摘要:Six representative parent rocks of sand, including limestone, quartzite, gneisses, granite, Basalt and Marble were selected to conduct a systematical research on the effects of various lithologies of manufactured sand on the workability, mechanism properties, volume stability and durability of manufacturedsand concrete. The experimental results show that the strength of manufactured-sand concrete is slightly higher than that of natural-sand concrete. Furthermore, substituting 15% cement of the concrete mixture with equal quantity of the six different lithology stone powder respectively, the data indicated that they can improve the concrete's workability, postpone the plastic cracking time, enhance the anti-cracking grade, and have no obvious effect on the properties of antifreeze and sulfate attack resistance but reduce the capability to resist chloride ion penetration. Moreover, the differences in concrete's workability, mechanism properties, volume stability and durability caused by various lithologies of manufactured sand and stone powder were not significant and the influence of lithology variety on the macro properties of concrete could be neglected eventually.
摘要:Five types of KNO_3-NH_4VO_3-rare earth metal nitrate(K-V-rare earth metal) catalysts supported on a-porous alumina ceramic substrates were prepared by a coating method. All the catalysts were characterized by X-ray diffraction and thermogravimetry/differential scanning calorimetry. Catalytic activities were evaluated by a soot oxidation reaction using a temperature-programmed reaction system. The experimental results show that the addition of rare earth metal compound could obviously improve the catalytic activities of the K-V-based catalysts. The proper ratio of K-V-rare earth metal catalysts can not only lower the soot onset ignition temperature, but also quicken the soot oxidation rate. The crystalline phases formed by K, V, and rare earth metal are stable.
摘要:A high-pressure hot-pressing process was applied to densify a commercial boron carbide-titanium diboride (B4C-TiB2) powder mixture.Nearly fully dense (98.6%) materials were obtained at 1700℃ under a pressure of 100MPa.Compared to the sintering temperature required to achieve similar results when a pressure of only 30MPa was applied,the sintering temperature was found to decrease by about 200℃ under pressure of 100 MPa.Analysis of the thermodynamics and microstructure showed that the plastic deformation of the B4C grains induced by high pressure dominated the densification mechanism when high pressure was applied.Furthermore,higher pressure resulted in remarkably improved mechanical properties of the composites,which could be traced back to the generation of stacking faults in the B4C grains and aggregation of TiB2.
摘要:The influences of different nano-SiO_2(NS) contents on the mechanical properties and rheological behavior of sulfoaluminate cement(SAC) based composite materials were studied.Results show that with increasing content of NS,the apparent viscosity,and shearing strength of fresh paste gradually increase but the fluidity decreases.With a dosage of 3.0%NS,the tensile and flexural strengths of mortars at 56 days were increased by 87.0%and 84.6%,respectively,compared with that in the absence of NS,indicating that the toughness of hardened mortars is significantly improved.Besides,the exothermic peaks of hydration are obviously increased and will earlier occur,and the second and the third peaks appear 2.61 hours and 2.56 hours earlier,respectively than that in the absence of NS,and the hydration of SAC before 8 hours is accelerated.The forming mechanism of strengths was revealed by scanning electron microscopy(SEM),hydration heat,X-ray diffraction(XRD) and derivative thermogravimetry(DTG).The micro-aggregate filling effect and nucleation effect at early age and weak pozzolanic effect at late age of NS make the microstructure more compact,which obviously enhances the strength of SAC mortars.
摘要:Three cement pastes were prepared with the fixed water-binder ratio and different fly ash contents. The compression test and electrical resistivity measurement of the paste mixes were conducted during 48 h. The changes of the CH content and the non-evaporable water content in the cement-fly ash hydration systems with time were obtained by the thermal gravimetric analysis. The experimental results show that dilution effect of fly ash as micro-filler is dominant mechanism before 48 h, which appears to decrease in the CH content and the non-evaporable water content, also in compressive strength and electrical resistivity, with the increase of fly ash replacement. The relationships between CH content, non-evaporable water content and electrical resistivity show that electrical resistivity can be the indicators of hydration products CH and non-evaporable water. The correlation of the compressive strength fc and the electrical resistivity ρcan be obtained as fc=8.3429ρ=6.7088 for the period of 48 h. The early age compressive strength can then be predicted by electrical resistivity measurement.
摘要:The aging behavior of single lap joints(SLJ) in hygrothermal cycles was investigated and compared by using a shearing strength test, Fourier transform infrared spectroscopy(FTIR), thermogravimetric analysis(TG/DTG), energy dispersive spectrometry(EDS) and scanning electronic microscopy(SEM). The temperature/relative humidity was set at 80 ℃/95% and –40 ℃/30% for 20 cycles, 40 cycles, and 60 cycles(one cycle was 12 hours), respectively. The experimental results show that hygrothermal aging significantly decreases the failure strength of adhesive joints. However, the failure displacement increases as the number of aging cycles increases. In addition, hygrothermal aging changes the failure mode of the adhesive joints from a cohesive fracture in un-aged adhesive layers to an interfacial failure of aged adhesive joints.
摘要:We report the synthesis of gold nanorods (NRs) by seed-mediated growth method. A small amount of different shapes such as triangles, hexagons and a large amount of rods are obtained by varying the proportion of seed to metal salt, adding NaOH to growth solution as well as using the seed solution of CTAB-capped agent. The gold nanorod (NR) formation yield is improved. Meanwhile, the growth mechanism of high yield gold NRs is discussed. The high quality single size NRs can be separated from polydisperse samples using surfactant-assisted nanorod self-assembly. The gold NRs synthesized were characterized by transmission electron microscopy (TEM) and UV-vis spectroscopy.
摘要:The Fe-based amorphous coatings were produced by air plasma spraying. The as-sprayed coatings were heat-treated at the temperature of 573, 873, and 1 023 K, respectively. The crystallization and wear behavior of the heat-treated amorphous coatings were investigated. It was found that the amorphous-nanocrystalline transformation appeared when the as-sprayed coatings were treated at 853 K. The crystallization process had completed and a coating with microcrystallines was formed when the treatment temperature reached 1 023 K. The resultant amorphous and nanocrystalline composite coatings exhibited superior wear resistance compared to crystalline coating. It is attributed to fine grain strengthening of formed nanocrystallines.
摘要:The purpose of the present study was to evaluate the effect of Al2O3 content on the fracture property of all-ceramics ZrO2. To improve the all-ceramics ZrO2 restoration mechanics properity ,96 samples containing 0,5,10 and 15 wt% of Al2O3 particles were prepared by cold isostatic pressing (200 MPa) and 1 550 ℃ sintered .The phase was analyzed by X-ray diffraction analysis and the bulk densities of the samples were made using Archimedes principle. Samples were randomly divided into four groups. In each group, 24 specimens were prepared so that the angle between notch and specimen's long axis is 90o and 60o. Notch depths were 1 mm for all samples. Samples were loaded with three-point bending method. 90° cut samples were used to measure fracture toughness while 60°cut samples were used to observe fracture curve by taking points on the fracture extension path under microscope, plotting points on coordinates, generating fitting curve by software 'Origin', and analyzing the microstructure of the specimen fracture surfaces by scanning electron microscopy (SEM).The results show that the increment of Al2O3 has insignificant effect on the densification of all-ceramic ZrO2.XRD analysis shows that the specimen is comprised of t-ZrO2 and α- Al2O3 before fracture while fracture surface is m-ZrO2, t-ZrO2 and a-Al2O3. ZrO2 containing 10% Al2O3 has the optimum mechanical properties and unconspicuous crack propagation and distribution. The observations may provide a reference for the materials selection, shape design , and production process of all-ceramic crown and bridge.
摘要:The effect of thermal wave at the initial stage for non-conductive Al_2 O_3 powders compact in field assisted sintering technique(FAST) was investigated. The Lord and Shulman type generalized thermoselastic theory was introduced to describe the influence of thermal-mechanical interaction, as well as the heat transport and thermal focusing caused by thermal wave propagation. The expression of vacancy concentration difference of the particles was deduced by considering transient thermal stress. Subsequently, the relationship between activation energy and vacancy concentration difference was obtained. The mechanism of surface diffusion, volume diffusion, simultaneous surface and volume diffusion was analyzed. The numerical simulations indicate that low sintering temperature can obtain high local temperature by the superposition effect of thermal wave. Vacancy concentration differences were improved during FAST compared with hot-pressure and pressureless sintering, thereby decreasing the sintering time. By contrast, the activation energy declined with the decrease of vacancy concentration difference in the neck growth process.
摘要:The surface passivation mechanism of nanocrystalline silicon powder was studied. The liquid nitrogen/argon was used as the medium to prepare the nanocrystalline silicon powder, using a cryomilling technology. The X-ray diffraction, transmission electron microscopy, plasma emission spectroscopy and infrared spectrum were used to analyze the prepared samples, and density functional theory was used to investigate the cryomilling process. For nanocrystalline silicon powder cryomilled with liquid N2, the amorphous outer layer with N element is formed on the surface, and chemisorption caused by the formation of Si-N-Si bond leads to the surface passivation, although physisorption also be confirmed, the Si-N bond is steady after exploded in air for 30 days and no new bond is observed. For nanocrystalline silicon powder cryomilled with liquid Ar, no new chemical bond is observed, Ar element absorbs on the surface of the prepared powder only through physisorption, and after exploded in air for 30 days, a Si-O bond can be observed obviously.
摘要:Bauxite-tailings is aluminosilicate wastes and is used in polymer as fillers. But its intrinsic low-whiteness has limited its large-scale application in polymeric materials. Conventional methods for whitening bauxite-tailings were ineffective. A new method, which involved reactions with phosphoric acid and calcinations, was proposed to whiten bauxite-tailings. Two process routes were employed to whiten bauxitetailings, which were pre- and post-reaction calcination. While the process of pre-reaction calcination was able to achieve a whiteness of 87% from bauxite-tailings, it required high-energy post-treatment processes such as washing, milling and drying. The process of post-reaction calcination, on the other hand, resulted in an increase in whiteness of bauxite-tailings from 19% to 73%. This was achieved using 4.5% of added phosphorous, 40% acid concentration, reaction time of 2.5 hours and a calcination temperature of 600 ℃. The resultant did not require any further processing, and was therefore industrially feasible. The whitened tailings consisted of corundum, anatase, and quartz. When compared to calcined tailings, whitened tailings possessed a higher content of amorphous material, the disappearance of red iron minerals, and the emergence of a new phase of AlPO 4.
摘要:ZrB_2-SiC based ultra-high temperature ceramic(UHTC) struts were firstly proposed and fabricated with the potential application in the combustor of scramjets for fuel injection and flame-holding for their machinability and excellent oxidation/ablation resistance in the extreme harsh environment. The struts were machined with electrospark wire-electrode cutting techniques to form UHTC into the desired shape, and with laser drilling to drill tiny holes providing the channels for fuel injection. The integrated thermal-structural characteristic of the struts was evaluated in high-temperature combustion environment by the propane-oxygen free jet facility, subject to the heat flux of 1.5 MW/m^2 lasting for 300 seconds, and the struts maintained integrity during and after the first experiment. The experiments were repeated for verifying the reusability of the struts. Fracture occurred during the second repeated experiment with the crack propagating through the hole. Finite element analysis(FEA) was carried out to study the thermal stress distribution in the UHTC strut. The simulation results show a high thermal stress concentration occurs at the hole which is the crack initiation position. The phenomenon is in good agreement with the experimental results. The study shows that the thermal stress concentration is a practical key issue in the applications of the reusable UHTC strut for fuel injection structure in scramjets.
摘要:Three different methods were applied to study the alkali content of gelpores in cement. In the closed system, the concentration of K+, Na+ and OH - have not reduced with the increase of age. In the open system, the diffusion and transferring of K+ and Na+ towards free space leads to the de-crease of total alkali content. In the micro-analysis system, the contents of K+ and Na+ in the first hy- drated layer of ground granulated blastfurnace slag (GBFS) are very low, while the contents of calcium and magnesium are relatively high. This phenomenon shows that the mechanism of GBFS preventing alkali aggregate reaction (AAR) is: when GBFS is dissolved by alkali medium, SiO2 and Al2O3 are dissolved into the cement matrix, then around GBFS particles form reaction rings rich in Ca2+ and Mg2+, and the C-S-H gel of positive charges formed in the area repulses K+ and Na+, which are forced to transfer to the mortar’s matrix, pore or mortar sample surface. The transferred K + and Na+ form alkali gel products with other dissolved ions, then become evenly distributed in the mortar sample and react with Ca(OH)2 in pore solutions to form (Na,K)x-2z·zCa·(SiO2)y·(OH)x gel products; and thus changes the AAR gel products’ structure. The gel products will not expand, and so they can delay expansion destruction.
摘要:A novel kind of denture base nanocomposite was prepared by polymethyl me-thyacrylate(PMMA) and cethyltrimethylammonium bromide modified organic montmorillonite (OMMT).The dispersion of montmorillonite in the polymer matrix was characterized by x-ray diffraction (XRD) and transimission electron microscope (TEM).The content of residual MMA in nanocomposites and the amount of MMA released to water from nanocomposites were determined by gas chromatography (GC).The analysis of TEM and XRD showed that exfoliated-intercalated and intercalated nanocomposites were formed when the content of OMMT was 3% and 5% in the PMMA powder respectively.The results of GC showed that the residual MMA increased with the increase of OMMT content in the polymer matrix.After 7 days in water,the amount of MMA re-leased into water from the nanocomposites tended to be stable.The results of one-way ANOVA and t-test showed that OMMT gave a significant increase of residual MMA concentration (p<0.05) in nanocomposites.
摘要:The starch-g-lactic acid copolymer was synthesized with catalysis of sulfuric acid by onestep process, and the structure of starch-g-lactic acid copolymer was characterized by means of IR, 13C-NMR, HMBC, XRD, and SEM. The experimental results show that the maximum grafting degree of starch can reach 75% when the starch-g-lactic acid copolymer is activated at 80 ℃ for 2 h and reacted with lactic acid at 90 ℃ for 4 h in vacuum.
摘要:The effects of different sintering addictives on the preparation of CaF2 transparent ceramics were studied. Transparent CaF2 ceramics were fabricated by vacuum sintering and hot isostatic pressing (HIP) method, using CaF2 nanopowders synthesized by chemical precipitation method as raw materials. The nanopowders and transparent ceramics were studied using X-ray diffraction, transmission electron microscopy (TEM), scanning electron microscopy (SEM) and spectrophotometer. The experimental results indicated that the obtained nanopowders presented normal distribution with grain size about 30 nm; transmittance of CaF2 transparent ceramics was 39% and 26% at 1100 nm for LiF and NaF as sintering addictives, respectively, with corresponding mean grain size 188 μm and 44 μm. Loss of transmission could be attributed to the residual closed porosity. Sintering mechanism was liquid-phase sintering at pre-stage, then solid-phase sintering at later stage, as well as solid solution of lithium ions and sodium ions in the CaF2 lattice structure.
摘要:A series of poly(ethylene oxide)(PEO) membranes with star-like structures for CO_2/H_2 separation were prepared by the photo-polymerization method. The structure of PEO membrane was characterized by Fourier transform infrared spectroscopy(FTIR). The thermal property and inter-segmental distance of polymer chain were investigated by differential scanning calorimetry and wide-angle X-ray diffraction, respectively. The density was determined by hydrostatic weighing method. The gas permeability, solubility and diffusivity of CO_2 and H_2 were investigated in the star-like PEO membranes. The relationship between gas permeation performances and physical properties was also discussed. The membrane exhibits outstanding CO_2 permeability(about 9.7×10^(-11) cm^3(STP) cm/cm^2/s/Pa) and CO_2/H_2 selectivity(about 11) compared with other membranes.
摘要:The dimpling defects caused by conventional hemispherical punch in doubly curved sheet metal reconfigurable die forming process were considered.The rotatable cubic punch (RCP) was developed to suppress the dimpling defects more effectively and conveniently.The former punch contacts with the work-piece through a point-surface contact and the latter punch contacts with the work-piece through a surface-surface contact.A series of stamping experiments were carried out using three different punches (hemispherical punch,RCP,chamfered-RCP) with three different loads.Some finite element simulations about the stamping experiments were carried out.The dimple scales were evaluated through the dimple depths.The corresponding data were obtained by 3-D scanning and FE result analysis respectively.A 3-D plate forming machine was developed,in which chamfered-RCP was adopted.Plate forming experiments were carried out on this machine.The stamped samples show a clear basis for the performance of chamfered-RCP.The study provided a means to guide the design of punches for dimpling suppression used in reconfigurable die.
摘要:Re-swelling capacity is a key factor influencing the self-sealing efficiency of superabsorbent polymers (SAPs) in concrete.In this paper,a new parameter (re-swelling ratio,η),the volume ratio of the crack which was filled with the expansive SAPs and the dry SAPs,was given to quantify the re-swelling capacity of a single SAPs particle.An innovative immersion test was used to study the η value of SAPs in the hardened cement paste with an artificial crack.Moreover,the influence of the crack width and particle size on the sealing efficiency of SAPs in the cracked paste was investigated by a water permeability test.The results showed that the mass ratios of the expansive SAPs in an artificial crack were less than those in a free state.The η value of SAPs in the hardened paste with an artificial crack increased with the increase of the crack width due to the restricting effects of the crack.The expansive SAPs in the cracked paste could totally seal or partly seal the crack within the original void.Moreover,the sealing efficiency of SAPs slightly increased with the rise of the crack width (0.25 to 0.5 mm) and the reduction of the particle size.This research demonstrates that the crack width in concrete and the particle size of SAPs are the key factors influencing the re-swelling behavior of SAPs which should be taken into consideration when designing the self-sealing concrete containing SAPs.
摘要:The strain effects of the Zn1-xMgxO substrate on the bands structure of wurtzite Nb-doped Zn O bulk materials have been investigated using fi rst-principles calculations based on density functional theory. Firstly, the band gap increases gradually with increasing Nb contents in unstrained Nb-doped Zn O, which is consistent with the experimental results. Secondly, the band gap decreases with increasing substrate stress in Nb-doped Zn O/Zn1-xMgxO. Splitting energies between HHB(Heavy Hole Band) and LHB(Light Hole Band), HHB and CSB(Crystal Splitting Band) in Zn0.9167Nb0.0833O/Zn1-xMgxO almost remain unchanged with increasing substrate stress, while decrease slightly in Zn0.875Nb0.125O/Zn1-xMgxO. In addition, detailed analysis of the strain effects on the effective masses of electron and hole in Nb-doped Zn O/Zn1-xMgxO is also given.
摘要:Durability design of recycled high performance concrete(RHPC) is fundamental for improving the use rate and level of concrete waste as coarse recycled aggregate(CRA). We discussed a frostdurability-based mix proportion design method for RHPC using 100 % CRA and natural sand. Five groups of RHPC mixes with five strength grades(40, 50, 60, 70 and 80 MPa) were produced using CRA with four quality classes, and their workability, 28 d compressive strengths and frost resistances(measured by the compressive strength loss ratio and the relative dynamic modulus of elasticity) were tested. Relationships between the 28 d compressive strength, the frost resistance and the CRA quality characteristic parameter, water absorption, were then developed. The criterion of a CRA maximum water absorption limit value for RHPC was suggested, independent of its source and quality class. The results show that all RHPC mixes achieve the expected target workability, strength, and frost durability. The research results demonstrate that the application of the proposed method does not require trial testing prior to use.
摘要:Two novel non-fullerene small molecule acceptors were prepared with the conjugated backbone of 5 H-fluoreno[3, 2-b:6, 7-b’] dithiophene carrying the electron deficient unit of dicyanomethylene indanone(DICTFDT) and rhodanine(TFDTBR), respectively. The two acceptors exhibited excellent thermal stability and strong absorption in the visible region. The LUMO level is estimated to be at-3.89 eV for DICTFDT and-3.77 eV for TFDTBR. When utilized as the acceptor in bulk heterojunction polymer solar cells with the polymer donor of PBT7-Th, the optimized maximum power conversion efficiency of 5.12% and 3.95% was obtained for the device with DICTFDT and TFDTBR, respectively. The research demonstrates that 5 H-fluoreno[3, 2-b:6, 7-b’] dithiophene can be an appealing candidate for constructing small molecular electron acceptor towards efficient polymer:non-fullerene bulk heterojunction solar cells.
摘要:Hexagonal boron nitride ceramic(h-BN) based on the nitridation of B powders was obtained by reaction sintering method. The effects of sintering temperature on the mechanical properties and microstructure of the resultant products were investigated and the reaction mechanism was discussed. Results showed that the reaction between B and N2 occurred vigorously at temperatures ranging from 1 000 ℃ to 1 300 ℃, which resulted in the generation of t-BN. When the temperature exceeded 1 450 ℃, transformation from t-BN to h-BN began to occur. As the sintering temperature increased, the spherical particles of t-BN gradually transformed into fine sheet particles of h-BN. These particles subsequently displayed a compact arrangement to achieve a more uniform microstructure, thereby increasing the strength.
摘要:Circulating fluidized bed combustion(CFBC) fly ash was mixed with cement or lime at a different ratio as a stabilizer to stabilize lake sludge.In order to understand the influences of stabilizers on the lake sludge properties,tests unconfined compressive strength,water stability and SEM observation were performed.The experimental results show that with the increase of the curing time,the strength of all the stabilized specimens increase,especially the samples containing cement.The strength of the specimens is decreased with the increasing of the CFBC fly ash/cement ratio,the optimum ratio between CFBC fly ash and cement is 2:3.The water stability of CFBC fly ash-cement based stabilizers is higher than those of cement and lime.Moreover,the lake sludge stabilization mechanism of CFBC fly ash-cement based stabilizers includes gelation and filling of the hydration products,i e,C-S-H gel and the AFt crystal,which act as benders to solidify those particles together and fill in the packing void of the aggregates.
摘要:The effects of superabsorbent polymers on the shrinkage and mechanical properties of concrete were investigated.Results of shrinkage tests show that SAP addition reduces the shrinkage.Especially,for the concrete with 3.84 kg SAP/m3 concrete,the shrinkage is little.Compressive strength of concrete with SAP is lower than that of concrete without SAP.However,for the concrete with SAP,the ratio of compressive strength at 90 days to one at 3 days is larger than that of concrete without SAP.
摘要:In order to consume the Yellow River sediment as much as possible and improve the longterm stability of the Yellow River, Yellow River sediment was utilized as the main raw material to produce a composite material. Ca(OH)_2 was used as alkali-activator to activate the active SiO_2 and Al_2O_3 compositions in Yellow River sediment. 10 wt% slag was added into the mixture to further improve the strength of the composites. The effect of activity rate of the Yellow River sediment and dosage of Ca(OH) _2 on the compressive strength of the Yellow River sediment-slag composite material at different curing ages was researched. XRD, SEM/EDS, light microscope and FTIR were used to further explore the products and the microstructure of the composite material. Results showed that the active ratio of sediment had a great influence on the compressive strength of specimen. In addition, the compressive strength of specimen increased with the increase of Ca(OH)_2dosage and curing age. When the dosage of Ca(OH)_2 was more than 5 wt% as well as the curing age reached 90 days, the compressive strength of the composite material could meet the engineering requirement. In the alkali-activated process, the main product was hydrated calcium silicate(C-S-H) gel, which filled up the gaps among the sediment particles and decreased the porosity of the specimen. Moreover, the CaCO_3 produced by the carbonization of the C-S-H gel and excess Ca(OH)_2 also played a role on the strength.
摘要:Li-doped Zn O thin films had been grown by radio frequency magnetron sputtering and then annealed under various annealing temperatures. The characteristics of Zn O films were examined by XRD, FESEM, Hall measurement and optical transmission spectra. Results showed that p type conduction was observed in Lidoped Zn O films annealed at 500-600 ℃ and the p type Zn O films possessed a good crystalline with c-axis orientation, dense surface, and average transmission of about 85% in visible spectral region.
摘要:ZrC coatings were deposited on graphite substrates by low pressure chemical vapor deposition(LPCVD) with the Br_2-Zr-C_3H_6-H_2-Ar system. The effects of deposition time on the microstructures and growth behavior of ZrC coatings were investigated. ZrC coating grew in an island-layer mode. The formation of coating was dominated by the nucleation of ZrC in the initial 20 minutes, and the rapid nucleation generated a fine-grained structure of ZrC coating. When the deposition time was over 30 min, the growth of coating was dominated by that of crystals, giving a column-arranged structure. Energy dispersive X-ray spectroscopy showed that the molar ratio of carbon to zirconium was near 1:1 in ZrC coating, and X-ray photoelectron spectroscopy showed that ZrC was the main phase in coatings, accompanied by about 2.5mol% ZrO_2 minor phase.