Electromagnetic interference shielding properties of polymer derived SiC-Si3N4 composite ceramics

SiC-Si3N4 composite ceramics are successfully fabricated by pyrolysis of ferrocene-modified polycarbosilane(PCS) mixed with inert filler Si3N4 powders, followed by thermal treatment from 1100℃ to 1400℃ in Ar atmosphere. The porosity of SiC-Si3N4 ceramics decreases to 6.4% due to the addition of inert filler Si3N4. And the content and crystallization degree of free carbon and SiC derived from PCS are improved simultaneously with the increase of thermal treatment temperature. Finally, the free carbon and SiC interconnect, forming the conductive network. As a result, the electromagnetic interference(EMI) shielding performance of the as-prepared ceramic annealed at 1400℃ reaches up to 36 d B, meaning more than99.9% of EM energy is shielded. The low porosity and high EMI shielding performance enable SiC-Si3N4 composite ceramics to be a promising electromagnetic shielding and structural material.

Improved shielding and filtering applied to immunity enhancement of underground gas sensors

In order to eliminate false alarms,issued by gas sensors in coal mining,caused by Electromagnetic Interference(EMI),both computer simulation and field measurements were introduced to analyze the underground EMI distribution.A simplified model of a sensor with metal enclosure was established and the effects of shielding properties about the enclosure aperture were studied.Because the haulage motor is the moving EMI source,varying with time,the onsite flameproof measuring instruments cannot accomplish synchronous measurements of electromagnetic field vectors.To simplify the field measurements,two sensors,one with a lead and the other without a lead,were chosen to conduct the contrasting measurements.The EMI current caused by the perforation lead was comparatively strong and therefore nickel zinc ferrite beads were used to cut off the EMI propagation paths.The peak value of the interference current was reduced by 20%-70% with the beads.After switching on the sensor power,the sen-sors still occasionally gave false alarms when the switch of nearby large-scale electric equipment was operated.A complex EMI filter was used and the EMI attenuated markedly.The running results demonstrated that false alarms had been eliminated.We con-clude that the improved shielding and filtering are highly significant in enhancing the immunity of the gas sensor.

From binary to ternary and back to binary:Transition of electromagnetic wave shielding to absorption among MAB phase Ni_(3)ZnB_(2)and corresponding binary borides Nin+1Bn(n=1,3)

Due to chemical inertness of nickel and boron,the preparation of nickel borides and corresponding layered ternary transition metal borides Ni_(3)ZnB_(2)(MAB phase)has always required high-temperature and/or high-pressure conditions.Yet,an innovative and efficient approach to preparing Ni_(3)ZnB_(2)at only 600℃and without applied pressure is presented in this study.It is discovered that by simply adjusting the temperature,a phase transition from Ni_(3)ZnB_(2)to Ni4B3 with a layered structure could be induced.This transition between the binary-component and the ternary-component brings about significant variation in electromagnetic wave(EMW)shielding/absorption performance of prepared borides.For instance,Ni2B has good EMW shielding performance(42.54 dB in X band)and Ni_(3)ZnB_(2)is of weak EMW shielding(13.43 dB in X band);Ni_(3)ZnB_(2)has poor EMW absorption performance(−5 dB)while Ni4B3 has excellent EMW absorption performance(−45.19 dB)at a thickness of 2.7 mm with effective absorption bandwidth(10.4 GHz).

Analysis on Interface Characteristics of FeNi_p/PP Nanocomposites

The FeNip/PP nanocomposites were successfully prepared by the two-step blending method and the permeable layer interface with thickness of 2 to 10 nm was formed on the surface of nanopowders. The interface is composed of the lattice and molecular chain, in which the polypropylene molecular chain enters the lattice defects and forms a cross-linked structure. The interface causes the FeNi nanopowders to be well compatible with the polypropylene matrix and uniformly dispersed in the matrix, and significantly improves the mechanical properties of composites. The tensile strength of 2 wt% FeNip/PP composites reached 38 MPa, 23% higher than that of pure PP resin. The shielding performance of 20 wt% FeNip/PP composites reached 9.8 dB in the frequency range of 1-100 MHz.

Thermally insulating robust carbon composite foams with high EMI shielding from natural cotton

Thermally insulating and fire-resistant carbon composite foams are prepared by consolidating natural cotton fibre dispersed in aqueous sucrose solution by filter-pressing followed by drying and carbonization.The compressive strength(5 kPa to 1.4 MPa)and thermal conductivity(0.069 to 0.185 W m^(-1)K^(-1))depend on the foam density(0.06 to 0.31 g cm^(-3))which is modulated by varying the sucrose solutions concentration(100 to 700 g L^(-1)).Partially flexible to the rigid transition of the carbon composite foams occurs at sucrose concentration above 200 g L^(-1).The tubular carbon fibre formed from cotton is welded at their contact points by the amorphous carbon produced from sucrose leading to partial flexibility at low sucrose concentration and advancement of fibre-to-fibre bonding area at higher sucrose concentration results in rigid foam.The porosity in the inter-fibre space and lumen of the carbonized cotton fibre contributes to the low thermal conductivity.The carbon composite foams prepared at a sucrose solution concentration of 500 g L^(-1) and above are amenable to machining using conventional machines and tools.The rigid carbon foams show EMI shielding effectiveness and specific shielding effectiveness in the ranges of 21.5 to 38.9 d B and 108-138 dB cm^(3) g^(-1),respectively.

Investigation ofmodified zinc borate glasses doped with BaO as a nuclear radiation-shieldingmaterial

Background Radiation protection and detection have been a main interest for researchers.The prepared glass samples were subjected to experimental investigations to evaluate their mechanical and attenuation properties.As a consequence,the values of the mass attenuation coefficient,total electronic cross section,the effective atomic number and an effective electron number were determined and utilized to assess the shielding effectiveness of the investigated glass samples.The mass attenuation coefficients of these samples were calculated theoretically using WinXcom program.Purpose Preparation of glass of borate with zinc and barium can withstand shock,heat and corrosion to be used as a radiation shield.Methods Glass samples were prepared by melt quenching technique.Density and molar volume measurements were obtained by applying the Archimedes principle.The hardness was measured by using a microhardness tester(Leco AMH 100,USA)for sample indentation.The thermal behavior of the glass samples was investigated by differential scanning calorimetry(DSC).Also,by using a scintillator detector(1.5″×1.5″NaI(Tl))exposed to ^(232)Th,^(137)Cs and ^(60)Co gamma ray sources with accuracy range 0.12%.Results The investigated glasses have relatively good gamma ray attenuation properties,water resistance ability and thermal stability with increasing barium oxide.So,they can be used in containers for keeping radioactive waste and radioactive sources.Conclusion The changes in the molar volume and density show approximately opposite linear trends.Also,μm is dependent on the chemical compositions of glass samples and energy of gamma rays.Good agreement between the experimentally obtained mass attenuation coefficient values and the corresponding theoretical predictions based on the known WinXcom program was observed.Additionally,the effect of gamma irradiation on this glass is minor because its impact on the hardness values and dissolution rate is extremely small.

Comparative Study on Processing Property Between CWW CO_2 Gas Shielded Welding and SAW

Cable welding wire（CWW）CO2gas shielded welding is an innovative process arc welding with high efficiency,high quality and low consumption,in which cable wire is used as consumable electrode.CWW CO2gas shielded welding and submerged arc welding（SAW）are used for contrast studies on processing property of high strength steel A36used in ship structure.The results show that the shapes of weld seam,using CWW CO2gas shielded welding and SAW,are good and no weld defect such as air hole,flaw,slag inclusion,incomplete fusion,lack of penetration and so on are found in the weld seam.Because the rotating of arc during CWW CO2gas shielded welding process has a strong stirring effect on molten pool,the grain in the heat affected zone（HAZ）of the joints,using CWW CO2 gas shielded welding,is small.Tensile failure positions of joints by CWW CO2gas shielded welding and SAW are all in the base metal,but tensile strength of CWW CO2gas shielded welding joint is higher than that of SAW joint by an average of 2.3%.The average impact energy of HAZ,using CWW CO2gas shielded welding and SAW,is almost equal,but the average impact energy of the weld seam using CWW CO2gas shielded welding is increased by 6%,and the average impact energy of the fusion line is increased by 7%.The 180°bending tests for the joints of CWW CO2 gas shielded welding and SAW are all qualified,and the joints hardness is all less than HV 355,but hardness of CWW CO2gas shielded welding wire welding joint near the fusion line is obviously lower.It can be concluded that the properties of CWW CO2gas shielded welding are better than those of the SAW joint,and CWW CO2gas shielded welding is suitable for welding high strength steel A36used in ship structure.

Effects of Particle Sizes on the Electromagnetic Property of Flaky FeSi Composites

Flaky FeSi absorbents with different size ranges were fabricated by sintering after mechanical milling process. X-ray diffraction （XRD） was used to analyze the particle crystal grain structure. The complex permittivity and permeability of FeSi/paraffin composites were measured in frequency of 2-12 GHz using a vector network analyzer, and the DC electric conductivity was measured by the standard four-point contact method, then microwave reflection loss （RL） and shielding effectiveness （SE） were calculated. It was obtained that c^-Fe appeared in the super-lattice diffraction peaks in XRD pattern. As the particles size decreased, the permittivity decreased due to the inferior microwave electrical conductivity and dielectric loss tangent, while the permeability increased due to the decrease of diameter-thickness ratio, which could be demonstrated in the comparison between the experiment and calculation results. When thickness is 1 mm, the composites with the smallest FeSi particles addition had a better absorbing property for the better impedance matching characteristic, and the minimum RL was -7.9 dB at 4.6 GHz. While the composites with larger FeSi particles addition had an excellent shielding property due to the higher permittivity, the SE value ranged from 15 dB to 30 dB at the frequency band.