Extruded polymeric HVDC cables have drawn great attention in modern power systems. This paper reviews the history and development of the polymeric HVDC cables and summarizes the key technical problems in extruded HVDC cables. It is pointed out that two key issues should be solved. One is the electric field inversion within HVDC cable insulation which is caused by the temperature and electric field dependent DC volume resistivity of the polymeric insulation materials. The other is the space charge behavior under multi-fields coupling, including charge injection, transportation, accumulation and dissipation characteristics. The following aspects need to be particularly concerned in the future: the interaction between temperature, electric field, space charge and DC volume resistivity under multi-fields coupling; mechanisms of nanoparticles doping on enhancing the properties of polymeric insulation material; the long-term operation characteristics of nanodielectrics; collaborative optimal regulation methods and theories on the properties of polymeric insulation material; and recyclable insulation materials for future HVDC cables.

Polymeric insulation materials for HVDC cables: Development, challenges and future perspective

Polypropylene (PP) has shown great potential as recyclable HVDC cable insulation material. This paper aims to investigate the effect of different nanoparticles on tuning the electrical properties of PP. Surface modified MgO, TiO 2 , ZnO and Al 2 O 3 nanoparticles with various contents were melt blended with PP. The microstructural morphology, dielectric properties, DC volume resistivity, space charge behavior and DC breakdown strength of the nanocomposites were investigated. It was found that the dielectric permittivity all increases with the increase of nanoparticle content, only the dielectric loss of TiO 2 /PP nanocomposites increases. For MgO and TiO 2 nanocomposites, the DC volume resistivity increases with the increase of nanoparticle content and then decreases, while it continues to increase for Al 2 O 3 nanocomposite. TiO 2 /PP nanocomposite with 5 phr nano-TiO 2 shows lower volume resistivity than PP. Space charge suppression and DC electric breakdown strength show similar variation trend, both increase with the increase of nanoparticle content and then decrease. MgO and TiO 2 nanocomposites show the most obvious space charge suppression and TiO 2 nanocomposites exhibit the highest DC breakdown strength, which is 43% higher than that of pure PP. Considering the electrical properties investigated, the optimal content for MgO, TiO 2 , ZnO and Al 2 O 3 nanoparticles is about 3, 1, 1 and 1 phr, respectively. Among these four kinds of nanoparticles, MgO and TiO2 nanoparticles are more capable than ZnO and Al 2 O 3 nanoparticles to modify the electrical properties of PP and more potential to be used as recyclable HVDC cable insulation material.

Effect of different nanoparticles on tuning electrical properties of polypropylene nanocomposites

Flexible polymer-matrix nanodielectrics have recently been attracting more attention in the fields of frontier research internationally since the beginning of the 21st century. It would accelerate a technical reformation for the applications in new energies, bioscience and biomedical engineering, artificial intelligence, electronic skin, wearable electronic clothing, Internet of things, and electrical insulation. This review summarises the recent development and achievements of nanodielectrics, including space charge suppression, high energy density storage, partial discharge resistance, non-linear field grading properties, high thermal conductivity, and future applications in biomedical area. Special attention is given to two representative interface models that are the Lewis's diffusion electrical double-layer model and Tanaka's multi-core model. At last, some issues related to the technology of nanodielectrics have also been discussed.

Past and future on nanodielectrics

In this paper, titanium dioxide (TiO2)/phenolphthalein-type polyimide (PPPI) composite was synthesized in situ polymerization, the diamino phenolphthalein type compound and dimethylacetamide as the matrix, and TiO2 as the filler. The mechanical property and thermal conductivity properties were used to characterize the properties of the obtained composite. A diamine monomer containing more benzene rings of phenolphthalein and an ether bond in its main chain were prepared, synthesized by a nucleophilic substitution reaction and a hydrazine hydrate reduction reaction. Polyimide composite had the same structure as that of a diamine monomer. And it was synthesized by solution imidization and heat imidization. The results showed that TiO2 improved the dispersibility, alignment and interfacial strength of the TiO2/PPPI composite. The mechanical property of polyimide composite was increased with the increasing TiO2 loading. And a suitable addition of TiO2 could enhance the mechanical properties of polyimide system. Compared to polyimide resin, the TiO2/PPPI composite had a higher thermal conductivity with the increasing of TiO2 content. All of these changes in properties were closely correlated with the TiO2/PPPI composite, which could form an interaction interface structure in the system.

In situ synthesis and preparation of TiO2/polyimide composite containing phenolphthalein functional group

Materials with superior thermal conduction, stable electrical insulation, and excellent mechanical strength that can be obtained at low cost are desirable for electronic packaging and thermal management in electrical engineering systems. There is therefore relentless interest in developing highly robust thermally conductive and electrically insulating polymer-based composites. Herein, a facile and environmentally friendly method to construct three-dimensional thermal transport channels in a poly(vinyl alcohol) (PVA) matrix composite is reported. The formation of thermal transport highways improves the thermal conductivity up to 4.79 W m−1 K−1 for alumina (Al2O3) loading of 45.4 vol% in the composites. The thermal conductivity achieved is the highest value for this Al2O3 content range compared with reported work, and is 1700% compared with that of pure PVA. In the meantime, the composites still retain outstanding electrical insulation properties and excellent mechanical strength. The heat-conducting and electrically insulating composites not only have promising applications in electronic packaging and electrical engineering systems, but the technically facile vacuum-assisted infiltration method is a new avenue for the mass production of spherical filler/polymer composites.

Effective thermal transport highway construction within dielectric polymer composites via a vacuum-assisted infiltration method

In the recent decades, the phenomena of space charge accumulation in the high voltage direct current (HVDC) insulation have been attracted more attention. In this paper, low density polyethylene (LDPE) nanocomposites filled with alumina nanoparticles (nano-Al2O3) were prepared employing melting blend method. Morphologies of nanoparticles and LDPE/Al2O3 nanocomposites were performed by scanning electron microscopy (SEM). Electrical properties of the LDPE nanocomposites were also investigated. Results shown that the nano-Al2O3 particles modified with vinyl silane coupling can effectively enhance the breakdown strength of LDPE nanocomposites. With the nano-Al2O3 particles loading, the volume resistivity of the LDPE nanocomposites was increased, while dielectric permittivity of the nanocomposites was decreased. Space charge of the LDPE nanocomposites was measured by pulsed electro-acoustic (PEA) method. The charge profiles indicated that space charge suppression of the LDPE nanocomposites was better than that of pure LDPE. The excellent insulation properties of the LDPE nanocomposites were attributed to the better interfacial adhesion between the surface-treated nano-Al2O3 particles and the LDPE matrix.

Preparation, microstructure and properties of polyethylene/alumina nanocomposites for HVDC insulation

Sagittal reconstruction of the atlantoaxial lateral mass complex with an intra-articular cage fusion technique for degenerative atlantoaxial instability.

Cartilage regeneration is dependent on cellular-extracellular matrix (ECM) interactions. Natural ECM plays a role in mechanical and chemical cell signaling and promotes stem cell recruitment, differentiation and tissue regeneration in the absence of biological additives, including growth factors and peptides. To date, traditional tissue engineering methods by using natural and synthetic materials have not been able to replicate the physiological structure (biochemical composition and biomechanical properties) of natural cartilage. Techniques facilitating the repair and/or regeneration of articular cartilage pose a significant challenge for orthopedic surgeons. Whereas, little progress has been made in this field. In recent years, with advances in medicine, biochemistry and materials science, to meet the regenerative requirements of the heterogeneous and layered structure of native articular cartilage (AC) tissue, a series of tissue engineering scaffolds based on ECM materials have been developed. These scaffolds mimic the versatility of the native ECM in function, composition and dynamic properties and some of which are designed to improve cartilage regeneration. This review systematically investigates the following: the characteristics of cartilage ECM, repair mechanisms, decellularization method, source of ECM, and various ECM-based cartilage repair methods. In addition, the future development of ECM-based biomaterials is hypothesized.

Preparation and Application of Decellularized ECM-Based Biological Scaffolds for Articular Cartilage Repair: A Review

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Papers

Preparation, microstructure and properties of polyethylene/alumina nanocomposites for HVDC insulation

Sagittal reconstruction of the atlantoaxial lateral mass complex with an intra-articular cage fusion technique for degenerative atlantoaxial instability.

Preparation and Application of Decellularized ECM-Based Biological Scaffolds for Articular Cartilage Repair: A Review