Epitaxial Ti3SiC2(0001) thin films have been deposited by dc magnetron sputtering from three elemental targets of Ti, C, and Si onto MgO(111) and Al2O3(0001) substrates at temperatures of 800–900°C. This process allows composition control to synthesize Mn+1AXn (MAX) phases (M: early transition metal; A: A-group element; X: C and∕or N; n=1–3) including Ti4SiC3. Depositions on MgO(100) substrates yielding the Ti–Si–C MAX phases with (101¯5), as the preferred orientation. Samples grown at different substrate temperatures, studied by means of transmission electron microscopy and x-ray diffraction investigations, revealed the constraints of Ti3SiC2 nucleation due to kinetic limitations at substrate temperatures below 700°C. Instead, there is a competitive TiCx growth with Si segregation to form twin boundaries or Si substitutional incorporation in TiCx. Physical properties of the as-deposited single-crystal Ti3SiC2 films were determined. A low resistivity of 25μΩcm was measured. The Young’s modulus, ascertained by nanoindentation, yielded a value of 343–370GPa. For the mechanical deformation response of the material, probing with cube corner and Berkovich indenters showed an initial high hardness of almost 30GPa. With increased maximum indentation loads, the hardness was observed to decrease toward bulk values as the characteristic kink formation sets in with dislocation ordering and delamination at basal planes.

Abstract:To develop functional sustainable epoxy resins, we report a novel epoxy resin (DEU-EP) with high net biobased content (70.2 wt%) derived from renewable eugenol. We comparatively study DEU-EP with a commercial bisphenol A epoxy resin (DGEBA) in the presence of a most representative aromatic diamine curing agent, 4,4'-diaminodiphenyl methane (DDM).Differential scanning calorimetry reveals that DEU-EP can be sufficiently cured by DDM at a slower rate than DGEBA. By applying an autocatalytic reaction model, we adequately simulate the curing rate of DEU-EP/DDM, and reveal its detailed kinetic mechanisms from model-free isoconversional analysis. Dynamic mechanical analysis shows that DEU-EP/DDM takes higher storage modulus up to ~97 o C than DGEBA/DDM with the glass temperature of 114 o C. Nanoindentation and thermogravimetric analysis demonstrate that compared with DGEBA/DDM, DEU-EP/DDM exhibits a 20%, 6.7% and 111% increase in Young's modulus, hardness and char yield, respectively. Microscale combustion calorimetry data show that DEU-EP/DDM expresses 55% and 38% lower heat release rate and total heat release than DGEBA/DDM, respectively.Macroscopically, the horizontal burning test approves DEU-EP/DDM can self-extinguish in a short time. Our results demonstrate that the eugenol building blocks and their arrangement greatly affect the cure behaviors of DEU-EP/DDM, and contribute significantly to its enhanced mechanical properties, high-temperature charring ability and chain motions at glass state, as well as the reduced flammability. To summarize, DEU-EP exhibits a high promise as a new sustainable epoxy monomer for fabricating high biobased content, high rigid and low flammable epoxy materials.

A novel bio-epoxy resin, TPEU-EP, was developed. It possesses good intrinsic flame retardancy, low smoke production, and excellent mechanical properties, showing high promise for application.

Harvesting biobased epoxy resins with improved thermomechanical properties (e.g., glass transition temperature T g and storage modulus), mechanical and dielectric similar and even superior to that of bisphenol A epoxy resin (DGEBA) is vital to many applications, yet remains a substantial challenge. Here we develop a novel eugenol-based epoxy monomer (TEU-EP) with a branched topology and a very rich biobased retention (80 wt %). TEU-EP can be well cured by 3,3′-diaminodiphenyl sulfone (33DDS) and the resultant TEU-EP/33DDS system can be considered as a “single” epoxy component, exhibiting adequate reactivity at high processing temperatures. Importantly, compared with DGEBA/33DDS, TEU-EP/33DDS achieves a 33 °C, 39% and 55% increment in the glass transition temperature, Young’s modulus, and hardness, respectively, and shows the improved creep resistance and dimensional stability. TEU-EP/33DDS is also characterized by the considerably reduced permittivity, dielectric loss factor, and flammability with high yield of pyrolytic residual. Overall, TEU-EP endows the cured epoxy with a number of the distinguished properties outperforming its DGEBA counterpart, and therefore may find practical applications in demanding and even cutting-edge areas.

No abstract

Growth of multilayer or superlattice thin films has shown various degrees of hardness enhancements often exceeding the individual hardness of the materials involved. Typically the hardness increases with decreasing wavelength until a maximum value is reached in the nm range, after which the hardness decreases with further decrease in wavelength 1,2 . Different theories have been developed in order to explain the observed increase in hardness. Koehler 3 showed theoretically a hardness increase for materials with a lamellar structure. This increase arises from image forces on dislocations due to a shear modulus difference between the layers. Other theories that tries to explain the observed increase in hardness are, coherency stress hardening 4-6 , where dislocation movement is restricted by the stress fields present at coherent interfaces within the multilayer. Also, the epitaxial stabilization effect has been demonstrated 7 , where a metastable structure for one of the layer materials is formed by pseudomorphic forces to the surface of the other layer during nucleation and growth thus creating a coherent interface, e.g., a normally amorphous material assuming crystalline structure for small layer thicknesses. For Orowan-like strengthening 8-11 , plastic deformation occurs by dislocation movement and bowing inside layers. Finally, in the case of Hall-Petch strengthening 12-13 hardness increases due to a reduction in grain size and thereby an increase in grain boundary density, grain boundaries which acts as dislocation obstacles.Multilayer thin films consisting of titanium nitride (TiN) and silicon nitride (Si 3 N 4 ) layers with compositional modulation periodicities between 3.7 and 101.7 nm have been grown on silicon wafers using reactive magnetron sputtering. Electron microscopy and X-ray diffraction studies showed that the layering is flat with distinct interfaces. According to the XRD studies (figure 1), the deposited TiN layers were crystalline and exhibited a preferred 002 orientation for layer thicknesses of 4.5 nm and below. For larger TiN layer thicknesses, a mixed 111/002 preferred orientation was present as the competitive growth favored 111 texture in monolithic TiN films. The TEM studies (figure 2) revealed that the Si 3 N 4 layers exhibited amorphous structure for layer thicknesses ≥ 0.8 nm, however, for the first time cubic crystalline silicon nitride phase was observed for layer thicknesses ≤ 0.3 nm. Formation of this metastable SiN x phase is explained by epitaxial stabilization to TiN. The microstructure of the multilayers displayed columnar growth within the TiN layers with intermittent TiN renucleation after each Si 3 N 4 layer. A nano-brick-wall structure was thus demonstrated over a range of periodicities. As-deposited films exhibited relatively constant residual stress levels of 1.3±0.7 GPa (compressive) independent of the layering. Nanoindentation was used to determine the hardness of the films, and the measurements showed an increase in hardness for the multilayered films compared to the ...

No abstract

The formation of cubic-phase SiNx is demonstrated in TiN∕SiNx multilayers deposited by reactive dual magnetron sputtering. Transmission electron microscopy examination shows a transition from epitaxially stabilized growth of crystalline SiNx to amorphous growth as the layer thickness increases from 0.3to0.8nm. The observations are supported by ab initio calculations on different polytypes, which show that the NaCl structure has the best lattice match to TiN. Calculations also reveal a large difference in elastic shear modulus between NaCl–SiNx and TiN. The results for phase structure and shear modulus offer an explanation for the superhardening effect determined by nanoindentation experiments.