Larger Average Grain Research Articles

Structure of superconducting layers in bronze-processed and internal-tin Nb3Sn-based wires of various designs

The microstructure of Nb3Sn layers in multifilamentary composites manufactured by bronze technology and the internal-tin (IT) method differing in the strand design has been studied. The IT strands of International Thermonuclear Experimental Reactor (ITER) type and high-Jc type are characterized by both the higher Sn concentration and the higher fraction of equiaxed grains than bronze-route composites. In all the samples under study, the fraction of equiaxed grains correlates with the average concentration of Sn in Nb3Sn layers. The Ti doping of the bronze matrix in the bronze-processed wires results in the reduction in average grain sizes of Nb3Sn grains and in a higher fraction of equiaxed grains with a higher Sn concentration, which leads to a higher non-Cu Jc (calculated as a ratio of critical current Ic to a cross-sectional area of a strand without stabilizing Cu), 997 A/mm2 at 12 T, 4.2 K, compared to a composite of the same design with artificially Ti-doped Nb filaments. The ITER type IT strand demonstrates a somewhat higher fraction of equiaxed Nb3Sn grains with much larger average grain sizes (120 nm). The grain coarsening in this IT strand results in a decrease in Jc compared to other ITER type strands studied. The highest values of Jc are ensured in the high-Jc type of IT wires with 7 extended tin sources by the highest fraction (92%) of equiaxed Nb3Sn grains despite their coarser sizes (92 nm).

Microstructure and Corrosion Behavior of Simulated Welding HAZ of Q315NS Steel in Sulfuric Acid Solution

The microstructure evolution and the corrosion behavior of welding heat affected zone (HAZ) of Q315NS steel in 50 wt % H2SO4 at 20 °C was investigated with thermal simulation technique, surface analysis and electrochemical tests. The microstructure of ferrite and pearlite was observed in base metal (BM), fine grained region (FGHAZ) and inter critical region (ICHAZ) while coarse grained region (CGHAZ) consisted of granular bainite. The CGHAZ exhibited the highest microhardness and the largest average grain size. The passivation process occurred on the surface of all specimens. Different microstructure give birth to different corrosion behaviors between CGHAZ and BM, FGHAZ, ICHAZ. The dense oxide film were formed on the surface of ferrite while oxide film with micro voids were formed on the surface of pearlite in BM, FGHAZ and ICHAZ after immersion in 50 wt % H2SO4 solution for 12 h. The rod-shaped corrosion product was formed on the surface of CGHAZ while the porous-structured corrosion product was formed on the surface of BM, FGHAZ and ICHAZ after immersion in 50 wt % H2SO4 solution for 72 h. The corrosion resistance of BM, CGHAZ, FGHAZ and ICHAZ increased during the first 12 h and then declined slowly with increasing immersion time. The BM had the best corrosion resistance while the CGHAZ had the lowest corrosion resistance throughout the corrosion process.

Statistically meaningful grain size analysis of CVD graphene based on the photocatalytic oxidation of copper

CVD graphene is a polycrystalline material, with grains typically smaller than a few microns. Although it is known that graphene’s grain size affects its properties, not much attention has been paid to developing methods similar to those existing for metals to perform grain size analysis. Reported strategies thus far are either limited to extremely small areas or incapable of providing real grain size statistics. Here we report a novel, straightforward laboratory-bench method that uses TiO2 suspended in water and UV light to selectively and precisely oxidize copper through graphene defects, resulting in well-contrasted darker copper oxide lines on a light copper background. The graphene grains are then visible via optical microscopy, and the quality is unprecedented. This allows for the statistical measurement of graphene grain size distributions across large areas, in accordance with ASTM standards and automated image analysis. Raman spectroscopy shows further that graphene is only moderately damaged after the photocatalytic process. Measurements of three monolayer CVD graphene samples with grain sizes ranging from 1.4 to 3.9 microns are presented, showing Gaussian distributions. For the first time, an experiment can be used to show how graphene grain size distributions become wider for larger average grain sizes, as it is common for polycrystalline materials.

Influence of Na doping on the magnetic properties of LaFeO3 powders and dielectric properties of LaFeO3 ceramics prepared by citric sol-gel method

The emergence of ferromagnetism in perovskite oxide LaFeO3 nanoparticles and colossal dielectric response in ceramics has inspired researchers to study the effect of various dopants on the magnetic and dielectric properties of LaFeO3 powders and ceramics. However, the influence of alkali element Na doping has not been studied yet, and the origin of such ferromagnetic behavior is still ambiguous. The primary objective of the present work is to elucidate the effect of Na doping on the magnetic properties of La1−xNaxFeO3 (x=0, 0.1 and 0.2) powders and dielectric properties of corresponding ceramics prepared by citric sol-gel method. FE-SEM results showed that the introduction of Na dopant actually resulted in the formation of nonstoichiometric La1−xFeO3 and x/2 Na2O. Compared to the canted AFM behavior for the pure powder, ferromagnetic behavior with enhanced magnetization of 2.11emu/g at 10kOe could be obviously observed at room temperature for the powder with x=0.2. XPS measurement suggested nonstoichiometric Fe/La ratio which leads to the distortion of lattice structure and enlarged canting angle between the two AFM coupled Fe sublattice should be responsible for the enhancement of magnetization in the Na-doped samples. Meanwhile, the introduction of Na dopant lowered the growth temperature of grains of the parent LFO and resulted in larger average grain size, which in turn leaded to great enhancement of ε′ into the order of 105 at 100Hz at the cost of high tanδ for the Na-doped ceramics.

Iron Oxide Films Prepared by Rapid Thermal Processing for Solar Energy Conversion

Hematite is a promising and extensively investigated material for various photoelectrochemical (PEC) processes for energy conversion and storage, in particular for oxidation reactions. Thermal treatments during synthesis of hematite are found to affect the performance of hematite electrodes considerably. Herein, we present hematite thin films fabricated via one-step oxidation of Fe by rapid thermal processing (RTP). In particular, we investigate the effect of oxidation temperature on the PEC properties of hematite. Films prepared at 750 °C show the highest activity towards water oxidation. These films show the largest average grain size and the highest charge carrier density, as determined from electron microscopy and impedance spectroscopy analysis. We believe that the fast processing enabled by RTP makes this technique a preferred method for investigation of novel materials and architectures, potentially also on nanostructured electrodes, where retaining high surface area is crucial to maximize performance.

Electrodeposition of Compact Tellurium Thick Films from Alkaline Baths

Compact semiconducting tellurium thick films (i.e., upto 50 μm) were electrodeposited at high rates (>100 μm h−1) with great current efficiency (>85%) by optimizing the electrolyte composition (TeO32− concentration and pH) and deposition potential. The preferred orientation of as-deposited Te films changed from (001) to (101) direction as the deposition potential becomes more cathodic. Average grain size ranged from 66 to 135 nm depending the deposition parameters. Electrodeposited Te films were p-type semiconductors with the carrier concentration ranged from ∼7.0 × 1018 to ∼3.1 × 1019 cm−3. The carrier concentration of Te films strongly dependent on the average grain size where larger average grain size resulted in a lower carrier concentration because of less structural defects.

Precipitation behaviors, texture and tensile properties of an extruded Mg-7Y-1Nd-0.5Zr (wt%) alloy bar with large cross-section

Precipitation behaviors, texture and tensile properties of an extruded Mg-7Y-1Nd-0.5Zr (wt%) (WE71) alloy bar with large cross-section of 230mm×140mm were investigated by hardness test, tensile test, optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), X-ray diffraction (XRD) macro-texture measurement. The bar was manufactured industrially through a procedure of “multi-direction forging (MDF)+extrusion+on-line quenching+T5 aging”. Totally different age-hardening behaviors are shown during T5 aging at 200 and 235°C. In the first 100h, T5 aging at 235°C brings about 13% increases in hardness, while T5 aging at 200°C results in 47% increase. During T5 aging at 200°C, β′ precipitates homogeneously nucleate within the matrix with high number density; however, during T5 aging at 235°C, β′ precipitates heterogeneously nucleate on discrete and sparse dislocations, resulting in chain-like arrangement of β′ precipitates with broad precipitate free zones in matrix. XRD macro-texture measurement illustrates that basal texture intensity of WE71 bar is much weaker than Mg-8Al-0.5Zn-0.15Mn (wt%) (AZ80) bar; the maximum basal texture intensities in the outer (O) and center (C) of WE71 bar are all about 3, while those of AZ80 bar are 17 and 14, respectively. EBSD micro-texture measurement demonstrates that the maximum texture intensities of C and O are 5.3 and 3.5, respectively. O has higher tensile properties than C because there are more un-dynamic-recrystallization (un-DRX) grains and thus larger average grain size in C. While stretching at room temperature (RT), extrusion direction (ED) in O has the best tensile properties, i.e. ultimate tensile strength (Rm)=368MPa, elongation (A)=5%, and normal direction (ND) in C has the lowest tensile properties, i.e. Rm=255MPa, A=2%. While stretching at 200°C, strength does not degrade much; ED in O still has the best tensile properties, i.e. Rm=326MPa, A=7%, and ND in C still has the lowest tensile properties, i.e. Rm=263MPa, A=4%.

Influence of sintering temperatures of ceramic targets on microstructures and photoelectric properties of titanium-doped ZnO nano-films

Thin films of Ti-doped ZnO (TZO) were prepared by RF magnetron sputtering using targets prepared with sintering temperatures in the range 1100–1500 °C; the microstructures and optoelectronic properties of the TZO targets and films were characterized by SEM, XRD, Hall Effect analysis, UV–VIS spectrophotometry and physical property measurement system. Results indicated that the target sintering temperature affected both the TZO targets and films. The Ti/Zn atomic ratios in the targets decreased progressively with increasing sintering temperature, but by a smaller amount in films prepared from them. XRD patterns showed that all films were preferentially oriented along the c axis at 2θ ~ 34° in their XRD patterns. The films sputtered with targets sintered at above 1300 °C were relatively smooth, and had larger average grain size. The target sintered at 1450 °C had the highest density. The best optoelectronic properties were found with the film sputtered from the target sintered at 1300 °C; this sample had superior crystal properties, high average optical transmittance (88.9%), and the lowest resistivity (8.47 × 10−4 Ω cm). Furthermore, the resistivity of all the films changed with temperature between 10 and 350 K, they experienced an initial decrease followed by an increase as the temperature was raised.

Grain refinement of wire arc additively manufactured titanium by the addition of silicon

This study demonstrates that silicon additions are effective in refining the microstructure of additive layer manufactured (ALM) titanium components. The addition of up to 0.75 wt% silicon to commercially pure titanium manufactured by wire arc ALM results in a significant reduction of the prior-β grain size. It is observed that silicon also reduces the width of the columnar grains and allows for the nucleation of some equiaxed grains through the development of constitutional supercooling and growth restriction. The grain size of the ALM components is compared to a casting process and it is found that the as-deposited microstructure produced during ALM exhibits larger average grain sizes. Using the Interdependence model for predicting grain size, it was determined that the population of nucleant particles that naturally occur in titanium, has comparable potency (i.e. ability to activate at a similar undercooling) regardless of the processing method, however, the ALM process contains fewer, sufficiently potent, nucleant particles than for the casting process due to the effect of subsequent cycles of remelting and heating.

Evaluating the Optoelectronic Quality of Hybrid Perovskites by Conductive Atomic Force Microscopy with Noise Spectroscopy.

Organic-inorganic hybrid perovskite solar cells have emerged as promising candidates for next-generation solar cells. To attain high photovoltaic efficiency, reducing the defects in perovskites is crucial along with a uniform coating of the films. Also, evaluating the quality of synthesized perovskites via facile and adequate methods is important as well. Herein, CH3NH3PbI3 perovskites were synthesized by applying second solvent dripping to nonstoichiometric precursors containing excess CH3NH3I. The resulting perovskite films exhibited a larger average grain size with a better crystallinity compared to that from stoichiometric precursors. As a result, the performance of planar perovskite solar cells was significantly improved, achieving an efficiency of 14.3%. Furthermore, perovskite films were effectively analyzed using a conductive AFM and noise spectroscopy, which have been uncommon in the field of perovskite solar cells. Comparing the topography and photocurrent maps, the variation of photocurrents in nanoscale was systematically investigated, and a linear relationship between the grain size and photocurrent was revealed. Also, noise analyses with a conductive probe enabled examination of the defect density of perovskites at specific grain interiors by excluding the grain-boundary effect, and reduced defects were clearly observed for the perovskites using CH3NH3I-rich precursors.

HiPIMS-deposited thermochromic VO2 films with high environmental stability

A comparative study of the environmental stability of high power impulse magnetron sputtered (HiPIMS) and radio frequency magnetron sputtered (RFMS) thermochromic vanadium dioxide thin films in a highly oxidizing environment has been performed. We observe that RFMS-deposited VO2 films quickly transform into V2O5 at 80°C and 100% relative humidity while HiPIMS-deposited films retain their thermochromic behaviour at least three times longer. Following a thorough analysis of the films, this increase in performance is ascribed to the films’ higher density and larger average grain size. The results indicate that the HiPIMS process provides sufficient durability for VO2 films without the need for a diffusion barrier used in typical everyday conditions. Still, when a 35nm thick SiNx diffusion barrier is added on top of the HiPIMS VO2 films, the thermochromic device shows less than 5% loss in thermochromic performance following extended accelerated ageing.

Microstructure and Mechanical Properties of Dissimilar Friction Stir Welding between Ultrafine Grained 1050 and 6061-T6 Aluminum Alloys

The ultrafine grained (UFGed) 1050 Al plates with a thickness of 2 mm, which were produced by the accumulative roll bonding technique after five cycles, were friction stir butt welded to 2 mm thick 6061-T6 Al alloy plates at a different revolutionary pitch that varied from 0.5 to 1.25 mm/rev. In the stir zone, the initial nano-sized lamellar structure of the UFGed 1050 Al alloy plate transformed into an equiaxial grain structure with a larger average grain size due to the dynamic recrystallization and subsequent grain growth. However, an equiaxial grain structure with a much smaller grain size was simultaneously formed in the 6061 Al alloy plates, together with coarsening of the precipitates. Tensile tests of the welds obtained at different welding speeds revealed that two kinds of fracture modes occurred for the specimens depending on their revolutionary pitches. The maximum tensile strength was about 110 MPa and the fractures were all located in the stir zone close to the 1050 Al side.

Grain growth in thin Al films during deposition from partially ionized vapor

Grain growth in thin Al films during deposition from partially ionized vapor flux with simultaneous self-ion bombardment was studied in this work. The films were deposited at constant ion energy of 940eV and total specific power of 0.4W/cm2 while the deposition time t of 6s to 246s and the resulting substrate temperature (Ts/Tm of 0.35–0.96) were varied. Thin continuous Al films exhibited normal grain growth through the entire experimental range of deposition time without limitation of grain growth by the film thickness effect. Three kinetic stages of the grain growth were observed within 100s of deposition time: the first one exhibits very slow grain growth, accelerated grain growth occurs in the second stage and then it rapidly changes to a retardation and stagnation mode in the third stage. Large average grain sizes Dg up to 11.3μm at film thickness of 1.4μm and integral grain growth rates up to 0.16μm/s were observed in this study. The experimental results were evaluated against various mechanisms of inhibition of grain growth. An estimate of the effective activation energy of the grain growth yields a value of 0.27eV which is lower than that of the bulk Al and much higher than the activation energy of surface self-diffusion on (111)Al monocrystal. The power law Dg=(k t)0.5 gives good match with experimental results in the initial deposition phase preceding the grain growth retardation, while another model that is based on the grain size dependent pinning force adequately explains the entire grain size dependence on time. It is deemed both ion enhanced film/surface interaction and impurities on one side and thermal grooves on another side contribute to the rapid retardation of the grain grooves commencing the second growth stage.

Effects of discharge power on the structural and optical properties of TGZO thin films prepared by RF magnetron sputtering technique

The transparent semiconductors of Ti and Ga-incorporated ZnO (TGZO) thin films were prepared by radio frequency (RF) magnetron sputtering onto glass substrates. The effects of discharge power on the physical properties of thin films are studied. Experimental results show that all nanocrystalline TGZO thin films possess preferential orientation along the (002) plane. The discharge power significantly affects the crystal structure and optical properties of thin films. When the discharge power is 200 W, the TGZO thin film has the optimal crystalline quality and optical properties, with the narrowest full width at half-maximum (FWHM) of 1.76×10−3 rad, the largest average grain size of 82.4 nm and the highest average transmittance of 84.3% in the visible range. The optical gaps of thin films are estimated by the Tauc’s relation and observed to increase firstly and then decrease with the increase of the discharge power. In addition, the optical parameters, including refractive index, extinction coefficient, dielectric function and dissipation factor of the thin films, are determined by optical characterization methods. The dispersion behavior of the refractive index is also analyzed using the Sellmeier’s dispersion model.

Sintering and electrical conductivity of gadolinia-doped ceria

Bulk specimens of Ce0.9Gd0.1O2-δ prepared with powders within a range of specific surface area were sintered in oxidizing, inert, and reducing atmospheres. The aim of this work is to investigate the effects of the sintering atmosphere on the microstructure and grain and grain boundary conductivities of the solid electrolyte. The lattice parameter determined by Rietveld refinement is 0.5420(1) nm, and the microstrain was found negligible in the powder materials. Specimens sintered in the Ar/4 % H2 mixture display larger average grain sizes independent on the particle size of the starting powders. The grain and grain boundary conductivities of specimens sintered under reducing atmosphere are remarkably lower than those sintered under oxidizing and inert atmospheres. The activation energy (∼0.90 eV) for total electrical conductivity remains unchanged with both the initial particle size and the sintering atmosphere.

A novel approach for Al2O3/epoxy composites with high strength and thermal conductivity

Insulating materials for electronic packaging should ideally have high strength and thermal conductivity. Ceramic/polymer composite is one of the candidates for such materials. In this work, Al2O3/epoxy composites with very high flexural strength and thermal conductivity were prepared using a new processing technique consisting of the gelcasting, sintering and vacuum infiltration methods. Al2O3 green bodies were first formed by gelcasting. After being degreased at 600 °C, the green bodies were sintered at different temperatures from 1200 °C to 1500 °C, and porous Al2O3 ceramic skeletons with different porosity were resulted. The porosity was readily controlled by the sintering temperature. Higher sintering temperature resulted in lower porosity and larger average grain size in the Al2O3 ceramic skeletons. Epoxy was vacuum infiltrated into the sintered porous Al2O3 ceramic skeletons to form Al2O3/epoxy composites by curing. High Al2O3 loadings up to 70 vol.% in the composites were achieved. The flexural strength and thermal conductivity of the Al2O3/epoxy composites were found to reach 305 MPa and 13.46 W m−1 K−1, respectively. These results are remarkably higher than those previously reported in the literature. This work provides an effective approach for fabricating high performance ceramic/polymer composites for electronic packaging.

Microstructure, surface morphology and optical properties of Na x Cu y Zn1−x−y O thin films

Na x Cu y Zn1−x−y O thin films were prepared by sol–gel method on quartz glass. The effect of Na (or Cu) doping on the microstructure, surface morphology, optical properties of the thin films was investigated. The Na0.1Cu0.01Zn0.89O thin film has the largest average grain size and the best preferential c-axis orientation, which may be attributed to the difference in ionic radius between Zn and Na (or Cu). The Na0.1Cu0.01Zn0.89O thin film also presents denser morphology with little voids and the smallest optical band gap, which may be attributed to band edge bending and the larger grain size. The largest intensity of A1(LO) modes may be due to the major structural defect concentrations. The quenching of UV and visible emissions, which may be attributed to the increase of non-radiative recombination transition, has been observed in the Na0.1Cu0.01Zn0.89O and Na0.1Cu0.02Zn0.88O thin films.

EFFECTS OF FORGING AND HEAT TREATMENTS ON STRESS CORROSION BEHAVIOR OF 316LN STAINLESS STEEL IN HIGH TEMPERATURE CAUSTIC SOLUTION

The reactor coolant piping in the third generation nuclear power plants of AP1000 is manufactured by integrally forging. Therefore, it is of vital importance to investigate the effects of forging and heat treatments on the stress corrosion cracking(SCC) resistance of 316 LN stainless steel(316LNSS), which is the candidate material for the reactor coolant piping in AP1000 nuclear power plants. In this work, electron back scattering diffraction(EBSD) and microhardness measurements(HV) were used to characterize the microstructure and residual strain of the as-received 316 LNSS, the forged and solution anneal treated 316 LNSS and the forged and stress relief treated 316 LNSS, respectively. The average grain size of the as- received 316 LNSS was the largest, and the forged 316 LNSS followed by solution anneal treatment and stress relief treatment showed no obvious differences on grain size. The as-received 316 LNSS exhibited the highest residual strain followed by the forged and stress relief treated 316 LNSS and then solution anneal treated 316 LNSS. Besides, the residual strain in the as- received316 LNSS concentrated on grain boundaries, while the residual strain in the forged and stress relief treated316 LNSS was characterized by a band-like distribution. The U-bend specimens were utilized to investigate the SCC behavior of the 3 kinds of 316 LNSS specimens in high temperature caustic solution. After SCC experiments, the crack morphologies of the 3 kinds of 316 LNSS specimens were examined by SEM. Then the macro and micro fracture morphologies were examined by OM and SEM, respectively. Grain morphology, residual strain and grain boundary character distribution near the SCC crack tip of the forged and stress relief treated 316 LNSS were investigated using EBSD. The results showed that the forged and solution anneal treated 316 LNSS exhibited the lowest SCC sensibility, while the as-received the highest, with the most cracks and the highest growth rate. The as-received and the forged and solution anneal treated 316 LNSS showed obvious intergranular cracking, while the forged and stress relief treated 316 LNSS showed a mixed cracking mode. The larger average grain size and higher residual strain, especially concentrating on the grain boundaries, were considered to be responsible for the highest SCC sensibility of the as-received 316 LNSS. Compared with the forged and stress relief treated 316 LNSS, the higher content of coincidence site lattice boundary(CSLB) and lower residual strain contributed to the lower SCC sensibility of forged and solution anneal treated 316 LNSS. The stress relief treatment failed to eliminate the band-like microstructure effectively, which disadvantaged the SCC resistance.

Dimethyl carbonate synthesis via transesterification of propylene carbonate with methanol by ceria-zinc catalysts: Role of catalyst support and reaction parameters

Ceria and zinc oxide catalyst were impregnated onto various oxide supports, namely Al2O3, TiO2 and SiO2, individually by deposition-coprecipitation method. The synthesized catalysts (CZA, CZS and CZT having supports Al2O3, TiO2 and SiO2, respectively) were characterized by X-ray diffraction (XRD), NH3- and CO2-temperature programmed desorption (TPD) and N2 adsorption. These catalysts were used for synthesis of dimethyl carbonate (DMC) from methanol and propylene carbonate in a batch reactor. CZS was found to have larger average grain size as compared to CZA and CZT. Composite oxides (catalysts) were found to contain individual phases of ZnO, CeO2 and some spinel forms of Zn, Ce along with their supports. CZS having highest basicity and surface area showed better catalytic activity as compared to CZA and CZT. Effect of reaction temperature and methanol/PC molar ratio on DMC yield was studied and a reaction mechanism has been discussed. Maximum DMC yield of 77% was observed with CZS catalyst at 170 °C with methanol/PC molar ratio of 10.

Influence of production methods on creep deformation of cold rolled 329LA lean duplex stainless steel in continuous annealing condition

The effects of processing methods, line tension, cold reduction and temperature on creep deformation of 329LA lean duplex stainless steel under continuous annealing condition are investigated. Two different processes, conventional continuous casting followed by hot rolling process and strip casting process, were used to make steel plates. Creep tests were carried out at 1010°C up to 1100°C using cold rolled specimens with various cold reductions for both kinds of specimen. It was found that continuous cast specimens show higher strain rate than strip cast specimens when tested with the same cold reduction and stress condition at all temperatures. Creep strain rate increases with the increase of line tension, temperature and the amount of cold reduction. The measured strain rate sensitivity values (m), around 0.5, show the superplastic behavior of both continuous and strip cast specimens at all test conditions, while the stress exponents (n), around 2, are consistent with the model of grain/phase boundary sliding as the rate controlling mechanism. The strip cast specimen has a very inhomogeneous microstructure which leads to larger average grain size, while the continuous cast specimen shows a uniform distribution of ferrite (δ) and austenite (γ). Electron back scattered diffraction (EBSD) results show that cold rolled texture is destroyed during heating to deformation temperature and δ/γ interface becomes high angle boundary which slides easily. Lastly, phase boundary sliding during creep deformation was confirmed by scanning transmission electron microscopy (STEM) in both specimens.