蒙脱土改性PU/EP IPN的研究

本文通过端异氰酸酯基聚氨酯预聚体与环氧树脂E-51形成了互穿网络,并通过热重分析仪(TGA)研究了完全固化后的互穿网络的热分解行为;透射电镜研究了IPN的相分离行为及用拉伸强度,断裂伸长率对其进行了力学性能的表征.结果表明,经过环氧树脂改性的聚氨酯的耐热性能比纯聚氨酯得到了提高,且力学性能也有所改善.并选取性能较好的PU/EP IPN,用纳米级有机蒙脱土对其进行了改性.研究发现,经有机纳米蒙脱土改性的PU/EP IPN的力学性能及耐热性有了进一步的提高.     

Thermal and dynamic mechanical properties of IPNS formed from unsaturated polyester resin and epoxy polyester

The interpenetrating polymer networks (IPNs) were formed by unsaturated polyester resin (UPR) polymerized by free radical initiators: benzoyl peroxide (BPO) or cumene hydroperoxide (CHP) and epoxy polyester (EP), cured with acid anhydrides: tetrahydrophthalic anhydride (THPA) or maleic anhydride (MA). IPNs consisting 10, 30, 50, 70, 90 wt% of EP were prepared. The effect of the EP component in the IPNs and the type of curing agent on the cure behavior, thermal, and viscoelastic properties have been investigated. The results showed that both EP content and used curing system influenced on studied properties. As the EP content increased, the glass transition temperatures (T _g) also increased. Moreover, higher values of tanδ_max and lower values of cross-linking density in a rubbery state (ν_e) of IPNs containing higher EP content, probably due to plasticization effect of EP component were observed. Additionally, more heterogeneous network structure (higher values of the full-width at half-maximum (FWHM) as the EP content decreased was prepared. The thermal and viscoelastic properties of the blends cured with BPO/MA or CHP/MA system were considerably better than those cured with BPO/THPA or CHP/THPA. The higher stiffness, ν_e, T _g and lower tanδ_max values were obtained. It was probably connected with the interactions of carbon–carbon double bonds of MA with vinyl monomer (styrene), UPR and radical initiators causing to obtain more cross-linked polymer network structure. This supposition was confirmed on basis of the cure reaction monitored by DSC. The chemical interactions between two components of the blends and epoxy hardener caused that the BPO/MA or CHP/MA cure systems influenced on the cure behavior of UPR and EP components in the IPNs. The exotherm peak temperature (T _max1) shifted to lower values compared to these in the neat UPR whilst T _max2 shifted to higher values than in the neat EP. However, the cure behavior of the UPR was not greatly affected by the presence of EP component when BPO/THPA or CHP/THPA cure systems were used due to the lack of chemical interactions between the components and their curatives.     

Fabrication and evaluation of mechanical and tribological properties of boron carbide reinforced aluminum matrix nanocomposites

In this study, fabrication and characterization of bulk Al–B₄C nanocomposites were investigated. B₄C nanoparticles were mixed with pure Al powder by ball milling to produce Al–B₄C powder. Al–B₄C powders containing different amounts of B₄C (5, 10 and 15 wt.%) were subsequently hot pressed to produce bulk nanocomposite samples. Consolidated samples were characterized by hardness, compression and wear tests. Results showed that the sample with 15 wt.% B₄C had the optimum properties. This sample had a value of 164 HV which is significantly higher than 33 HV for pure Al. Also, ultimate compressive strength of the sample was measured to be 485 MPa which is much higher than that for pure Al (130 MPa). The wear resistance of the nanocomposites increased significantly by increasing the B₄C content. Dominant wear mechanisms for Al–B₄C nanocomposites were determined to be formation of mechanical mixed layer on the surface of samples.     

Micro-structural evolution of phenol-formaldehyde resin modified by boron carbide at elevated temperatures

The phenol-formaldehyde (PF) resin was modified by boron carbide (B₄C). In order to investigate the modification effect of B₄C, the residue values of pure PF resin and B₄C modified PF resin were measured using thermal gravity. It was shown that the residue values of B₄C modified PF resin are 71.9% and 68.4% after being pyrolyzed at 700 and 1000 °C, respectively, which are obviously higher than those of the pure PF resin (62.9% and 60.5% at 700 and 1000 °C, respectively). The microstructure evolution of the modified resin at high temperatures was also investigated by scanning electron microscopy and energy dispersive analysis of X-rays. By means of the microstructure characterization, the modification reactions between the B₄C additives and the oxygen-containing volatiles, such as CO and H₂O, are demonstrated. The carbon and oxygen elements remained in the resin matrix in the forms of amorphous carbon and B₂O₃, respectively, resulting in the improvement of residue values and stability of the PF resin at high temperatures. The distribution of modification particles became well-proportioned gradually at the elevated temperatures, and the shape of ceramic additives changed into white spherules due to the surface tension.     

B4C/Al Composites Processed by Metal-assisted Pressureless Infiltration Technique and its Characterization

In order to improve the wettability between Al melt and B₄C ceramic preform during fabricating B₄C/Al composites by pressureless infiltration technique, trace amount of Ti particulates with high melting point was added into the starting materials as infiltration inducer. A simple and cost-effective method, metal-assisted pressureless infiltration technique, was developed to fabricate light-weight B₄C/Al composites. The microstructure, phases, and mechanical behavior of B₄C/Al composites were characterized by SEM, XRD, and mechanical property test. The density of the as-fabricated B₄C/Al composites was about 2.75 g/cm³ and the relative density of this kind of composites was over 97%. The as-fabricated B₄C/Al composites exhibited rather well wear resistance. The flexural and compressive strengths of the as-fabricated B₄C/Al composites were about 200 MPa and 670 MPa, respectively.     

Influence of high heat capacity substances doping on quench currents of fast ramped superconducting oval windings

The effect of high heat capacity substances doping on superconducting magnets ramp rate induced quench currents has been investigated for three oval windings. The windings were wound from Rutherford type cable (1.44 × 4.64 mm²), made of 10 multifilamentary NbTi strands 0.85 mm dia. Before application of electrical insulation and winding the cable was covered with epoxy resin with three different fillers: BN (boron nitride—a standard filler for preventing of epoxy cracking at low temperatures), and two rare-earth intermetallic compounds (HoCu₂ and CeCu₆). The specific heat of these compounds at liquid helium temperatures is extremely high. The volumetric fraction of these compounds was 2.9% of the total winding volume, corresponding to 4.5 times increase of averaged winding heat capacity for HoCu₂ and to 1.5 times increase for CeCu₆. At high ramp rates (∼4 kA/s, or ∼6.5 T/s) quench current of HoCu₂ doped winding was 35% higher than that for the BN doped one, while for CeCu₆ doped winding the quench current increase was 12%. Measured quench currents match values calculated with our theoretical model, in which whole doping substances enthalphy was taking into account. It indicates that the enhanced enthalphy is fully utilized in the used range of dB/dt.     

Thermophysical Properties of Ceramic Filler-Added BaO–ZnO–B<Subscript>2</Subscript>O<Subscript>3</Subscript>–P<Subscript>2</Subscript>O<Subscript>5</Subscript> Glass Composites for Barrier Ribs of Plasma Display Panels

The capability of BaO–ZnO–B₂O₃–P₂O₅ glass in hosting various ceramic fillers (up to 20 mass% of Al₂O₃, TiO₂, and ZnO) has been investigated. All the investigated filler-added glasses have demonstrated a reasonable densification at 550 °C to form stable ceramic filler-glass composites. Modifications of the thermophysical properties, such as coefficient of thermal expansion (CTE), glass transition temperature, and dilatometric softening temperature, by the addition of the fillers have been investigated and correlated to phase and microstructural evolution. The CTE of the fabricated composites with varying filler addition is well correlated with theoretical predictions based on the Turner equation considering the modification by phase evolution, which indicates the thermal property tuning potential of the BZBP-based glass composites for application to barrier ribs of plasma display panels.     

Sol–gel derived CaO–B2O3–SiO2 glass/CaSiO3 ceramic composites: processing and electrical properties

The CaO–B₂O₃–SiO₂ glass/CaSiO₃ ceramic (CBS/CS) composites were fabricated via sol–gel processing routes. Their densification behavior, structures and dielectric properties were investigated. The precursors of CBS glass and CS ceramic filler were firstly obtained via individual soft chemical route and then mixed together in various proportions. The results indicated that the structures of CBS/CS composites are characteristic of CS and CaB₂O₄ (CB) ceramic phases distributed in the matrix of glass phase at 800–950 °C. The CS ceramic phase not only acts as fillers, but nuclei for the crystallization of CBS glass as well such that the CS content exhibits an effect on the densification and dielectric properties of the composites. The CBS/CS composites with 10% CS sintered at 850 °C own dielectric properties of ε_r < 5 and tanδ = 6.4 × 10⁻⁴ at 1 MHz.     

CF/EP composite laminates with carbon black and copper chloride for improved electrical conductivity and interlaminar fracture toughness

An experimental study was conducted to improve the electrical conductivity of continuous carbon fibre/epoxy (CF/EP) composite laminate, with simultaneous improvement in mechanical performance, by incorporating nano-scale carbon black (CB) particles and copper chloride (CC) electrolyte into the epoxy matrix. CF/EP laminates of 65 vol.% of carbon fibres were manufactured using a vacuum-assisted resin infusion (VARI) technique. The effects of CB and the synergy of CB/CC on electrical resistivity, tensile strength and elastic modulus and fracture toughness (K_IC) of the epoxy matrix were experimentally characterised, as well as the transverse tensile modulus and strength, Mode I and Mode II interlaminar fracture toughness of the CF/EP laminates. The results showed that the addition of up to 3.0 wt.% CB in the epoxy matrix, with the assistance of CC, noticeably improved the electrical conductivity of the epoxy and the CF/EP laminates, with mechanical performance also enhanced to a certain extent.     

Low-temperature sintering and compatibility with silver electrode of Ba4MgTi11O27 microwave dielectric ceramic

Ba₄MgTi₁₁O₂₇ microwave dielectric ceramic was investigated using X-ray diffraction, scanning electron microscopy and dielectric measurement. The pure Ba₄MgTi₁₁O₂₇ ceramic shows a high sintering temperature (∼1275 °C) and good microwave dielectric properties as Q × f of 19,630 GHz, ?_r of 36.1, τ_f of 14.6 ppm/°C. It was found that the addition of BaCu(B₂O₅) (BCB) can effectively lower the sintering temperature from 1275 to 925 °C, and does not induce much degradation of the microwave dielectric properties. The BCB-doped Ba₄MgTi₁₁O₂₇ ceramics can be compatible with Ag electrode, which makes it a promising ceramic for LTCC technology application.     

Morphologies and properties of polyurethane/epoxy resin interpenetrating network nanocomposites modified with organoclay

Polyurethane/epoxy resin interpenetrating network nanocomposites containing various contents of organophilic montmorillonite (oM-PU/EP nanocomposites) were prepared by a sequential polymeric technique and an in situ intercalation method. Transmission electronic microscopy and scanning electronic microscopy analysis showed that the interpenetrating process of PU and EP increases the exfoliation degree of organophilic montmorillonite (oMMT), and that oMMT improves the compatibility and phase structure of polyurethane/epoxy resin interpenetrating polymer networks (PU/EP IPNs). Tensile test, differential scanning calorimetry and thermal gravity analysis proved that the mechanical and thermal properties of the oM-PU/EP nanocomposites are superior to those of the pure PU and PU/EP IPNs.     

聚氨酯和蒙脱土协同增韧增强环氧树脂

采用聚合物互穿技术与原位插层聚合相结合的方法制备了有机蒙脱土修饰的环氧树脂/聚氨酯互穿网络纳米复合材料.傅立叶红外光谱,X射线衍射及透射电镜分析表明剥离或插层的蒙脱土片层表面羟基能与环氧树脂/聚氨酯基体发生交联反应,并且均匀分散在环氧树脂/聚氨酯基体中.力学性能测试结果表明聚氨酯,蒙脱土的加入同时增加了环氧树脂的拉伸强度和断裂韧性.扫描电镜分析表明聚氨酯和蒙脱土协同增韧增强环氧树脂的主要原因为剪切屈服和微裂纹增韧.     

Low permittivity SrCuSi4O10-LMZBS glass composite for LTCC applications

The tetragonal gillespite type SrCuSi₄O₁₀ (SCS) was prepared by the conventional solid-state ceramic route. The SCS sintered at 1100 °C/6 h showed ε_r = 4.0 and tan δ = 1.1 × 10⁻³ at 5 GHz. The SCS has poor sinterability and the addition of lithium magnesium zinc borosilicate glass (20: Li₂O, 20: MgO, 20: ZnO, 20: B₂O₃, 20: SiO₂) lowered the sintering temperature and improved densification. The SCS ceramic with 5 wt.% LMZBS glass sintered at 900 °C has ε_r = 5.0 and tan δ = 1.9 × 10⁻³ at 5 GHz. The composite is chemically compatible with the common electrode material silver.     

Spark plasma sintered tantalum carbide: Effect of pressure and nano-boron carbide addition on microstructure and mechanical properties

TaC and TaC-1 wt.% B₄C powders were consolidated using spark plasma sintering (SPS) at 1850 °C and varying pressure of 100, 255 and 363 MPa. The effect of pressure on the densification and grain size is evaluated. The role of nano-sized B₄C as sintering aid and grain growth inhibitor is studied by means of XRD, SEM and high resolution TEM. Fully dense TaC samples were produced at a pressure of 255 MPa and higher at 1850 °C. The increasing pressure also resulted in an increase in TaC grain size. Addition of B₄C leads to an increase in the density of 100 MPa sample from 89% to 97%. B₄C nano-powder resists grain growth even at high pressure of 363 MPa. The formation of TaB₂/Carbon at TaC grain boundaries helps in pinning the grain boundary and inhibiting grain growth. The effect of B₄C addition on hardness and elastic modulus measured by nanoindentation and the indentation fracture toughness has been studied. Relative fracture toughness increased by up to 93% on B₄C addition.     

A low-firing microwave dielectric material in Li2O-ZnO-Nb2O5 system

LiZnNbO₄ ceramic was fabricated by the conventional solid state reaction method and its microwave dielectric properties were reported for the first time. The phase structure, microstructure, and sintering behavior were also investigated. The LiZnNbO₄ ceramic could be well densified at around 950 °C and demonstrated high performance microwave dielectric properties with a low relative permittivity ~ 14.6, a high quality factor (resonant frequency/dielectric loss) ~ 47, 200 GHz (at 8.7 GHz), and a negative temperature coefficient of resonant frequency approzmiately −64.5 ppm/°C. The LiZnNbO₄ ceramic is chemically compatible with Ag electrode material at its sintering temperature. It can be a promising microwave dielectric material for low-temperature co-fired ceramic technology.     

Carbon black/graphite nanoplatelet/rubbery epoxy hybrid composites for thermal interface applications

Hybrid composites were developed by dispersing carbon black (CB) nanoparticles and graphite nanoplatelets (GNPs) at 4–6 and 12–14 wt%, respectively, into rubbery epoxy resin. SEM analysis showed that CB particles improved the dispersion of GNPs in the hybrid composite. The thermal conductivity of 4 wt% CB/14 wt% GNP-15/rubbery epoxy hybrid composite, 0.81 W/m K, is ca. four times higher than that of rubbery epoxy. When silane-functionalised, the fillers reduced the viscosity of the hybrid dispersion and made the hybrid composite highly electrically insulating. Nevertheless, filler functionalisation decreased the composite’s thermal conductivity by only 16.6%. Compression testing showed that the hybrid fillers increased the compressive modulus and strength of rubbery epoxy by nearly two and three times, respectively. Overall, the hybrid composites with their thermal paste-type morphology, low viscosity, high compliance, improved thermal conductivity and, when fillers are functionalised, low electrical conductivity makes them promising materials as thermal interface adhesives.     

ZrSiO4 ceramics for microwave integrated circuit applications

The sintering temperature of ZrSiO₄ ceramic was optimized by studying the variation of density as a function of temperature. The dielectric properties were investigated at the radio and microwave frequencies. It has ε_r = 10.5, tan δ = 0.0016 (at 1 MHz), ε_r = 7.4, tan δ = 0.0006 (at 5.15 GHz) and τ_ε = 225 ppm/°C (at 1 MHz). The ceramic exhibited a negative coefficient of thermal expansion (CTE) of − 2.4 ppm/°C in the temperature range of 30-800 °C.     

Improvement of the fracture toughness of adhesively bonded stainless steel joints with aramid fibers at cryogenic temperatures

Adhesives should be reinforced with reinforcing fibers for the bonding of adherends at cryogenic temperatures because all the adhesives become quite brittle at cryogenic temperatures. In this work, the film-type epoxy adhesive was reinforced with randomly oriented aramid fiber mats to decrease the CTE (Coefficient of Thermal Expansion) of the adhesive and to improve the fracture toughness of adhesive joints composed of stainless steel adherends at the cryogenic temperature of −150 °C. The cleavage tests of the DCB (Double Cantilever Beam) adhesive joints were performed to evaluate the fracture toughness and crack resistance of the adhesive joints. Also, the thermal and mechanical properties of the fiber reinforced adhesive layer were measured to investigate the relationship between the fracture toughness of adhesive joints and fiber volume fraction of aramid fibers. From the experiments, it was found that the crack propagated in the adhesive with the stable mode of significantly increased fracture toughness when the film-type epoxy adhesive was reinforced with aramid fiber mats. The optimum volume fraction of aramid fibers was suggested for the film-type epoxy adhesive in the adhesive joint at the cryogenic temperature of −150 °C.     

The adhesive behaviour of poly(p-phenylene benzobisthiazole) (PBT)/epoxy composites

A modified scarf joint specimen was developed for characterizing the adhesive behaviour of poly (p-phenylene benzobisthiazole) (PBT) film/epoxy composites. This method subjected samples to varying amounts of normal stress (tensile or compressive) and shear stress. This resulted in the determination of two adhesive strengths; one in the absence of shear stress and one in the absence of normal stress. As a result, the dependence of the adhesive strength on the degree of normal stress was determined. The adhesive behaviour of PBT/epoxy composites was investigated at cure temperatures of 55, 85, 115 and 215°C. Adhesive strengths of 3.5 and 8.2 MPa were measured in the absence of shear and normal stress, respectively, for samples cured at 55° C. A decrease in adhesive strength with increasing cure temperature was attributed to residual cure and thermal stresses. The fracture of these composites was predominantly adhesive, resulting in a clean delamination of the PBT film from the epoxy surface. A modified Tsai-Wu failure criterion is suggested for these composites.     

Microwave dielectric properties and microstructures of the 11Li2O-3Nb2O5-12TiO2 ceramics with B2O3 addition

The effects of B₂O₃ addition, as a sintering agent, on the sintering behavior, microstructure and microwave dielectric properties of the 11Li₂O-3Nb₂O₅-12TiO₂ (LNT) ceramics have been investigated. With the low-level doping of B₂O₃ (≤2 wt.%), the sintering temperature of the LNT ceramic could be effectively reduced to 900 °C. The B₂O₃-doped LNT ceramics are also composed of Li₂TiO₃ss and “M-phase” phases. No other phase could be observed in the 0.5-2 wt.% B₂O₃-doped ceramics sintered at 840-920 °C. The addition of B₂O₃ induced no obvious degradation in the microwave dielectric properties but increased the τ_f values. Typically, the 0.5 wt.% B₂O₃-doped ceramics sintered at 900 °C have better microwave dielectric properties of ?_r = 49.2, Q × f = 8839 GHz, τ_f = 57.6 ppm/°C, which suggest that the ceramics could be applied in multilayer microwave devices requiring low sintering temperatures.