Improving Mechanical Properties of Tin+1-A-Cn (A=Al or Si) Max Phases Through Solid Solution Strengthening and Fiber Reinforcement

Ti2AlC and Ti3AlC2 belong to the family of MAX phase and they are considered to be good candidates for high-temperature structural materials as they show excellent oxidation resistance up to 1450oC, good damage tolerance and pseudo-ductile behavior. This Ph.D. dissertation reports on the reaction synthesis of Ti2AlC and Ti3AlC2 and improvements on their mechanical properties by either solid solution strengthening due to substitution of Al with Si, or reinforcement with alumina ceramic fibers. The reaction synthesis of Ti2AlC from Ti-Al-TiC powder mixture was investigated in the temperature range of 700oC and 1500oC using pulsed electric current sintering (PECS). A bulk high-purity Ti2AlC was successfully fabricated with fine grain microstructure through PECS in one step, possessing the highest fracture strength ever reported. However, no Ti2(Al1-xSix)C solid solution could be sintered from Ti-Al-Si-TiC using pressureless sintering as they are thermodynamically unstable. Because it is unpractical to improve the mechanical properties of Ti2AlC by substitution of Al with Si, they were reinforced by addition of Nextel^TM 720 and Nextel^TM 610 alumina fibers. 20vol.% of Nextel^TM 720-Ti2AlC and 20vol.% of Nextel^TM 610-Ti2AlC composites with uniform distribution of short fibers were successfully fabricated using colloidal processing and densification through PECS. With addition of alumina fibers, the elastic modulus of Ti2AlC was improved only slightly (by ~4.5%), while Vickers hardness and fracture toughness were enhanced significantly, i.e. for more than 35.8% and 15% in alumina fiber reinforced composites, respectively. In addition, results of mechanical testing in compression show that fabricated composites have higher compressive strength than the pure Ti2AlC, for more than 16% at both room temperature and 1100oC. Therefore, result of this study implies strongly that alumina fibers can be used to improve mechanical properties of the Ti2AlC at both room and high temperatures. In order to expand the application of Ti3AlC2 MAX phase at high temperatures, a series of high-purity Ti3(Al1-xSix)C2 solid solutions with 0<x<1 and fine and coarse grained structure were reaction sintered PECS. It was found that c-lattice parameter and elastic moduli (both Young's and shear modulus) increase linearly with increasing amount of Si in those solid solutions. Regardless of grain size, Vickers hardness results demonstrate significant hardening effect in Ti3(Al1-xSix)C2 solid solutions with a maximum around Ti3(Al0.5Si0.5)C2. At room temperature, the strengthening effect was found to be marginal for fine grained (FG) structure with improvement of only 7.3%, but significant strengthening effect was observed in coarse grained (CG) one of ~25.8%. Above brittle-to-plastic transition (BPT) temperature, the solid solution strengthening effect diminishes and the strength of solid solutions is still higher than that of Ti3AlC2 but less than Ti3SiC2. Meanwhile, Ti3Al0.6Si0.4C2 forms a protective alumina oxide layer at 1200oC with comparable oxidation resistance to Ti3AlC2

Fatigue performance analysis and experimental study of steel trusses integral joint based on multi-scale FEM

The fatigue performance of steel truss integral joint is analyzed based on multi-scale FEM. Numerical results are verified with experimental ones. The fatigue performance of steel truss integral joint is analyzed using a sea-crossing suspension bridge as its background. The 1:2 scale model fatigue test is taken. After 2e6 time load loops, the stress situation of integral joint is stable and fatigue performance is satisfactory for requirements. The multi-scale FEM, whose main researching part adopts three-dimensional elements while the connecting component adopts the common beam element, can reduce the computational cost. The  connection elements are used to connect the beam elements and  threedimensional elements. The connection element has two joints, one of which has six degrees of freedom, three translation degrees of freedom  and three rotational degrees of freedom, all degrees of freedom being  coupled. The multi-scale FEM and the experimental one yield very close results The multi-scale FEM can provide an accurate simulation of the main research part and ensure the fatigue life forecast so as to simplify the calculation. This method is high efficient and feasible. The multi-scale FEM provides a new and reliable method for the fatigue performance of structure analysis

Validity and reliability of the toxic leadership behaviors of nurse managers scale among Chinese nurses

ObjectivesToxic leadership is increasingly becoming common in the nursing field, but the measurement tools are lacking. Therefore, this study aimed to translate the toxic leadership behaviors of nurse managers (ToxBH-NM) scale into Chinese and test its psychometric properties among Chinese nurses.MethodsThe data for this study were obtained from a cross-sectional survey of 1,195 nurses. Exploratory factor analysis (EFA) and confirmatory factor analysis (CFA) were used to examine the structural validity of the ToxBH-NM. The following psychometric properties of the scale were assessed: content validity, criterion validity, internal consistency reliability, and test–retest reliability.ResultsThe Chinese version of the ToxBH-NM (C-ToxBH-NM) scale had two dimensions and 30 items. The correlation coefficients between the scores of each item and the total scores were 0762–0.922 (p &lt; 0.001), and the range of the CR determination values of all the items were 8.610–18.998, with statistical significance (p &lt; 0.001). The total content validity index (CVI) was 0.996, the average CVI was 0.996, and the item-level CVI was 0.875–1.000. Two common factors were identified in the EFA, and 81.074% of the variation was explained cumulatively. The CFA showed that all the fitting indexes reached the standard, and the model fit degree was good. When the Chinese version of the Destructive Leadership Scale was used as calibration, the correlation coefficient was 0.378 (p &lt; 0.001). The Cronbach’s alpha coefficients of the overall scale were 0.989 and of the two dimensions were 0.969 and 0.987, respectively, with a split-half reliability of 0.966 and test–retest reliability of 0.978.ConclusionThe research results show that the C-ToxBH-NM scale has good reliability and validity and can be used to evaluate the severity of toxic leadership behavior among nursing managers

Exciton Dynamics in Suspended Monolayer and Few-Layer MoS2

Femtosecond transient absorption spectroscopy and microscopy were employed to study exciton dynamics in suspended and Si3N4 substrate-supported monolayer and few-layer MoS2 2D crystals. Exciton dynamics for the monolayer and few-layer structures were found to be remarkably different from those of thick crystals when probed at energies near that of the lowest energy direct exciton (A exciton). The intraband relaxation rate was enhanced by more than 40 fold in the monolayer in comparison to that observed in the thick crystals, which we attributed to defect assisted scattering. Faster electron-hole recombination was found in monolayer and few-layer structures due to quantum confinement effects that lead to an indirect-direct band gap crossover. Nonradiative rather than radiative relaxation pathways dominate the dynamics in the monolayer and few-layer MoS2. Fast trapping of excitons by surface trap states was observed in monolayer and few-layer structures, pointing to the importance of controlling surface properties in atomically thin crystals such as MoS2 along with controlling their dimensions

Ultrahigh Thermoelectric Performance by Electron and Phonon Critical Scattering in Cu 2 Se 1‐x I x

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/102251/1/adma201302660.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/102251/2/adma201302660-sup-0001-S1.pd

Rhizosphere Microenvironments of Eight Common Deciduous Fruit Trees Were Shaped by Microbes in Northern China

The rhizosphere microenvironment is the site of nutrient circulation and microbial community formation, and thus is an ongoing topic of research. Although research on this topic is extensive, studies into the rhizosphere microenvironment of fruit trees remain rare. To elucidate the mechanisms driving the fruit tree rhizosphere microenvironment, we assessed soil physicochemical properties, enzyme activities, the community-level physiological profile (CLPP) and microbial diversity in rhizospheric soils of eight common deciduous fruit trees in northern China. We found that the available minerals, pH, enzyme activities, microbial utilization of six types of carbon (C) substrates, and microbial diversity in the rhizosphere varied among tree species. Redundancy analysis (RDA) showed that rhizosphere microenvironmental parameters (ammonia nitrogen content, soil pH and invertase activity) were closely related to the soil microbial community. Further analysis revealed that the soil microbial utilization of six C sources, nitrate nitrogen content, and invertase activity were negatively correlated with Ambiguous species and Alternaria; however, these groups were positively correlated with pH. The ammonia nitrogen content was positively correlated with C source utilization and negatively correlated with Ambiguous, Lysobacter, Nitrospira, Alternaria, Fusarium, and Colletotrichum. Interestingly, invertase was closely linked to the microbial community, especially fungal diversity, and was positively correlated with plant-beneficial microbes such as Mortierella, Geomyces, Lysobacter, and Chaetomium, but negatively correlated with pathogenic microbes such as Alternaria, Fusarium, and Colletotrichum. Hence, rhizosphere soil physicochemical properties, enzyme activities and microbial community were significantly affected by tree species. Additionally, a variety of environmental factors were closely related to the microbial community in the rhizospheric soils of eight species of deciduous fruit trees

Breakdown Walkout in Polarization-Doped Vertical GaN Diodes

We demonstrate the avalanche capability and the existence of breakdown walkout in GaN-on-GaN vertical devices with polarization doping. By means of combined electrical and optical characterization, we demonstrate the following original results: 1) vertical p-n junctions with polarization doping have avalanche capability; 2) stress in avalanche regime induces an increase in breakdown voltage, referred to as breakdown walkout; 3) this process is fully-recoverable, thus being related to a trapping mechanism; 4) temperature-dependent measurements of the breakdown walkout identify $\text{C}_{N}$ defects responsible for this process; and 5) capacitance deep level transient spectroscopy (C-DLTS) and deep level optical spectroscopy (DLOS) confirm the presence of residual carbon in the devices under test. A possible model to explain the avalanche walkout is then proposed