The challenge the researchers set out to address is fundamental to how direct current cables fail. In an HVDC transmission system, insulation must withstand hundreds of kilovolts across just a few centimeters of polymer. Under such intense fields, charge carriers are injected from the electrodes into the material, where they become trapped, migrate, and accumulate as space charge. This trapped charge distorts the local electric field, creating hot spots that can be far more intense than the applied field itself. Over years of service, these distortions accelerate aging, trigger partial discharges, and ultimately drive the insulation toward breakdown. Any material that hopes to serve as the next generation of cable insulation must therefore suppress charge injection, slow carrier mobility, and keep the internal field as uniform as possible.

Polypropylene has long been attractive for this role. It offers excellent electrical properties, high melting temperature, low cost, and, crucially, it is thermoplastic and fully recyclable, unlike cross-linked polyethylene, which is permanently cured and difficult to reprocess. Impact polypropylene copolymer, which blends a polypropylene matrix with an ethylene-propylene rubber phase for toughness, is a particularly promising variant. Yet until now, its optimization has been hampered by a lack of rigorous, quantitative correlations between structure and dielectric performance. Modifications were often evaluated only at the macroscopic level, leaving researchers to guess at the molecular and microscopic mechanisms responsible. The Tianjin team, led by You Wu, Zhonglei Li, and Boxue Du, set out to close that gap.

Their approach centered on a deliberately designed monomer. AOB packs three chemically distinct features into a single molecule: benzene rings, carbonyl groups, and ether bonds. Each of these contributes polarizable electrons and permanent dipoles, and together they create energy states within the polymer that can capture injected charge carriers. Using an aqueous suspension grafting method, the researchers covalently attached AOB onto the polypropylene chains, ensuring that the modifier would not simply migrate or bleed out of the insulation over decades of thermal and electrical stress, a known weakness of physically blended additives.

To understand what the grafted molecules were doing at the smallest scale, the team turned to density functional theory calculations and Kelvin probe force microscopy. The DFT simulations showed that the specific molecular architecture of AOB introduces deep traps for both electrons and holes, corresponding to the highest occupied and lowest unoccupied molecular orbitals of the modified system. These deep energy wells capture charge carriers that would otherwise roam freely through the polymer. The experimental signature came from KPFM measurements of surface potential decay: when the modified material was charged and then allowed to discharge, the surface potential lingered far longer than in the unmodified polymer. The decay time stretched from 3.9 seconds in the pristine material to 9.2 seconds after grafting, a clear indication that carriers were being held in place rather than moving and recombining rapidly.

Deep traps might sound like a liability, since trapped charge is precisely what forms space charge accumulation. The subtlety lies in where and how the charge is held. By capturing carriers close to where they are injected and immobilizing them in a dense distribution of shallow-to-deep trap sites, the grafted molecules prevent large, coherent packets of charge from building up deep inside the insulation. The net effect, confirmed at the macroscopic scale, was the dramatic 85 percent reduction in space charge accumulation. Slower carrier mobility also translated directly into lower measured conductivity, since conduction in these polymers is largely a matter of charge transport through amorphous regions and along interfaces.

The second half of the story unfolds at the mesoscale, in the polymer’s crystalline morphology. When AOB was grafted onto the material, it acted as a nucleating agent, prompting the formation of more numerous, smaller spherulites and increasing the overall crystalline density. This refinement matters because crystalline and amorphous regions of a semicrystalline polymer have different dielectric constants. Wherever these phases meet, the mismatch in permittivity causes local distortion of the electric field, concentrating stress at phase boundaries. Using finite element simulations built on a Voronoi algorithm to model the realistic geometry of the spherulitic structure, the researchers quantified this effect: the morphological changes induced by AOB reduced the dielectric mismatch between the crystalline and amorphous phases, cutting interfacial electric field distortion from 43 percent to 21 percent.

What distinguishes this study from much of the prior literature is how these scales were connected. Rather than treating molecular, microscopic, and macroscopic observations as separate narratives, the team first established direct quantitative correlations within each specific scale and only then integrated them into a comprehensive model. The deep traps characterized at the molecular scale explain the suppressed injection and mobility seen in surface potential measurements; the refined spherulitic morphology measured at the mesoscale explains the homogenized internal field; and the combination of these two mechanisms accounts for the macroscopic outcomes. The synergy is the point. Trap capture alone would slow charge but leave field distortions intact, while morphology refinement alone would homogenize the field but do nothing about the sheer quantity of injected charge. Together, they attack the failure cascade from both ends.

The macroscopic results follow logically from that dual mechanism. With fewer carriers injected and those carriers immobilized, space charge accumulation collapsed by 85 percent. With carrier transport suppressed throughout the bulk, conductivity dropped by an order of magnitude, which in turn reduces Joule heating and thermal aging of the insulation. And with the internal field distribution smoothed by the refined crystalline structure, the material withstood a direct current breakdown strength 30 percent higher than the unmodified polymer before failure. Each of these metrics is directly relevant to cable design, where higher breakdown strength allows thinner insulation walls, and lower conductivity and space charge translate into longer service life and higher permissible operating temperatures and voltages.

Beyond the specific numbers, the study offers a template for rational materials design in electrical insulation. Instead of screening additives empirically, the framework demonstrated here, from molecular orbital engineering through trap spectroscopy and morphological modeling to macroscopic dielectric testing, provides a quantitative theoretical basis for predicting how a given chemical modification will perform. As grids worldwide expand high-voltage direct current transmission to move renewable electricity over long distances, the demand for recyclable, high-performance insulation will only grow. A modified polypropylene that can be grafted, characterized, and optimized with this level of mechanistic clarity brings that future measurably closer, and suggests that the next generation of power cables may be built on plastics engineered one molecular feature at a time.

Subject of Research: Multiscale structural modulation of grafted polypropylene for high-voltage direct current cable insulation

Article Title: Enhanced dielectric performance of polypropylene for hvdc cable insulation via 4‑allyloxybenzil grafting for multiscale structural modulation

Article References: Wu, Y., Li, Z., Wang, H., Cao, G., Zheng, Z., Du, H., & Du, B. (2026). Enhanced dielectric performance of polypropylene for hvdc cable insulation via 4‑allyloxybenzil grafting for multiscale structural modulation. Advanced Composites and Hybrid Materials. https://doi.org/10.1007/s42114-026-02115-9

Image Credits: AI Generated

DOI: 10.1007/s42114-026-02115-9

Keywords: polypropylene, HVDC cable insulation, dielectric properties, 4-allyloxybenzil, chemical grafting, space charge, deep traps, spherulites, breakdown strength, density functional theory, Kelvin probe force microscopy, multiscale modeling

The material at the center of the study is a glass-ceramic based on the PSBNT system, a lead-containing niobate-titanate composition that crystallizes into what researchers call tetragonal tungsten bronze niobate, a crystal structure prized in dielectric applications. Glass-ceramics like this are made by preparing a glass, crystallizing it into a ceramic phase, and then sintering the resulting powder into a dense solid. The problem has always been that sintering these niobate-based powders requires high temperatures, and high temperatures bring unwanted consequences: exaggerated grain growth, evaporation of volatile components such as lead, and porosity that undermines both dielectric performance and electrical breakdown strength. The team, led by Yunhe Yi of Northeastern University in Shenyang together with colleagues from the GRINM Group, GRIMAT Engineering Institute, the General Research Institute for Nonferrous Metals, and the Shanghai Institute of Ceramics, attacked this problem with a sintering aid called PZABS glass, a low-melting additive designed to soften and flow well below the temperature at which the main ceramic phase would normally densify.

The manufacturing route is as important as the chemistry. Rather than pressing dry powder into pellets, the researchers used tape casting, a scalable industrial process in which the powder is mixed with solvents, binders, and plasticizers into a slurry and spread into thin, uniform sheets. Tape casting is the same technique used to fabricate multilayer ceramic capacitors, where hundreds of thin dielectric layers are stacked and co-fired together. That connection matters because any improvement demonstrated in a tape-cast system translates more directly to real capacitor manufacturing than results obtained on conventionally pressed bulk ceramics. Prior work in the field, including studies on binder burnout shrinkage and compaction in multilayer capacitor fabrication, has shown how sensitive the final microstructure is to the details of this processing chain, which is precisely why the authors chose to investigate densification behavior in a tape-cast format.

The central variable in the experiments was the amount of PZABS glass added to the PSBNT powder, and the results traced a classic goldilocks curve. With no additive or too little of it, the ceramic particles sintered sluggishly, leaving pores scattered through the microstructure. As the glass content rose, the liquid phase formed during firing filled the gaps between particles, lubricated their rearrangement, and accelerated mass transport, driving the density up and the porosity down. At an addition of just 2 weight percent PZABS, the glass-ceramic reached its best state: a density of approximately 4.84 grams per cubic centimeter and a porosity of only about 0.30 percent, remarkably low numbers for a sintered niobate glass-ceramic. The liquid glass essentially acted as a transient glue, wetting the particle surfaces and pulling the compact together before it crystallized or vaporized away.

But the story turns when the glass dose climbs further. At 6 weight percent PZABS and above, the researchers observed the precipitation of a secondary phase, lead niobate-titanate with the formula Pb2(Nb0.67Ti0.33)2O6.67, alongside the dominant tetragonal tungsten bronze phase and minor silica. Excess glass also worsened densification rather than helping it, a counterintuitive result that the authors attribute to the way too much liquid phase disrupts the sintering balance, promoting secondary crystallization and degrading the microstructural uniformity that dense ceramics require. In other words, the sintering aid is a tool with a narrow effective window: enough to flow and fill pores, but not so much that it changes the phase chemistry of the material. This densification-versus-phase-stability tradeoff is a recurring theme in glass-assisted sintering, and the study quantifies it cleanly for this system.

The dielectric measurements tell the practical story. The optimally doped 2 weight percent sample achieved a relative dielectric permittivity of about 949, a high value that reflects both the intrinsic polarizability of the tungsten bronze niobate crystals and the near-absence of porosity, since air-filled pores dilute permittivity and concentrate electric fields. Just as importantly for energy storage, the sample withstood a breakdown strength of roughly 220 kilovolts per centimeter. Breakdown strength is the ceiling on how much voltage a capacitor can take before it fails, and dense, pore-free microstructures are the key to raising it, because pores and defects act as failure initiation sites. Combining high permittivity with high breakdown strength is the central balancing act of dielectric energy storage research, since the stored energy density scales with the square of the breakdown field.

Those two quantities together delivered the headline numbers of the study: a maximum energy density of 1.03 joules per cubic centimeter and a recoverable energy density of 0.82 joules per cubic centimeter. The gap between the two reflects hysteresis losses in the ferroelectric response, energy that is dissipated as heat rather than returned during discharge, and minimizing that gap is what efficiency means in this context. The researchers also tracked how the additive changed the character of the polarization response. Increasing PZABS content enhanced the relaxor behavior of the glass-ceramic, with the diffuseness exponent gamma rising from 1.81 to 1.96. Relaxor ferroelectrics have smeared-out, frequency-dependent transitions and slim polarization loops, which generally means lower hysteresis loss and better energy-storage efficiency, so the shift toward stronger relaxor character is a meaningful microstructural lever, not just a curiosity.

Placed in context, the work connects to a broad and active effort across the ceramics community to use glass additives as sintering accelerants and property modifiers. Previous studies have employed calcium-aluminosilicate glasses in strontium barium niobate ceramics, phosphorus pentoxide doping to enhance densification of SrO-BaO-Nb2O5-SiO2 tungsten bronze glass-ceramics, and zinc-bismuth-silicate glasses in barium titanate systems, while low-temperature co-fired ceramic technology has long relied on glass phases to bond ceramic layers at reduced temperatures. What distinguishes the new study is the combination of a deliberately low-melting PZABS glass, a tape-casting route compatible with multilayer device fabrication, and a systematic mapping of how additive content controls the full chain from thermal behavior and phase evolution to microstructure and final dielectric and ferroelectric performance. The authors also situate their findings within recent work on defect engineering and crystallization control in glass-ceramics, where managing the defects formed during crystallization has emerged as a parallel strategy for boosting energy storage.

There are honest caveats. The material contains lead, which raises environmental and regulatory questions in commercial electronics, even as much of the field pushes toward lead-free alternatives such as sodium niobate and bismuth-based systems. And an energy density of about one joule per cubic centimeter, while respectable for a sintered glass-ceramic, remains below the targets for the most demanding pulsed-power applications, where researchers pursue several joules per cubic centimeter through strategies like configurational entropy tuning and viscous polymer processing. The value of this study lies less in a record number and more in the demonstration of a controllable, industrially relevant processing knob: a two-weight-percent glass addition that simultaneously maximizes density, preserves the desired crystal phase, and tunes the relaxor response. The work was supported by the Beijing Natural Science Foundation, and the corresponding authors are Qingmeng Zhang and Zheng Lu.

For engineers designing the next generation of capacitors, the message is that sintering aids deserve the same careful optimization as the ceramic chemistry itself. The difference between the best and worst formulations in this study was only a few weight percent of glass, yet it separated a dense, high-permittivity, high-breakdown material from a porous one littered with secondary phases. As demand grows for compact, efficient energy storage in electrified transport and renewable power systems, such processing-level insights, validated in tape-cast form factors that mirror real device fabrication, are exactly the kind of incremental but transferable knowledge that moves dielectric materials from laboratory promise toward manufacturing reality.

Subject of Research: Low-melting glass-assisted sintering of PSBNT niobate glass-ceramics for dielectric energy storage

Article Title: Low-melting glass-assisted sintering of PSBNT glass-ceramics: densification, microstructure evolution and dielectric properties

Article References: Yi, Y., Chen, J., Zhao, Y., Zhou, M., Zhang, Q., & Lu, Z. (2026). Low-melting glass-assisted sintering of PSBNT glass-ceramics: densification, microstructure evolution and dielectric properties. Journal of Materials Science. https://doi.org/10.1007/s10853-026-13841-2

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13841-2

Keywords: glass-ceramics, sintering, tape casting, dielectric materials, energy storage, tungsten bronze, relaxor ferroelectrics, sintering aids, porosity, breakdown strength, capacitors, materials science