analysis of the “step-by-step” double-Schottky barrier model, it indicates that the direction of the electric field determines the conduction band bending direction, while the electric field strength determines ΦSB. This in turn serves to regulate the overall structure and electronic properties of the heterojunction. Notably, under reverse bias, charges will decreased due to neutralization effect at the heterojunction interfaces of Ga2O3-SiC and SiC-WO3, consequently reducing the barrier height (Fig. 2c, d, e). When reverse biases of −0.2 V/Å, −0.4 V/Å, and −0.6 V/Å are applied, the barrier heights decrease from 0.94 eV (zero-bias value) to 0.88 eV, 0.6 eV, and 0.45 eV, respectively. When the barrier height is reduced to zero, abundant electrons can flow through the heterojunction interface and become electrically active. However, the heterojunction system will maintain insulating properties under low forward bias and only actively conducts when the bias reaches the threshold voltage (Supplementary Fig. S7). Above results confirm the stable “step-by-step” double-Schottky barrier within the nanofibers, with a stepwise barrier height. Compared to SiC, the flanking Ga2O3 and WO3 effectively enhance the barrier height, while the wide bandgap of Ga2O3 significantly improves the insulating properties of the material under low bias (Supplementary Table S1). It is important to note that the DFT models employed in this work represent idealized crystalline interfaces of SiC-Ga2O3-WO3. In contrast, the experimental TEM characterization reveals a more complex nanostructure containing grain boundaries, defects, and transition regions. The purpose of these simplified models is to elucidate the fundamental electronic interactions at SiC-Ga2O3 interface and SiC-WO3 interface. The agreement between the calculated trends (e.g., charge redistribution and barrier height) and the macroscopic electrical properties suggest that these interfacial electronic interactions are a primary governing factor of field grading performance.

Electrical performances of the SiC-GaW
To explore the mechanism of space charge transfer process and the distribution in the heterojunction interfaces, the charge density difference of SiC-GaW hybrid system without bias voltage is calculated by DFT (Fig. 3a). Upon contact between SiC, Ga2O3 and WO3, the Fermi level shift will be induced due to the differing electrostatic potential heights27. Once integrated into a unified electronics system, the Fermi level equilibrates to a consistent elevation (Fig. 3b). Electrons accumulate and deplete in the space charge region at the interfaces, leading to a redistribution of charge and the formation of an internal electric field, thereby establishing a Schottky barrier. Specifically, at the SiC-Ga2O3 interface in Fig. 3a, electrons transfer from the O atoms in Ga2O3 to the Si atoms in SiC. The charge accumulation region formed on the Si atom surface and the charge depletion region on the O surface collectively constitute the SiC-Ga2O3 interfacial electric field. Meanwhile, at the SiC-WO3 interface, electrons transfer from the C atoms in SiC to the O atoms in WO3. The charge accumulation region formed on the O atom surface and the charge depletion region on the C surface collectively constitute the SiC-WO3 interfacial electric field. As the forward and reverse biases increase, the distribution of electrons mainly changes at the SiC-WO3 interface because Ga2O3 is of high resistivity and can hardly produce free electrons under low bias voltage (Supplementary Fig. S8). However, once the bias voltage reaches a critical value, the SiC-WO3 interface fully conducts, while a significant voltage drop will be applied across the Ga2O3 side. Above discussion agrees well with the proposed “step-by-step” double-Schottky barrier model.

As barrier height is a vital factor determining the field grading performance, we experimentally investigated the band structure of SiC and SiC-GaW nanofibers with various doping concentrations employing combined characterizations of reflection electron energy loss spectroscopy (REELS) and ultraviolet photoelectron spectroscopy (UPS). The band gap energy of SiC and SiC-GaW nanofibers are depicted from the high-resolution REELS spectra where the difference between the low energy loss peak energy and elastic scattering peak energy represents the bandgap (Supplementary Fig. S9). Combined with the binding energy obtained from UPS onset and cutoffs spectra (Supplementary Fig. S10), the working function (WF) and Schottky barrier heights are estimated and presented in Fig. 3c. One can see that the doping content of Ga2O3 has a negative effect on WF while that of WO3 has a positive effect, also providing strong evidence that Ga2O3 has a relatively high Fermi level and WO3 has a relatively low Fermi level. Upon increasing Ga2O3 or WO3 doping contents, the Fermi level is correspondingly raised or lowered, respectively (Supplementary Table S2). In addition, note that the barrier height of SiC is estimated to be 0.44 eV, but due to the “step-by-step” double-Schottky barrier constructed within SiC-GaW nanofibers, the barrier heights of all SiC-GaW nanofibers have been improved to above 0.6 eV, which will significantly improve the nonlinear J-E characteristics.

To verify that a stable Schottky barrier was developed in the SiC-GaW nanofiber, the nonlinear electrical properties of a single nanofiber under forward and reverse biases were directly measured using conductive atomic force microscope (C-AFM, Fig. 3d). The bias was applied to the nanofiber to obtain I(V) characteristics, in which all SiC-GaW nanofibers present desirable nonlinear performance (Fig. 3e). As the doping concentration of Ga2O3 or WO3 increases, the switching voltage significantly decreases. This decrease can be attributed to the change of the grain size within the nanofiber, which will be discussed in detail in FGMs section12,14. Notably, the lower conduction band position and relatively narrower bandgap of WO3 have a more significant effect on reducing the switching voltage, which agrees well with the DFT discussion. The nonlinear coefficients of SiC-GaW nanofibers with different doping concentrations are all above 11, indicating that the successful construction of the “step-by-step” double-Schottky barrier significantly increases the barrier height of SiC, and the polarity reverse has little effect on the nonlinear I(V) characteristics (Supplementary Table S3). Specifically, the switching voltage and the nonlinear coefficient varies by no more than 0.2 V and 0.7, respectively, indicating that the Ga2O3 and WO3 dopants are uniformly distributed in the synthesized SiC-GaW nanofibers, thereby constructing a stable double-Schottky barrier.

Electric field grading behavior of the nanocomposites
SiC-GaW nanofiber mats (NFMs) were fabricated via large-scale electrospinning technology and incorporated into a typical polymer insulating material (epoxy resin) with parallel, vertical and rolled-up alignments (Fig. 4a, b). While randomly dispersed SiC-Ga15W15 nanofibers do not exhibit nonlinear behavior, aligned NFMs exhibit desirable nonlinear performance due to continuous percolation pathways (Fig. 4c). Notably, all FGMs with parallel stacked NFMs have higher Ec and lower α than those with vertical and rolled-up configurations (Supplementary Fig. S11 and Table S4). This phenomenon stems from: (1) thin epoxy layers bearing voltage drops, and (2) more compact fiber structures in rolled-up NFMs creating shorter percolation paths (Fig. 4d). Besides, the Ec of FGMs with vertical or rolled-up NFMs will decrease whatever doped with Ga2O3 or WO3 (Supplementary Fig. S12 and Table S5). Despite the ultra-low loading (0.3 vol%), the FGMs still present desirable nonlinear J(E) characteristics (Fig. 4e). The Ec values for EP/SiC-Ga15W15 and EP/SiC-Ga10W20 in the same loadings are nearly identical, indicating that the Ec is directly related to the total mass fraction of the doped metal oxide. Additionally, another advantage of these FGMs is the high and stable nonlinear coefficients, all of which are around 10, satisfying the requirements for electrical overstress control31 (Supplementary Table S6). Schottky barrier calculation confirmed higher barriers in our NFMs versus conventional FGMs, explaining their superior field grading performance (Fig. 4f; Supplementary Figs. S13-S15).

To the best of our knowledge, the resultant nonlinear coefficient exceeds all reported SiC-based FGMs16,32–38, as shown in Fig. 4g. While some FGMs that doped with ZnO microvaristors achieve nonlinear coefficients as high as 25, this invariably requires high filler loadings that compromise processability14,39,40. In contrast, graphene oxide filled FGMs exhibit a relatively high nonlinear coefficient (~16) at low loadings (3 vol%) due to high aspect ratio37. Building upon above work, the present study utilizes high-aspect-ratio nanofibers, with the optimal nanocomposite demonstrating excellent insulation-conductive transition ability under recorded ultra-low loading (0.3 vol%). However, the breakdown strength cannot be ignored in electronic industry. For FGMs, the breakdown strength must exceed twice the switching field to guarantee the proper functioning32. We measured the breakdown strength of EP/SiC-Ga10W20 and EP/SiC-Ga15W15 in various loadings, indicating that all composites exhibit breakdown strengths 3 to 5 times higher than their switching field (Supplementary Fig. S16 and Table S6). It demonstrates the promising potential for diverse applications. Besides, based on the measured steady-state current limit preventing thermal breakdown in FGMs during dielectric breakdown, we calculated the saturation current at pre-breakdown region and power rating applicable for electrostatic protection applications (Supplementary Fig. S17). The rated power ranges from 100 to 250 W, with corresponding switching field between 220 V/mm and 575 V/mm, making these materials suitable for various semiconductor devices including MOSFETs (100~200 V), GaAsFETs (100~300 V), thyristors (600~1000 V), SCL (680~1000 V) structures and CMOS (250~2000 V) integrated circuits. Therefore, the power rating and switching field of FGMs can be precisely customized according to detailed ESD protection applications. Despite the promising nonlinear electrical performance, the industrial adoption of these fibrous FGMs may face challenges due to historical concerns over material safety, warranting future efforts on secure encapsulation or alternative morphologies.

Practical application of the nanocomposites
To visually observe the voltage-controlled nonlinear J-E properties of FGMs, we designed an LED circuit (Fig. 5a; Supplementary Note). At threshold voltage UcR, L4 illuminated first, confirming lowest switching field of rolled-up NFMs. The rapid L4 brightening versus slower L3 response at UcP reflect the superior nonlinearity (higher α) of rolled-up NFMs (Supplementary Movie S1). LEDs with random nanofiber composites remained dark, verifying their linear behavior. The unique nonlinear electrical transport property also enables FGMs to present excellent electrostatic charge release performance, which is verified by electrostatic discharge (ESD) gun (Fig. 5b). The charge release performance is primarily dominated by Ec (See the Supplementary Note), as presented in Fig. 5c. When applying 150 V/mm (below Ec) on the FGMs sample, the charge decays slowly through the high bypass resistance from intinial Q0 of 11 nC (Fig. 5d). By contrast, when increasing the applied charge pulse to 18 nC (corresponding to 250 V/mm, higher than Ec), the residual charge in the intial state will be rapidly released to a lower level and then slowly decays (Fig. 5e). A similar charge release process is also observed from the FGMs with 0.3 vol% rolled-up NFMs (Fig. 5f, g). Consequently, the FGMs can be precisely tailored to meet diverse application requirements by optimizing their composition and microstructure for specific operating conditions. To evaluate the barrier stability under transient high-current and electrical stresses conditions, the EP/SiC-Ga10W20 sample was subjected to multiple ESD pulses and repeated electrical stresses, respectively. The post-stress electrical characteristic (nonlinear coefficient and switching field) results confirm that the composite maintains its functional barrier behavior without significant performance degradation, indicating excellent resilience against pulse-induced heating effects (Supplementary Fig. S18). This approach effectively mitigates the degradation of device stability caused by the mismatch between doped NFMs and the epoxy matrix.

Methods
Materials
The following chemicals used in this work were purchased from Sigma-Aldrich: Poly(ethylene oxide) (PEO, average Mv ~8,000,000 (nominal)), Gallium(III) acetylacetonate (Ga(acac)3), Chloroform (CHCl3), epoxy resin (Bisphenol A diglycidyl ether, D.E.R.TM 332), hexahydro-4-methylphthalic anhydride (96%, mixture of cis and trans, MHHPA) and 2-ethyl-4-methylimidazole (EMI). Polycarbosilane (PCS, Mn~1300) was purchased from SAIFEI Co. Ltd., China. Tungsten oxide (WO3, ~20 nm) nanoparticles were obtained from Shanghai Xiangtian Nano-Materials Co. Ltd., China. All chemicals were used as received without further purification.

Electrospinning process and assembly of NFMs
Preceramic polymer mothod is an effective approach to convert the polymer precursor to polymer-derived ceramics (PDC) via pyrolysis. Polycarbosilane (PCS) is a well-established precursor for polymer-derived SiC, while Gallium(III) acetylacetonate (Ga(acac)3) is a metallic organic precursor for the preparation of Ga2O3 via pyrolysis. During the electrospinning of the precursor fiber, we added the Ga(acac)3 and WO3 nanoparticle to PCS solution. The Ga(acac)3 and WO3 nanoparticle were uniformly dispersed in the PCS precursor fiber. After pyrolysis, the PCS was converted to SiC with Ga2O3 and WO3 uniformly incorporation.

In detail, the electrospinning solution was prepared using PCS, PEO, Ga(acac)3 and WO3 according to a certain weight ratio in CHCl3 (wt/vol%). The mixture was thoroughly and vigorously stirred using an electromagnetic stirrer over 5 h at room temperature to ensure the PCS and PEO dissolved completely to form a homogenous solution. Then, the solution was sonicated for 30 mins to ensure the homogenous dispersion of WO3 particles in the solution. Then the fabrication of PCS/Ga(acac)3/WO3 nanofiber mat was carried out with a needleless electrospinning machine (Nanospider NS LAB, ELMARCO, Inc., Czech). The solution cartridge rate was set at 200 mm/seconds, the applied voltage was 60 kV, and collector distance was 24 mm. The as-spun nanofiber mat shown in Fig. 4 can be easily peeled off from the roller collector and transferred to air oven curing at 200 °C for 5 h. Then the nanofiber mat was assembled into different orientation: parallel, vertical and rolled-up NFMs. At last, assembled SiC-Ga2O3-WO3 NFMs were pyrolyzed in a tube furnace under flowing argon to various temperatures at a ramp rate of 1 °C/min to 1300 °C and held at the temperature for 2 h. Rolled-up SiC NFMs, SiC-Ga2O3 NFMs and SiC-WO3 NFMs were also prepared for comparison.

Preparation of assembled NFMs/epoxy nanocomposites
In this work, NFMs/epoxy nanocomposites were fabricated through vacuum-assisted impregnation, which has been widely employed to achieve complete wetting of delicate reinforcements with minimal fiber damage41–43. The MHHPA and EMI were added into epoxy resin in a certain weight ratio (epoxy: MHHPA: EMI = 100: 90: 0.5) for vigorously stirring for 1 h at 60 °C. Then the parallel stacked NFMs, vertically stacked NFMs and Rolled-up NFMs were immersed into the epoxy solution until thoroughly infiltration of epoxy resin. The mixture was transferred into a vacuum for continuously degassing at 60 °C for 6 h. Finally, epoxy nanocomposites were placed on a stainless mold and cured at 100 °C for 2 h, 115 °C for 1 h and 135 °C for 1 h. The fabricated epoxy nanocomposites were named as EP/SiC-GaxWy, where x and y are the weight ratio (wt%) of Ga2O3 and WO3 in NFMs, respectively. The mass fraction of doped Ga2O3 and WO3 were determined by the weight ratio of PCS, Ga(acac)3 and WO3 nanoparticles. The volume fractions of NFMs with different alignment in epoxy nanocomposites were decided by the parallel stacked density, the vertical stacked density and the rolled-up density, respectively.

Characterization
For the electrospinning SiC-Ga2O3-WO3 nanofibers, the morphologies are observed by JEOL 7600 field emission scanning electron microscope (FESEM) and high-resolution transmission electron microscopy (HRTEM). Element mapping of single SiC-Ga2O3-WO3 nanofiber was also obtained during the HRTEM observation. To characterize the phase and structure, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses were conducted on a Bruker D2 Advance diffractometer with Cu-Kα radiation and a Thermo Kalpha spectroscopy with Al-Kα, respectively. To identify the band structure and calculate the barrier height, ultraviolet photoelectron spectroscopy (UPS) and reflection electron energy loss spectroscopy (REELS) analyses were performed in Thermo Scientific ESCALab 250Xi and Xi+, respectively. The nonlinear I-V characteristic of a single SiC-Ga2O3-WO3 nanofiber was measured by a conductive atomic force microscope (C-AFM).

For NFMs/epoxy nanocomposites, SEM was used to characterize differently assembled orientation of NFMs in epoxy nanocomposites. The nonlinear I-V characteristic of the nanocomposite sample was measured by the power source Keithley 2410C, which can provide a step-up DC voltage from 0 to 1100 V and measure the current I flowing through the sample at each voltage point. The dielectric breakdown strength was evaluated using a linear voltage ramp rate of 200 V/s. For each specimen, ten independent measurements were conducted, with the resulting data analyzed using two-parameter Weibull distribution statistics. The charge dissipation property of the FGMs was performed on an experiment setup, in which an ESD gun is applied to generate a charge surging with controllable overvoltage amplitude and a 100 MΩ HV probe is for monitoring the overvoltage waveform on the sample (Fig. 5b).

DFT calculations
Our DFT simulations was calculated from quantum mechanics based on the PBE flavor of density functional theory in which exchange and correlation were described using the generalized gradient approximation (GGA), and London dispersion (van der Waals attraction) was treated by the low gradient correction44–46. These periodic QM calculations were performed using the VASP package47,48. We found that a kinetic energy cutoff of 500 eV for the plane wave expansions gives excellent convergence of the total energies, energy differences, and structural parameters in structure optimization. The convergence criteria were a 1×10−6 eV energy difference for solving the electronic wavefunction and 1×10−3 eV/Å force for structure optimization. The simulation model was two-dimensional layered structure with 15 Å vacuum layer, and 3×3×1 Monkhorst-Pack k-point mesh for Brillouin zone integration was used. It should point out that the 20 nm WO3 particle was modeled as a thin sheet to reduce the computational cost of DFT calculations in this study. Since our study focuses on the interfacial electronic structure between Ga2O3, SiC, and WO3, which is primarily governed by local atomic coordination and electronic coupling at the contact plane rather than the three-dimensional morphology of the particles—properties preserved in the thin-sheet approximation. In addition, the calculations based on GGA for PBE for solids with Hubbard U (GGA+U) were performed by applying Hubbard corrections Up=9 eV for O 2p state. The DFT+U method can correct the density delocalization errors (DDE) issues so that accurate structure and properties can be precisely predicted.

The modeled system incorporates a vacuum layer along the z-direction, thereby eliminating the need to consider electron transport paths in this dimension. Following the orthogonal k-point labeling convention established in reference 49, the high-symmetry points in the reciprocal space of our band structure are mapped as shown in the figure below, where b1 and b2 represent the reciprocal lattice vectors along the x- and y-directions, respectively. The band structure in this work uses the Γ-X-S-Y-Γ path.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This is interesting work. The authors have created a hybrid fiber with a mixture of Schottky barriers to try to stabilize the non-linear field grading properties for charge dissipation. I have not seen this combination of materials before.\nThe authors do a nice job of creating fibers, and measuring the properties including the local field as a function of current of individual fibers using conducting AFM. The AFM results correlate well with their macroscopic results.\nA few concerns:\nI am not sure why the materials are called SAD, when really they are just a form of non-linear field grading materials. The authors should use the term used in the field of field grading materials.\nThe authors are missing some references in this field where others have been able to tune properties using particle size (Onneby et al.), or graphene oxide degree of oxidation (Wang et al.) or a key review of these materials (Donzel et al.) – which shows non-linear coefficients as high as 25 (much higher than in this work). The authors shoudl reflect these results in the graph comparing non-linear coefficients.\nThe paper is about twice as long (in words) than it needs to be. The authors are making unnecessary claims that can’t always be supported. The paper would be stronger if it was shortened and only the key points made needed to show the novelty of the results.\nThe electronics industry is very wary of using fibers. Asbestos created huge challenges and while the higher aspect ratio is valuable in terms of lower loading, it typically also leads to a much larger decrease in overall breakdown strength (not measured here), and could have health implications.\nI am concerned that the DFT does not properly reflect the structure in the fibers. The WO3 is a 20 nm particle and was modeled as a thin sheet. While the results are perhaps suggestive of what is happening in the composite fibers, the limitations of this modeling should be described.\nSEM does not have micron scale markers. And in the supplemental section, typically the lower region would not be published and a micron marker that is legible would be added. If I am reading the micron markers correctly for the SEM in the supplemental work, these are not nanofibers, but about 1micron diameter fibers. If that is so, the authors should reflect and not claim nanofibers.\nThe x-ray diffraction patterns are confusing. The WO3 peak only shows up for Ga2O3 15, WO3 10, and not for any other loadings. It seems that peak would be of higher intensity as the WO3 loading increases. Similarly, the SiC peaks are not always visible. I don’t know if the heat treatment didn’t lead to a full transformation or if there is a change in crystalline structure due to the mixing, but the authors should address this.\nFigure S5 is missing (d)-(f)\nThe methodology is sound, and likely could be reproduced.\nThe comment on the first page that in nanocomposites there inevitably defects between the polymer and particle is not correct any more than there are defects between the fiber and the polymer. That statement should be removed.", "reviewer_2": "General comment on the main contributions.\nThe article presented by the authors deals with the development of composite materials with non-linear conduction properties to deal with electrostatic discharge phenomena or voltage induced failure in electrical systems. The main approach is to develop a SiC-based fiber containing Ga and W oxides by thermal treatment of electrospun fibers. The electrical, structural and electronic properties of the characterised fibers are discussed and a new alternative model for Schottky barrier modulated conduction phenomena is proposed. The fibers are incorporated into a polymer matrix in different configurations (arrangement and concentration) to characterize non-linear properties and composite performance in a test object. The reported non-linear coefficient stands out compared to other reports containing only SiC particles, especially at a low concentration. Although extremely high non-linear coefficients are not obtained, the results are interesting because high values of non-linear coefficients (>25) can be obtained on composites with microvaristors composed of polycrystalline ZnO, with inherently high-volume concentrations.\nComments to the authors:\nIt is advised to make a thorough review if the terminology used concering the application fields. In my opinion, many terms might be changed or rephrased slightly to increase precision and readability. Complementary advice on the terminology can be provided by a fellow colleague working directly on some of the cited applications.\nVoltage in electrostatic discharges is a concern, but total power dissipation is a key factor, so the current density that can drive through the material must be high enough. In switching field materials and composites, the voltage threshold and nonlinearity coefficient are used to describe the behavior. For ESD protection, the main goal is to achieve very high nonlinear coefficients and the highest possible saturation current, hence increasing the power dissipation to meet the requirements of the application. Target values for switching field threshold, nonlinearity coefficients and power rating are application dependent. It is recommended that some typical targets be clearly stated. The authors justify part of the strategy of the study by aiming to lower the number of contacting interfaces between particles, as high particle loading might induce instabilities on the electrical performance. However, by moving to fibrous material, these number of possible contacting points are significantly increased as the aspect ratio increases. This would have a consequence of observing similar properties at much lower concentration (as shown from results in the present work).\nThe heterostructure SiC-WO3-Ga203 is derived from a pyrolysis and crystallization of electrospun precursors. SiC already is used as non-linear material, but the rationale to produce the SiC, WO3, and Ga203O3 and Ga2O3 secondary phases is not presented.\nThe produced nanofibers are composed of an assembly of individual phases of SiC, WO3, and Ga203. Simulations carried out to estimate the band structure of the junctions assume that no inter-diffusion or the transition region is well defined between phases. However, this can cannot be inferred unequivocally from the presented evidence. The proposed model of layer-by-layer double Schottky barrier is to be corroborated after more in-depth structural characterizations to fine tune the DFT simulations. Materials produced by other synthesis paths, such as ALD (or other) should also confirm this new proposed model.\nWhile non-linear coefficient is linked to Schottky barrier height, other parameters are also known to have significant impact, such as microstructure, heterogeneity, junction definition and distribution, number of conductive paths.\nAuthors propose that the presence of Ga2O3 in the heterostructures gives the advantage of lower current densities below the switching field. Data presented is not clear enough to confirm this statement.\nSince there is an impressive work on the synthesis of nanofibers, their characterization and the composite preparation. It is advised to narrow the scope of the article to present to reader a detail insight on the proposed nanomaterial preparation and switching field properties.\nThe proposed novel step-by-step barrier mechanism is hinted in a published work from the authors with another material configuration. Since the new investigated materials are suspected to have a new type of barrier transport mechanism, the authors could target these results and discussions in a separate and more in-depth article.\nStatement from the reviewer:\nThe detailed evaluation on the methodologies for DFT simulation and the adjustment from the UPS/REELS into band structure, fall out of my field of expertise.", "reviewer_3": "This paper addresses the simulation, fabrication, and testing of carbide-derived nanofiber mats with double Schottky barriers that can transition between insulating and conductive properties. The effect can be modulated by adjusting the nanofiber mat content. The effect is achieved with low volume fractions of the nanofibers (0.3 vol%). This material is significant in its electrostatic protective capabilities for electronic devices.\nMy main concern with the manuscript is the DFT modeling. In the main manuscript, the authors state that there may be DFT errors due to the self-interaction effects in the simulation. The authors are correct on this point, especially in the presence of oxygen atoms in crystalline systems, density delocalization errors (DDE) can be significant. There are methods to correct for this, most prominently the DFT+U method. This method is routinely used to correct the DDE issues so that accurate structure and properties can be predicted. I read through the manuscript and the SI, and I don’t see any mention of the use of methodologies to correct DDE, either the DFT+U method or other methods. Because these systems are highly correlated, I feel that this is crucially important for accuracy.\nUnless the authors conduct a study to fully understand the DDE, I don’t see how the results of the simulations can be reliable."}, "author_response": "Response: We thank the reviewer for their positive evaluation of this work.\n\n[... Authors' full point-by-point response to Round 1 ...]"}, {"round": 2, "reviewer_comments": {"reviewer_1": "Thanks for this significant rewrite.\n1. I still maintain that these are not intelligent materials and any mention of that should go away. The authors name them \"Field Grading Materials\" but field grading materials have been around a long time and this is an iteration on that using doped materials. (so please adjust that language where it sounds like you are claiming to define FGM to read something like, \"these are well known as FGM.\") I understand that they adapt, butthe use of intelligent here sounds great, but it suggests that this is a new type of material and it isn't. The organization of the fillers adds a nice twist to this and the work is interesting.\n2. The authors may also want to look at this paper. \"Graphene Oxide Filled Nanocomposite with Novel Electrical and Dielectric Properties.\" It is from quite awhile ago, but has some similar behaviors including the impact of interesting FGM properties at low loading due to the aspect ratio.\n3. While the bottom part of the SEMs was removed, a micron marker (or scale marker) was not added. This should be done. See figure 4.\n4. I maintain that many industries will be very hesitant to use fibers of any kind, but that is just based on my conversations with european industry and clearly not universal.", "reviewer_3": "The authors have satisfactorily addressed my concerns.", "reviewer_4": "This manuscript presents an innovative approach to developing field-grading materials (FGMs) for electrostatic protection in advanced electronic devices.\nThe authors demonstrate promising electrical properties at low loading and provide comprehensive DFT analysis alongside experimental validation. However, several technical issues need addressing before publication.\n1. The DFT calculations model idealized interfaces between crystalline phases, but TEM images suggest more complex nanostructures with potential grain boundaries, defects, and transition regions. How do you reconcile the simplified computational model with the actual fiber microstructure?\n2. The XRD results shown in Figure S3 indicate that the FWHM of the SiC-Ga15W10 peak has significantly increased, demonstrating enhanced amorphous phase characteristics. This exhibits a phenomenon distinct from other samples (SiC-Ga10W20, SiC-Ga10W15, etc.). Please add an explanation for this.\n3. The study of the band structure reveals a pronounced indirect band gap characteristic in the SiC-GaW hybrid system. Does this affect the behavior of charge carriers? The authors are requested to provide further explanation of this phenomenon.\n4. The Brillouin zone paths depicted in the band structure shown in Figure S7 and Figure 2 require further clarification. It is recommended to provide additional explanation in reciprocal space and cite supporting literature.\n5. Your power ratings assume steady-state conditions, but electrostatic discharge events involve high-current pulses that can cause heating. Have you tested thermal stability and barrier behavior under repeated high-current pulses?\n6. Practical electrostatic discharge protection requires discharge cycles without performance degradation. How do the heterojunction interfaces behave under repeated electrical stress?\n7. There are still some challenges in the mechanical integration. How do you prevent fiber damage during composite fabrication? What is the impact on mechanical properties and thermal expansion mismatch that could affect device reliability?"}, "author_response": "Response: Thank you very much for approving our efforts.\n\n[... Authors' full point-by-point response to Round 2 ...]"}, {"round": 3, "reviewer_comments": {"reviewer_1": "My concerns have been addressed", "reviewer_4": "The revised manuscript can be accepted as its current state."}, "author_response": ""}]

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