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The current study focused on improving the properties of polylactic acid (PLA) for wider application in load‐bearing scenarios. Various methods were explored to optimize the interaction between PLA and natural fibers, particularly wood fibers (WFs). Alkalized and epoxy‐impregnated WFs were evaluated against untreated WFs and cellulose fibers in both injection molding (IM) and fused deposition modeling (FDM). FTIR analysis revealed the removal of hemicellulose and lignin in alkalized WFs and uniform epoxy curing. Addition of fibers reduced PLA's thermal stability while acting as nucleating agents. Additionally, fibers augmented the storage modulus of biocomposites, with alkalized fibers exhibiting the highest tensile modulus in IM. FDM samples with a 0° raster angle showed superior impact resistance compared to IM counterparts. Moreover, raster angle significantly influenced FDM biocomposite properties, enhancing the tensile strength and modulus of untreated WF and cellulose fibers at 0°. Although FDM did not produce alkalized WF samples, epoxy impregnation emerged as a promising method for enhancing PLA/WF composite mechanical properties in the IM process, offering valuable insights for composite material development. [ABSTRACT FROM AUTHOR]

Polymer nanodielectrics render a great material platform for exhibiting the intrinsic nature of incorporated particles, particularly semiconducting types, and their interfaces with the polymer matrix. Incorporating the oxide fillers with higher loading percentages (>40 vol%) encounters particular challenges in terms of dispersion, homogeneous distribution, and porosity from the process. This work investigated the dielectric loss and electrical conduction behaviors of composites containing semiconducting ZnO varistor particles of various concentrations using the epoxy impregnation method. The ZnO varistor particles increased the dielectric permittivity, loss, and electrical conductivity of the epoxy composites into three different regimes (0–50 vol%, 50–70 vol%, 70–100 vol%), particularly under an electric bias field or at higher temperatures. For lower loading fractions below 50 vol%, the dielectric responses are dominated by the insulating epoxy matrix. When loading fractions are between 50 and 70 vol%, the dielectric and electric responses are mostly associated with the semiconducting interfaces of ZnO varistor particles and ZnO–epoxy. At above 70 vol%, the apparent increase in the dielectric loss and conductivity is primarily associated with the conducting ZnO core forming the interconnected channels of electric conduction. The foam-agent-assisted ZnO varistor particle framework appears to be a better way of fabricating composites of filler loading above 80 vol%. A physical model using an equivalent capacitor, diode, and resistor in the epoxy composites was proposed to explain the different property behaviors. [ABSTRACT FROM AUTHOR]

Polymer nanodielectrics render a great material platform for exhibiting the intrinsic nature of incorporated particles, particularly semiconducting types, and their interfaces with the polymer matrix. Incorporating the oxide fillers with higher loading percentages (>40 vol%) encounters particular challenges in terms of dispersion, homogeneous distribution, and porosity from the process. This work investigated the dielectric loss and electrical conduction behaviors of composites containing semiconducting ZnO varistor particles of various concentrations using the epoxy impregnation method. The ZnO varistor particles increased the dielectric permittivity, loss, and electrical conductivity of the epoxy composites into three different regimes (0–50 vol%, 50–70 vol%, 70–100 vol%), particularly under an electric bias field or at higher temperatures. For lower loading fractions below 50 vol%, the dielectric responses are dominated by the insulating epoxy matrix. When loading fractions are between 50 and 70 vol%, the dielectric and electric responses are mostly associated with the semiconducting interfaces of ZnO varistor particles and ZnO–epoxy. At above 70 vol%, the apparent increase in the dielectric loss and conductivity is primarily associated with the conducting ZnO core forming the interconnected channels of electric conduction. The foam-agent-assisted ZnO varistor particle framework appears to be a better way of fabricating composites of filler loading above 80 vol%. A physical model using an equivalent capacitor, diode, and resistor in the epoxy composites was proposed to explain the different property behaviors.

The high-temperature superconducting (HTS) racetrack pancake (RP) coil is recommended to be used as a field coil in rotating machines such as generators and motors. Epoxy impregnation is normally required in the field coil to protect it against mechanical stresses due to the variation in the magnetic field and to enhance its mechanical stability to withstand rotor vibrations. However, the thermal conductivity of epoxy is very low compared with that of HTS. Consequently, when quenching is induced in an epoxy-impregnated coil, the Joule heating is poorly dissipated outward because of the absence of thermal drain to dissipate the heat in liquid nitrogen (LN2), which damages the coil through heat accumulation. We designed cooling channels (pores) through the surface of an epoxy-impregnated coil to boost the circulation of LN2 in the coil during the cooling process and to improve the operation of the coil during quenching. In this study, cooling-down and quench tests were performed on the epoxy-impregnated RP coils to investigate the effect of cooling channel on the thermal-quench characteristics of the RP coils. Moreover, the minimum quench energy and normal-zone propagation velocity of the hotspot induced in the coils were discussed on the basis of the quench-test results.

The intra-layer no-insulation (LNI) method is effective for protecting a layer-wound REBCO coil from a quench. However, a recently reported hoop stress modification (both increase and decrease) due to screening currents is a potential problem causing conductor degradation during charging. Epoxy impregnation can suppress the hoop stress modification and in this work, we develop prototypes of epoxy-impregnated LNI-REBCO coils to be charged at 77 K. Degradation of the coil performance due to cooling down can be prevented by mechanically isolating each layer. The coil shows a reasonably short field delay and current bypassing behavior, which are major technical benefits of the LNI-REBCO coil. A structural analysis with screening current-induced electromagnetic forces qualitatively suggests that the epoxy-impregnated LNI-REBCO coil can support a compressive hoop stress which causes conductor degradation due to buckling. The tensile hoop stress, however, is still very high at the upper and lower ends of each layer, causing fracture of a superconducting layer; i.e., additional reinforcement is required. [ABSTRACT FROM AUTHOR]

We have designed and built a small, conduction cooled, 1.5 T high-temperature superconductor (HTS) magnet. The magnet coils have been wet-wound with a novel conductive epoxy resin system to achieve a derivative of the no-insulation coil winding method. The epoxy is filled with copper powder to reliably set the turn-to-turn contact resistivity and diamond powder to accurately space the coil turns apart, as well as match the filled epoxy thermal contraction rate to that of the HTS. The magnet is designed to act as a test bed to measure sudden discharge rates and hence validate models of coil behaviour using the filled epoxy system we have developed. A lumped-element electrical model was used to predict the coil contact resistivity needed to bring the magnetic field of the 1.5 T magnet down to a near-zero level within 0.5 s. The conductive epoxy blend was tuned to give a contact resistivity of 1 × 10−6 Ω m2, based on previous measurements of coil contact resistivity at 77 K. Once the magnet was wound and tested at 40 K, we found the magnet's sudden discharge time constant was 500 ms rather than 69 ms as was predicted from the 77 K measurements. The discrepancy was traced to the temperature dependence of the contact resistivity. With further testing of the individual coils this was accounted for and the model adjusted. The predicted discharge time was 2 s, in good agreement with the measured sudden discharge time of the magnet. [ABSTRACT FROM AUTHOR]

For the economical construction of fatigue loaded structures with textile reinforced concrete (TRC), it is necessary to investigate the fatigue behavior of the materials. Since next to the tensile load-bearing behavior, the bond behavior of a material is crucial as well, the present paper deals with the bond fatigue of TRC with epoxy-impregnated carbon textiles. First, static tests are carried out to determine the sufficient anchorage length of the investigated material combination. Afterwards, the influence of cyclic loading on the necessary anchorage length, deformation, stiffness, and residual strength is investigated. The results of the cyclic tests are summarized in stress-number of cycles to failure (S-N) diagrams. In the end, it can be said that the cyclic loading has no negative impact on the necessary anchorage length. If specimens withstand the cyclic loading, there is no difference between their residual strength and the reference strength. The failure of specimens occurs only at high load levels, provided that the anchorage length is sufficient. [ABSTRACT FROM AUTHOR]

For the economical construction of fatigue loaded structures with textile reinforced concrete (TRC), it is necessary to investigate the fatigue behavior of the materials. Since next to the tensile load-bearing behavior, the bond behavior of a material is crucial as well, the present paper deals with the bond fatigue of TRC with epoxy-impregnated carbon textiles. First, static tests are carried out to determine the sufficient anchorage length of the investigated material combination. Afterwards, the influence of cyclic loading on the necessary anchorage length, deformation, stiffness, and residual strength is investigated. The results of the cyclic tests are summarized in stress-number of cycles to failure (S-N) diagrams. In the end, it can be said that the cyclic loading has no negative impact on the necessary anchorage length. If specimens withstand the cyclic loading, there is no difference between their residual strength and the reference strength. The failure of specimens occurs only at high load levels, provided that the anchorage length is sufficient.

The critical current (Ic) of epoxy impregnated YBCO coil is prone to degrade due to the cumulative radial tensile stress. Winding tension has the advantage to reduce the amount of epoxy resin and decrease the radial tensile stress during cool down process. In this paper, the dry and wet coils wound on copper and polytetrafluoroethylene (PTFE, Teflon) bobbin materials with various winding tensions were manufactured and tested. With increasing the winding tension, the winding strain increased but the thermal strain reduced. The low coefficient of thermal expansion for bobbin and epoxy impregnation could decrease thermal contraction of the innermost layer of coil. The winding tension effectively reduced the degradation of epoxy impregnated YBCO coil. The bobbin materials and winding tension of 10–50 N had no effect on the degradation of Ic. A proper winding tension of 20 N was successfully used to fabricate an epoxy impregnated YBCO coil of 30 turns without degradation [ABSTRACT FROM PUBLISHER]

For the conduction cooling application, epoxy impregnation is inevitable to enhance the thermal conduction. However, there have been several research results on the delamination problem with coated conductor and the main cause of the delamination is related with the different thermal contraction between epoxy, the insulation layer and the weak conductor. To avoid this problem, the amount of epoxy and insulation layer between conductors should be minimized or removed. Therefore, no insulation (NI) winding method and impregnation after dry winding can be considered to solve the problem. The NI coil winding method is very attractive due to high mechanical/thermal stability for the special purpose of DC magnets by removing the insulation layer. In this paper, the NI coil winding method and vacuum impregnation are applied to a HTS coil to avoid the delamination problem and enhance the mechanical/thermal stability for the conduction cooling application. Through the charging/discharging operation, electric/thermal characteristics are investigated at 77 K and 30 K. [ABSTRACT FROM AUTHOR]

The A15-type superconductors (\Nb3\Sn, \Nb3 \Al) have been used to fabricate the 10-15-T high-field superconducting magnets. Due to the strain sensitivity of this type of conductors, minimization of the strain in conductors is an important issue to realize the highest critical current density during operation. It is also known that the superconductors in epoxy-impregnated coils exhibit much less critical current degradation than wire samples when subjected to transverse compressive stress, due to the stress buffer effect of the epoxy and insulation materials among strands. This paper presents analytical and numerical investigation of the stress and strain distributions in A15-type superconducting coils, which are usually wound with copper-stabilized Rutherford cable and are epoxy impregnated. Analytical formulas have been derived to calculate the strain in superconductors from the average strain in coils. In addition, a FEM simulation model has been built to get a clear view of the strain distribution between superconductors and the surrounding insulation materials. [ABSTRACT FROM PUBLISHER]

Abstract: Degradation of the epoxy impregnated YBCO coil performance is due to tensile radial stress concentration on the outer edge of the conductor during cool down. This stress acts as a cleavage stress and opens the conductor edge, fracturing the YBCO layer. The fracture propagates to another edge of the conductor, resulting in degradation of the coil performance. Degradation of the epoxy impregnated YBCO coil is eliminated, if we use a polyimide-electrodeposited YBCO-coated conductor: tensile radial stress concentration on the outer edge of the conductor is reduced due to plastic deformation of the ductile polyimide. Polyimide electrodeposition onto the YBCO-coated conductor is reliable, uniform, easy to apply, and can be extended to larger YBCO magnets, removing the risk of coil degradation. [Copyright &y& Elsevier]

Abstract: Now that YBCO-coated conductors have been commercialized, a number of YBCO coils have been developed. However, their basic performances have not been systematically investigated so far. Here, we demonstrate that of a YBCO double pancake coil. The critical current of an epoxy impregnated YBCO double pancake coil was substantially degraded, i.e. the normal voltage appears above 8A, only 18% of that for the dry coil. It was inferred that degradation occurs if the cumulative radial stress developed during cool down exceeds the critical transverse stress for the YBCO-coated conductor (typically 10MPa). Under these conditions, the conductor was debonded at the interface between the buffer layer and YBCO layers, or fractured in the YBCO layer itself, causing cracks on the YBCO layer, resulting in a significant decline of the critical current. These negative effects are suppressed if the coils are dry wound or impregnated with paraffin, as the bonding strengths between turns are negligible and therefore turns are separated if the cumulative radial stress tends to be tensile. For non-circular coils in which epoxy impregnation is inevitable, degradation due to cumulative tensile transverse stress is still the major problem. [ABSTRACT FROM AUTHOR]

Diameter of the superconducting ac composite strand is small, typically 0.1 mm, because the strand must be twisted in a very short pitch to reduce coupling losses. Therefore, current capacity of the single strand is small and in the range of -10 A. In large-scale electric power apparatus, the conductors must be able to carry large currents and hundreds of these composite strands should be bundled. The composite strand is highly unstable and susceptible to a very small disturbance due to a frictional wire motion because the main matrix of the wire is highly resistive CuNi. For stable operation of an ac superconducting winding, every strand in the bundle cable should be fixed firmly. An effective technique to fix strands is to impregnate the winding by epoxy. In this case, the ac losses in the winding are to be cooled by heat conduction through the epoxy. Therefore, it is important to estimate the temperature rise of the winding to discuss the fact that the epoxy impregnation technique is applicable to the ac superconducting electric power apparatus. In this paper, the mechanism of thermal conduction of the epoxy-impregnated winding at the 4.2 K region is discussed based on experimental data and the temperature rise of a large-scale cable bundles by 7 × 7 × 7 strands calculated considering thermal resistivity at the interface between the epoxy and the strand. The calculated value agrees well with the measured value. [ABSTRACT FROM AUTHOR]

• The cooling channels through epoxy impregnated racetrack pancake coil were successfully made. • The cooling channels improve the cooling rate of the epoxy impregnated RP coil. • The Coil with pores through epoxy experienced less degradation of their superconducting properties. • The cooling channels improved thermal stability of the RP coil. • The Coil with cooling channels exhibited faster NZPVs. The high-temperature superconducting (HTS) racetrack pancake (RP) coil is recommended to be used as a field coil in rotating machines such as generators and motors. Epoxy impregnation is normally required in the field coil to protect it against mechanical stresses due to the variation in the magnetic field and to enhance its mechanical stability to withstand rotor vibrations. However, the thermal conductivity of epoxy is very low compared with that of HTS. Consequently, when quenching is induced in an epoxy-impregnated coil, the Joule heating is poorly dissipated outward because of the absence of thermal drain to dissipate the heat in liquid nitrogen (LN 2), which damages the coil through heat accumulation. We designed cooling channels (pores) through the surface of an epoxy-impregnated coil to boost the circulation of LN 2 in the coil during the cooling process and to improve the operation of the coil during quenching. In this study, cooling-down and quench tests were performed on the epoxy-impregnated RP coils to investigate the effect of cooling channel on the thermal-quench characteristics of the RP coils. Moreover, the minimum quench energy and normal-zone propagation velocity of the hotspot induced in the coils were discussed on the basis of the quench-test results. [ABSTRACT FROM AUTHOR]

• Direct shear behavior of Textile-reinforced cementitious mortar was investigated. • The TRM material showed ductile behavior when subjected to direct shear. • A crack-shear slip model for TRM was developed and verified with the test results. This study experimentally investigated the characteristics of carbon fiber textile-reinforced mortar (TRM) under direct shear. The main test parameters of this study include the fiber reinforcement ratio of carbon fiber textile, the surface treatment used between mortar matrix and the fiber textile, and the inclination angle of the fiber filaments. Three fiber reinforcement ratio of carbon fiber textile of (0.17, 0.35, and 0.53) % in the mortar matrix were investigated. Two different surface treatment methods were employed to improve the bond performance of carbon fiber textiles in the mortar matrix: epoxy impregnation only, and sand coating after epoxy impregnation. Two inclination angles of fiber filaments of 0°/90° and –45°/45° were investigated. The test results showed that the textile reinforcement within cementitious mortar matrix affected the failure modes and the shear performance of test specimens in terms of cracking and ultimate shear stress, crack deformation, and residual shear stress in the pre- and post-peak stages. Moreover, a crack-shear slip model for cementitious mortar reinforced by carbon fiber textile was developed considering the material characteristics of the mortar and the fiber textile, and its prediction showed good agreement with the experimental results. [ABSTRACT FROM AUTHOR]