High Insulation Resistance Research Articles

Nanosized Hexagonal Boron Nitride and Polyethylene Glycol-Filled Leathers for Applications Demanding High Thermal Insulation and Impact Resistance.

Leather is a niche material used for upholsteries, gloves, and garments due to its high durability, flexibility, and softness properties. The inclusion of nanoparticles in the leather matrix provides multifunctionality for high-performance applications. Herein, we synthesized hexagonal boron nitride (h-BN) nanoparticles via a single-step hydrothermal synthesis and treated the leather after dispersing in polyethylene glycol (PEG) to yield h-BN/PEG-treated leathers. Atomic force microscopy and high-resolution transmission electron microscopy analysis ascertained the particle size of 30-50 nm for as-synthesized h-BN nanoparticles. h-BN nanoparticles along with PEG were successfully incorporated into the leather matrix, and this was confirmed by surface and morphological studies using field emission scanning electron microscopy/energy-dispersive X-ray analysis and Fourier transformed infrared spectroscopy. Leathers treated with h-BN/PEG were studied for insulation against heat and cold, and the results displayed improved thermal insulation properties compared to the control leathers. The dynamic mechanical analysis of control and treated leathers demonstrated higher storage modulus, loss modulus, and tan δ values for h-BN/PEG-treated leathers, signifying an increased energy absorption and dissipation potential, which was further ascertained by the low-velocity drop-weight impact resistance test. Thus, the results of this study open up new prospects for h-BN/PEG-treated leathers in strategic applications demanding high thermal insulation and impact resistance properties.

Simultaneously Achieving Colossal Permittivity, Ultralow Dielectric Loss Tangent, and High Insulation Resistivity in Er-Doped SrTiO3 Ceramics via Oxygen Vacancy Regulation.

High-performance Sr1-xErxTiO3 (x = 0-0.014) ceramics were sintered in different atmospheres using the conventional solid-state reaction method. The phase structure and micromorphology of ceramics were analyzed using X-ray diffraction and scanning electron microscopy. Meanwhile, the Sr1-xErxTiO3 (when x = 0.012) ceramic sintered in hydrogen attains a colossal permittivity (132 543 @1 kHz, 157 650 @1 MHz) and ultralow tan δ (0.009 @1 kHz, 0.03 @1 MHz) and has good frequency stability (20 Hz to 2 MHz) and temperature stability (-180 to 425 °C). X-ray photoelectron spectroscopy, electron paramagnetic resonance, and impedance analysis show that the colossal permittivity and ultralow dielectric loss are attributed to the defect dipoles and defect clusters [TiTi'-VO••-TiTi'], [ErSr•-TiTi'], [2ErTi'-VO••], and [ErSr•-ErTi']. The insulation resistivity is determined by the grain boundary. The dielectric properties of samples sintered in hydrogen are excellent, and then, the oxidation method is used to backfill the oxygen vacancy (VO••), thus improving the insulation resistivity (2.8 × 1014 Ω cm) of the grain boundary. In addition, the diffusion mechanism of ceramic VO•• from low, medium, and high temperatures was studied by monitoring VO•• behavior in real time. The results reveal that the diffusion coefficient of VO•• in the grain boundary is greater than that in the grain; as a result, as the external oxygen partial pressure rises, the VO•• escapes first from the grain boundary. When the external oxygen partial pressure decreases, oxygen atoms enter the grain boundary region first and backfill oxygen vacancies.

Influence of MnO2 addition on the dielectric properties of 0.95MgTiO3-0.05CaTiO3 ceramics sintered in a reducing atmosphere

The increasing cost of palladium has demonstrated the significant value of using inexpensive base metals as inner electrodes for RF/microwave multilayer ceramic capacitors. However, solving the co-sintering problem of dielectric ceramics with base metals such as copper is highly challenging, as sintering in a reducing atmosphere is required to prevent the oxidation of the base metals. In this work, the feasibility of sintering nonreducible MnO2-doped 0.95MgTiO3-0.05CaTiO3 (95MCT) ceramics in a reducing atmosphere (H2-N2, pO2=1×10−10∼1×10−11MPa) at 1000 °C was investigated. The effects of the MnO2 additive (1.6–2.4 mol%) on the phase structures, dielectric properties, and thermal stability were studied. Thermally stimulated depolarization current (TSDC) analysis and X-ray photoelectron spectroscopy (XPS) were conducted to characterize the oxygen vacancies and Mn ion valence state in MnO2-doped ceramics, indicating that Mn plays an important role in the dielectric properties, especially the extrinsic dielectric loss. With the optimized addition of 2.0 mol% MnO2, due to the decrease in oxygen vacancy concentration, the samples with εr=20.22 exhibited the attractive properties of a high insulation resistivity of IR =3.65×1014 Ω∙cm and a low dielectric loss of tanδ=0.036%, meeting the C0G specifications. The results show that the 2.0 mol% MnO2-doped 0.95MgTiO3-0.05CaTiO3 ceramic is an outstanding nonreducible dielectric material for base-metal electrode RF/microwave multilayer ceramic capacitor applications with low cost, excellent dielectric properties, and high reliability.

Evolution of inverter ground impedances for PV modules with various backsheet types

AbstractHerein, we present results that emphasize the importance of retaining high insulation resistance in polymer backsheets for the reliable operation of photovoltaic installations. By correlating inverter monitoring data, meteorological data, and spectroscopic information from backsheet materials of photovoltaic modules, we derive performance and degradation rates for inverters connected to photovoltaic modules with several backsheet types. The feasibility of evaluating the degradation status of photovoltaic modules in the field through spectroscopic analysis of the water content in backsheet and encapsulant is demonstrated. We show that an increase in water content is a sign for reduced leakage resistance. Ground impedance was measured alongside relative humidity and temperature for three specific backsheet types. Inverters with polyamide and fluorinated coating/polyethylene terephthalate backsheets showed a significant drop in ground impedance at high humidity, RH > 70%. A loss rate of −2 kΩ/day in ground impedance was found for inverters connected to modules with certain fluorinated backsheets. This value is twice as high as for inverters connected to modules with polyamide backsheets. Time series show that the sensitivity to moisture and water ingress increases with time, especially for modules with polyamide and certain fluorinated backsheets. The worst degradation scenario observed had an annual yield loss of 27% due to inverter outages on humid days. About 25% of inverter trip alerts after 8 years of operation included modules with the specific fluorinated backsheet. In general, the study revealed a distinct backsheet impact on ground impedance and operation of inverters and the resulting yield showing an important role of the choice of polymers for reliable and long‐lasting photovoltaic systems.

Novel SrTiO3-based ceramics with colossal permittivity, low dielectric loss, and high insulation resistivity via defect engineering

It is a great challenge to high colossal permittivity (CP, Ɛr) in polycrystal ceramics together with low dielectric loss (tanδ) and high insulation resistivity (ρ) due to the miniaturization of electrical devices. In this contribution, the defect dipoles [Ti4+•e−CeSr••−e•Ti4+] and [CeSr•−e•Ti4+] have been actively induced in the Sr1-1.5xCexTiO3 (SCT) systems by doping Ce ions with amphoteric nature to address this issue. The ultra-stable dielectric performance under the external stimulus, with Ɛr = 12,406 (with low variation of < ±10%) and tanδ <0.08, can be achieve in the x = 0.01 sample within the temperature range of 10–200 °C and at the frequencies ranging from 20 Hz to 1 MHz, accompanied by high insulation with ρ > 1010 Ω•cm with low degradation rate under harsh test conditions of 2–4.5 kV/cm. This work paves a path by multisite defect engineering to achieve high-performance CP ceramics.

Investigation of thermal property of plasma‐polymerized fluorocarbon thin films

AbstractThe physical property changes in the plasma‐polymerized fluorocarbon (PPFC) thin films near the melting temperature (Tm) were investigated through chemical structure analysis. The PPFC thin films, produced by semi‐crystalline polytetrafluoroethylene (PTFE)‐based polymer sputtering target, form amorphous polymers with outstanding properties, such as high water repellency and high optical transmittance, electrical insulation, and chemical resistance. Interestingly, in‐situ X‐ray photoelectron spectroscopy analysis revealed that the PPFC thin films maintained their physical properties at temperatures above the Tm in vacuum and exhibited properties similar to those of bulk PTFE while maintaining amorphous structure. Therefore, this thin film can be applied to various devices and surface coatings manufactured at high temperatures.

Designing multi-aromatic ring epoxy composites to integrate high insulation and high heat resistance performances by electron-induced effect

Multi-aromatic ring epoxy composites (MAREC) are prospective electronic packaging materials owing to their high heat resistance. However, the presence of numerous conjugated π bonds lead to poor insulation performance of MAREC, limiting its further application. Here, we propose a unique strategy to design MAREC to integrate high insulation and high heat resistance performances by electron-induced effect. The results show that the substitution of H atoms of benzidine by F atoms and –CF3 groups can lead to strong electron-induction effect. When all H atoms on the aromatic ring of benzidine are replaced by F atoms (8F), the electron-induced effect is the strongest (energy gap = 5.56 eV). The highest occupied molecular orbital of fluorenyl biphenyl exists by head to head overlap (similar to a σ bond), which significantly weakens the free sharing ability of the electrons on the conjugate π bonds of the aromatic ring. Although the reaction activity of –NH2 groups of 8F is low, the strong electron-induction effect of 8F can be fully harnessed by combining CF (two –CF3 groups replacing the H atoms of benzidine). The designed MAREC shows excellent insulation performance (AC breakdown strength = 39.18 kV/mm) and heat resistance performance (Tg = 234.5 °C, Td = 396.6 °C). Furthermore, the dielectric constant and dielectric loss are low (3.37@50Hz and 0.0035@50Hz, respectively). We successfully fabricated materials with enhanced insulation and heat resistance performances, which can be used as packaging materials for large-scale integrated circuits, new energy power equipment, and energy storage devices.

Novel strategy and multi-scale modelling of integrated multifunctional composite for thermal protection under extreme environment

In order to overcome the shortcomings of the heritage thermal protection materials with only a single function as well as the macroscopic evaluation method lacking the consideration of manufacture parameters, in this study, a novel strategy of integrated multifunctional composite with an overall 3-Dimensional reinforcement weave fabric and multifunctional zones was prepared, and a new multi-scale computational modelling for evaluating its thermal protection performance was established. The properties of constitute materials were characterized, as well as the thermal and ablative behavior of the integrated multifunctional composite were tested by oxyacetylene test. The new modelling took the detail weaving parameters as well as the physical and chemical mechanisms of the composite into consideration. The model was solved by our FORTRAN codes. The experimental and simulation results showed that this new integrated composite had high structural efficiency, thermal insulation and recession resistance. The multi-scale computational modelling can be applied on the evaluation of the performance of the integrated woven composite in a bottom-up manner, which gives a refined design method for thermal protection materials.

Dielectric properties of BaTiO3-based ceramics are tuned by defect dipoles and oxygen vacancies under a reducing atmosphere

BaTiO3-2 mol% MnYbO3-2 mol% CaZrO3-x mol Nb2O5 (x = 0, 0.3%, 0.5%, 0.7%) ceramics were successfully prepared via the conventional solid‐state method, followed by sintering in a reducing atmosphere. The effect of Nb2O5 doping on the crystal structure, micromorphology and dielectric properties of sintered samples was studied, and the correlation mechanisms between the microstructure and dielectric properties were further determined. For x = 0.5 mol%, optimal dielectric properties with high permittivity (ԑr ∼ 3570), low dielectric loss (tanδ ∼ 0.013) and large insulation resistivity (ρv ∼ 4.95 × 1011 Ω cm) are obtained. On this basis, the temperature coefficient of the capacitance (ΔC/C25°C) ranges between ±15% over the temperature range from −55 °C to 125 °C, satisfying the EIA X7R standard. Analysis shows that the increased permittivity of the doped ceramics at room temperature is due to enhanced short-range hoping polarization triggered by the formation of defect dipoles ([MnTi″−2NbTi·]× and [YbTi′−NbTi·]×) and the release of oxygen vacancies. The high insulation resistivity is related to the grain size and electron migration in the ceramics. In addition, the improvement in the temperature stability of the ceramics is ascribed to a decrease in the Curie temperature. The polarization mechanism presented in this work can be extended to BaTiO3 materials, and the results have important application value for BME-MLCC devices.

Studies on Epoxy Resin Modified with Organic Agents

Epoxy composites are one of the most versatile thermosetting polymers widely used due to their strong soldering resistance, high insulation, and thermal resistance. A wide range of raw materials can be used for the preparation of epoxy resins, thus offering a variety of resins with highly controllable performance characteristics. Essentially, epoxy resins are low molecular weight liquids with two or more epoxide functional groups. To get superior properties of the composite material, a broad variety of inorganic and organic modification agents are used. For this study, two protein substances, gelatin, and wheat gluten were used to change the basic properties of three types of epoxy resins. The specific heat of modified epoxy resin materials was performed by using Differential Scanning Calorimetry (DSC) instrument. The thermal analysis was developed on two heatingcooling cycles and the specific heat was evaluated for each segment of the cycle analysis. Also, the mechanical behavior of organic modified polymers subjected to compression was studied using INSTRON compressive testing machine.&#x0D;

Mechanical properties and thermal conductivity of lightweight and high-strength carbon-graphite thermal insulation materials

Thermal insulation composites are widely used in civil and military applications; however, it is difficult to achieve the synergy of multiple technical objectives such as lightweight, thermal insulation, high pressure resistance and high-temperature resistance by adopting traditional preparation techniques. In this study, a novel carbon-graphite thermal insulation material was rapidly prepared by exploiting the micro-thermal press additive manufacturing forming technology, and these multiple objectives were simultaneously achieved by introducing a large number of closed pores. It was found that the percentage of closed pores in the carbon-graphite insulation was increased by increasing the forming density or the amount of thermosetting phenolic resin added, but the thermal conductivity increased in parallel with the compressive strength, while the addition of pre-covered expandable graphite was able to achieve the synergy of high compressive strength and low thermal conductivity. When the content of thermosetting phenolic resin was 25 wt%, forming density was 1.2 g·cm−3, and expandable graphite was clad twice, the prepared carbon-graphite insulation exhibited a closed porosity/porosity ratio, compressive strength, and thermal conductivity of 48.92%, 16.432 MPa, and 0.743 W·m −1 K−1, which has the advantages of lightweight, high compressive strength, heat insulation and high-temperature resistance and has good prospects for industrial applications.

Improvement of thermomechanical characteristics of multilayer biomaterial of sheep wool and clay

The use of green renewable energy is an element of energy efficiency and sustainable construction in buildings to conserve other energy sources and reduce our carbon footprint. This research focuses on the sheep's wool valorization as an abundant raw material. This composite developed in solid bricks for buildings offers high thermal insulation and mechanical resistance. Thanks to the new technique development based on multi-layer reinforcement with sheep's wool yarn. This experimental work concerns physicochemical, thermal, and mechanical characterizations. The effect of the wool yarns was significant on the thermomechanical behavior of the developed biomaterial. We compared this study to other eco-material studies targeting thermal insulation for energy efficiency.

Preparation of defect-free alumina insulation film using layer-by-layer electrohydrodynamic jet deposition for high temperature applications

Ultra-high temperature thin-film sensors（UTTSs）have drawn enormous attention due to their wide application prospect in multiple physical parameters monitoring in harsh environment. As an important part of UTTSs, the insulation film between workpiece and sensors plays a crucial role to ensure the accuracy of UTTSs. High insulation resistance (IR) is essential for the reliability improvement of the UTTSs and the reduction of their testing error. However, the defects in the insulation film will form the conductive path between the sensors and their mounting surface at high temperature, leading to a rapid IR drop and stability deterioration of the insulation film. In this work, 10 μm thick alumina film was prepared with suspension made out of alumina gel and alumina nano-powder using layer-by-layer electrohydrodynamic jet (E-jet) deposition (LLED). By adopting this technique, the process defects in one layer can be remedied in situ by the next deposition layer. The prepared film shows satisfactory insulation performance at elevated temperature, and IR of the film can reach 80 MΩ, 3 MΩ and 38 kΩ at 500 °C, 800 °C and 1200 °C, respectively. The thickness-normalized IR of the alumina film at 800 °C in this work is at least 7.8 times that obtained by other methods reported in the literatures. Moreover, after subjected to three thermal cycles from room temperature to 1200 °C, the IR of the film can still remain at 14 kΩ. The high performance of the alumina insulation film in this work can be attributed to the effective reduction of the minor defects in the LLED deposition process.

Colossal permittivity (Nb, Mg) co-doped BaTiO3 ceramics with excellent temperature stability and high insulation resistivity

High performance colossal permittivity ceramic materials are indispensable for the development of electronic devices. In this work, Mg and Nb co-doping BaTiO3 ceramics were prepared by solid-state reaction under reducing atmosphere. The polarization mechanism, dielectric properties and insulation mechanism were investigated. The defect dipoles ([MgTi″−VO••]× and [MgTi″−2NbTi•]×) could be formed inside co-doping BaTiO3 ceramics with various content of MgO, which is the source of colossal permittivity. Appropriate MgO content doping is favorable to enhance short-range hopping, thus improve grain resistance, grain boundary resistance and grain boundary activation energy. The 1.0 at% MgO modified BaTiO3-0.01Nb2O5 ceramics possess excellent performance at room temperature with colossal dielectric constant (ԑr = 4.5 × 104), low dielectric loss (tanδ<0.018), and high insulation resistivity (ρv = 6.52 × 1011 Ω cm). Meanwhile, good temperature stability is obtained in 1.0 at% MgO modified BaTiO3-0.01Nb2O5 ceramics, which satisfies the Electronic Industries Association (EIA) X8R (−55–150 °C, △εr/ε25≤ ±15%) specification. This study suggests that the defect-dipoles related polarization mechanism can be used to design the colossal permittivity BaTiO3-based ceramics.

Structure and properties of lightweight magnesia refractory castables with porous matrix

To improve the energy-saving capacity of magnesia refractory castables for working lining of high-temperature kilns, this study presents the researches on microstructure and properties of lightweight magnesia refractory castables with porous matrix fabricated by direct foaming method. The results show that formation of closed-pores in the matrix significantly enhanced high-temperature thermal insulation performance of castables with minor changes of slag corrosion resistance. The thermal conductivity of the lightweight magnesia castables at 1000 °C was below 1.2 W/m·K, which is 47.8% lower than that of the referenced magnesia castable. The increasing content of SDS (foaming agent, over 0.02 wt%) led to increments of size and number of large-sized pores, resulting in the significantly decreased density and mechanical performances. The slag resistance mechanism reveals that, in addition to intergranular penetration, the accumulation of slag and penetration between adjacent pores were the major ways of slag mass transfer in lightweight magnesia castables. In conclusion, controlling the size (below 53.2 μm), number and distribution of closed-pores in the matrix is effective to realize the coupling of high thermal insulation, mechanical properties and slag resistance for lightweight magnesia castables used in the metallurgical field.

The self-compensation mechanism in barium titanate ceramics with colossal permittivity

Based on adjusting the relationship between polarization and charge carriers, barium titanate (BaTiO3) ceramics with colossal permittivity and high insulation resistivity are prepared by solid-state reaction approach. Modified by reducing atmosphere (0.1%H2/N2) and donor-ions doping, different polarization modes are discussed via treatment with impedance analysis and dielectric data, then the relationship between dielectric properties and charge carriers is established. It is illustrated that the interfacial polarization and hopping polarization can be adjusted along with different charge compensation modes by changing Nb5+ content from 0.5 mol% to 3 mol%, corresponding to colossal permittivity and insulation resistivity, respectively. Thus, the approach provides an adjustable mechanism for balancing the colossal permittivity and insulation resistivity in BaTiO3 ceramics. The colossal permittivity can be achieved by controlling oxygen vacancies, and the distribution of oxygen vacancies is further adjusted by the donor-ions. Meanwhile, the self-compensation is found in the grains to improve the grain resistance. On this basis, single-layer microchip capacitors of BaTiO3-based ceramics are available in the future.

Charge compensation in rare earth doped BaTiO3-based ceramics sintered in reducing atmosphere

Rare earth with radius ranges from 0.868 Å to 1.36 Å are introduced in BaTiO3(BT) -based ceramics by solid-state reaction method, representing different charge compensation modes. Based on X-ray diffraction (XRD) and electron paramagnetic resonance (EPR) measurement, the behavior and solid solubility of La3+, Ho3+, Y3+, and Yb3+ in BT-based ceramics are systematically analyzed, then the charge compensation mechanism in this system is ascertained. Meanwhile, the function of dielectric properties with temperature at different concentrations is measured, which can be reasonably clarified by the Curie–Weiss and modified Curie–Weiss laws. Through the excellent dielectric performance, it is proved that rare earth ions play a role in improving the performance of BT-based ceramics, and the defect complexes formed therein are conductive to insulation resistivity in reducing atmosphere. Among all samples, the Yb3+ doped BT-based ceramic possesses a relatively good comprehensive performance with high dielectric contant (ε > 2600), low dielectric loss (tanδ = 1.83%) and high insulation resistivity (4.38 × 1011 Ω·cm). Consequently, the defect compensation mechanism provides an effective regulatory method for BT-based ceramics, establishing a theoretical basis for multi-layer ceramic capacitor (MLCC).

Defect chemistry and colossal dielectric behavior of Nd-modified SrTiO3 lead-free ceramic materials

Colossal permittivity (CP) ceramics have a great potential in the development of miniaturization, integration and lightweight of electronic components. In this work, NdxSr1-xTiO3 (0.000 ≤ x ≤ 0.014) (NST) ceramics are successfully prepared via sintering in N2. Simultaneously microstructure and dielectric properties of NST ceramics are characterized. It can be found that 0.010 ≤ x ≤ 0.014 ceramics show CP (>20000, 20–1 × 106 Hz) and low dielectric loss (<0.04, 20–1 × 106 Hz). In particular, x = 0.010 ceramics exhibit CP (33475, 1 kHz), high insulation resistivity (5.37×108 Ω cm, DC 100 V), low dielectric loss (0.01, 1 kHz) and excellent frequency (20–1 × 106 Hz) and temperature (25–420 °C) stability. In addition, the origin of CP behavior is discussed in NST ceramics based on the characterizations of EPR, XPS, XRD, SEM and complex impedance. The main reason is that the coupling of point defects (TiTi', NdSr⋅, VO⋅⋅) forms the defect dipoles (TiTi'−NdSr⋅, TiTi'−VO⋅⋅−TiTi') and the defect clusters (VO⋅⋅−3TiTi'−NdSr⋅), which produces the electron pinning effect and restricts the long-range movement of the carriers. Additionally, the grain boundary with high insulation resistivity plays a major role in the resistance of ceramics, and it also acts as a high barrier layer to limit the movement of several delocalized carriers.

Advanced lightweight steel floor towards high sound insulation and fire resistance

Industrialized lightweight buildings have great advantages associated with lower indirect costs, minimum construction wastes and faster construction processes. However, their main weaknesses lie in their acoustic performance and fire resistance. This paper develops and assesses a novel lightweight steel floor with high fire resistance and high acoustic performance for buildings. Acoustic measurements through laboratory tests were carried out according to ISO 10140:2010 and ISO 717:2013. Fire resistance was tested according to EN 1365–2:2000. The results demonstrate that using an appropriate structural and resilient layer through an optimized design, insulating the floor surface from the surrounding structure, separating structural systems between layers, minimizing flanking transmissions through resilient strips and joint sealing between boards, a Weighted sound reduction index of Rw (C; Ctr) ≥ 65 (−6; −14) dB, and a Weighted normalized impact sound pressure level of Ln,w (CI) ≤ 49,9 (4) dB can be achieved, along with a fire resistance of REI45.

Colossal permittivity and high insulation resistivity in Dy- modified SrTiO3 lead-free ceramic materials with low dielectric loss

High performance colossal permittivity (CP) ceramic materials are indispensable for the development of electronic devices. In this work, excellent Sr1-xDyxTiO3 (0.000 ≤ x ≤ 0.018) ceramic samples were prepared through a traditional solid phase method. It can be found that the insulation resistivity of 4.7 × 107 Ω/cm3 (DC 100V), the CP of 26712 and the low dielectric loss of 0.0065 (1 kHz) are obtained in x = 0.008 ceramic samples sintered in N2. Based on the analysis of XRD, XPS and comparison of sintering in different atmospheres, the excellent dielectric property is mainly determined by defect dipoles associated with oxygen vacancy (eg.TiTi'−VO⋅⋅−TiTi'). These defect dipoles limit the long-range transition of electrons and result in electronic pinning effect. Additionally, it can be perceived that the insulation resistivity of 1.12 × 109 Ω/cm3 (DC 100 V), the CP of 23134 and the low dielectric loss of 0.0098 (1 kHz) are achieved in x = 0.008 ceramic samples after reoxidation. According to the analysis of complex impedance spectroscopy, the improvement of the insulation resistivity of x = 0.008 ceramic samples is mainly attributed to the increase of the grain boundary resistance.