The relationship between fracture toughness and Yttria content in modern zirconia ceramics was revised. For that purpose, we evaluated here 10 modern Y2O3-stabilized zirconia (YSZ) materials ...currently used in biomedical applications, namely prosthetic and implant dentistry. The most relevant range between 2-5 mol% Y2O3 was addressed by selecting from conventional opaque 3 mol% YSZ up to more translucent compositions (4−5 mol% YSZs). A technical 2YSZ was used to extend the range of our evaluation. The bulk mol% Y2O3 concentration was measured by X-Ray Fluorescence Spectroscopy. Phase quantification by Rietveld refinement considered two tetragonal phases or an additional cubic phase. A first-account of the fracture toughness (KIc) of the pre-sintered blocks is given, which amounted to 0.4 – 0.7 MPa√m. In the fully-densified state, an inverse power-law behavior was obtained between KIc and bulk mol% Y2O3 content, whether using only our measurements or including literature data, challenging some established relationships. A linear relationship between KIc and the fraction of the transformable t-phase was established within the range of 30–70 vol%.
Lithium disilicate glass-ceramics show great potential in the fabrication of anisotropic structures due to their high-strength needle-form crystalline phase subjective to processing-induced ...alignment. Here we explore this strategy by focusing on the mechanistic aspects of fracture. Through different variations of the fracture toughness test we demonstrate the effect of bulk crystal orientation on fracture energy, toughening mechanisms, crystal size and aspect ratio. Using the anisotropic Poisson's ratio of the Li2Si2O5 crystal phase obtained by Resonant Ultrasound Spectroscopy, we apply the new geometry factor developed by Strobl et al. to provide a more accurate insight into the anisotropic crack propagation behavior in LS2 glass-ceramics. Raman spectra and X-Ray Diffraction patterns are resolved for the Li2Si2O5 phase aligned at plane parallel, anti-plane and random orientations.
We show that Li2Si2O5 crystallites oriented perpendicular to the crack growth plane tended to induce large scale crack deflection unless the crack was forced into a straight path, thereby promoting crystallite fracture, increased KIc-values and accentuated R-curve behavior. Crystal aspect ratio and residual stresses in the glass have been identified as important influencing factors on crack growth behavior and toughening mechanisms.
•Lithium disilicate (LS2) glass-ceramics were used to demonstrate the effect of crystal alignment on mode-I fracture toughness;•Li2Si2O5 crystallites were aligned in different orientations to the crack plane by pressing the LS2 melt using two different pressing strategies;•The use of a variety of specimen geometries and loading conditions allowed an insight into their R-curve behaviors;•The effect of residual stresses in the glass and the crystallite aspect ratio was determined;•Crystallites oriented perpendicular to the crack plane showed increased fracture resistance.
In this study, two biodegradable Mg-Zn-Ca alloys with alloy content of less than 1 wt % were strengthened via high pressure torsion (HPT). A subsequent heat treatment at temperatures of around 0.45
...led to an additional, sometimes even larger increase in both hardness and tensile strength. A hardness of more than 110 HV and tensile strength of more than 300 MPa were achieved in Mg-0.2Zn-0.5Ca by this procedure. Microstructural analyses were conducted by scanning and transmission electron microscopy (SEM and TEM, respectively) and atom probe tomography (APT) to reveal the origin of this strength increase. They indicated a grain size in the sub-micron range, Ca-rich precipitates, and segregation of the alloying elements at the grain boundaries after HPT-processing. While the grain size and segregation remained mostly unchanged during the heat treatment, the size and density of the precipitates increased slightly. However, estimates with an Orowan-type equation showed that precipitation hardening cannot account for the strength increase observed. Instead, the high concentration of vacancies after HPT-processing is thought to lead to the formation of vacancy agglomerates and dislocation loops in the basal plane, where they represent particularly strong obstacles to dislocation movement, thus, accounting for the considerable strength increase observed. This idea is substantiated by theoretical considerations and quenching experiments, which also show an increase in hardness when the same heat treatment is applied.
Mechanical properties and microstructure were compared for zirconium diboride and two zirconium diboride solid solutions containing 3 and 6 at% tantalum diboride. X-ray diffraction indicated that the ...ceramics were nearly phase-pure and that tantalum dissolved into the ZrB2 lattice to form (Zr,Ta)B2 solid solutions. Microstructural analysis indicated that samples achieved nearly full relative density with average grain sizes that ranged from 3−5 μm. The three compositions had similar values of Young’s modulus (510−531 GPa), shear modulus (225−228 GPa), Vickers hardness (15.2–16.4 GPa), and flexural strength (391−452 MPa). Fracture toughness ranged from 2.6 to 3.7 MPa m1/2 and with increasing tantalum content, the fracture mode changed from predominantly intergranular to predominantly transgranular. Diboride solid solution materials had comparable properties to the single metal diboride, but differences in microstructure, secondary phases, and strain state among the three ceramics partially obscured the actual effects of the solid solution on fracture behavior.
The present study reports the structural, electronic, elastic, and thermoelectric properties of Cu12Sb3.9Te0.1S13-xSex (x = 0, 0.1, 0.5, 0.75 and 1) tetrahedrite. The thermal conductivity was reduced ...due to charge carrier compensation (by Te), weakening of the Sb-S bond and local chemical disorder (induced by Se), resulting in a low mean sound velocity. Simultaneously, the power-factor was maintained high from an enhancement of the density of states near the Fermi level due to Se. Consequently, a maximum figure of merit ~0.84 at 673 K was achieved for the Cu12Sb3.9Te0.1S12.5Se0.5 sample due to concomitant optimisation of the power-factor and thermal conductivity.
Display omitted
Microscale and nanoscale mechanical resonators have recently emerged as ubiquitous devices for use in advanced technological applications, for example, in mobile communications and inertial sensors, ...and as novel tools for fundamental scientific endeavours. Their performance is in many cases limited by the deleterious effects of mechanical damping. In this study, we report a significant advancement towards understanding and controlling support-induced losses in generic mechanical resonators. We begin by introducing an efficient numerical solver, based on the 'phonon-tunnelling' approach, capable of predicting the design-limited damping of high-quality mechanical resonators. Further, through careful device engineering, we isolate support-induced losses and perform a rigorous experimental test of the strong geometric dependence of this loss mechanism. Our results are in excellent agreement with the theory, demonstrating the predictive power of our approach. In combination with recent progress on complementary dissipation mechanisms, our phonon-tunnelling solver represents a major step towards accurate prediction of the mechanical quality factor.
Covalently functionalized graphene derivatives were synthesized via benchmark reductive routes using graphite intercalation compounds (GICs), in particular KC8. We have compared the graphene ...arylation and alkylation of the GIC using 4-tert-butylphenyldiazonium and bis(4-(tert-butyl)phenyl)iodonium salts, as well as phenyl iodide, n-hexyl iodide, and n-dodecyl iodide, as electrophiles in model reactions. We have put a particular focus on the evaluation of the degree of addition and the bulk functionalization homogeneity (H bulk). For this purpose, we have employed statistical Raman spectroscopy (SRS), and a forefront characterization tool using thermogravimetric analysis coupled with FT-IR, gas chromatography, and mass spectrometry (TGA/FT-IR/GC/MS). The present study unambiguously shows that the graphene functionalization using alkyl iodides leads to the best results, in terms of both the degree of addition and the H bulk. Moreover, we have identified the reversible character of the covalent addition chemistry, even at temperatures below 200 °C. The thermally induced addend cleavage proceeds homolytically, which allows for the detection of dimeric cleavage products by TGA/FT-IR/GC/MS. This dimerization points to a certain degree of regioselectivity, leading to a low sheet homogeneity (H sheet). Finally, we developed this concept by performing the reductive alkylation reaction in monolayer CVD graphene films. This work provides important insights into the understanding of basic principles of reductive graphene functionalization and will serve as a guide in the design of new graphene functionalization concepts.
Black phosphorus intercalation compounds (BPICs) with alkali metals (namely: K and Na) have been synthesized in bulk by solid‐state as well as vapor‐phase reactions. By means of a combination of in ...situ X‐ray diffraction, Raman spectroscopy, and DFT calculations the structural behavior of the BPICs at different intercalation stages has been demonstrated for the first time. Our results provide a glimpse into the very first steps of a new family of intercalation compounds, with a distinct behavior as compared to its graphite analogues (GICs), showing a remarkable structural complexity and a dynamic behavior.
Black to the future: The bulk intercalation of black phosphorus with alkali metals (namely: K and Na; black spheres) has been developed, showing a complex dynamic structural behavior. By in situ Raman spectroscopy and DFT calculations straightforward fingerprints for the identification of black phosphorus intercalation compounds (BPICs) have been demonstrated.
The thermoelectric efficiency of skutterudite materials can be improved by lowering the lattice thermal conductivity via the uniform dispersion of a nanosized second phase in the matrix of filled ...Co4Sb12. In this work, nanocomposites of Ba0.3Co4Sb12 and InSb were synthesized using ball-milling and spark plasma sintering. The thermoelectric transport properties were studied from 4.2 to 773 K. The InSb nanoparticles of ∼20 nm were found to be dispersed in the Ba0.3Co4Sb12 grains with a few larger grains of about 10 μm due to the agglomeration of the InSb nanoparticles. The +2 oxidation state of Ba in Co4Sb12 resulted in a low electrical resistivity, ρ, value of the matrix. The enhancement of the Seebeck coefficient, S, and the electrical resistivity values of Ba0.3Co4Sb12 with the addition of InSb can be credited to the energy-filtering effect of electrons with low energy at the interfaces. The power factor of the composites could not be enhanced compared to the matrix because of the very high ρ value. A minimum possible lattice thermal conductivity (0.45 W/m·K at 773 K) was achieved due to the combined effect of rattling of Ba atoms in the voids and enhanced phonon scattering at the interfaces induced by nanosized InSb particles. As a result, the (InSb)0.15 + Ba0.3Co4Sb12 composite exhibited improved thermoelectric properties with the highest zT of 1.4 at 773 K and improved mechanical properties with a higher hardness, higher Young’s modulus, and lower brittleness.
Herein we report the synthesis of covalently functionalized carbon nano-onions (CNOs) via a reductive approach using unprecedented alkali-metal CNO intercalation compounds. For the first time, an in ...situ Raman study of the controlled intercalation process with potassium has been carried out revealing a Fano resonance in highly doped CNOs. The intercalation was further confirmed by electron energy loss spectroscopy and X-ray diffraction. Moreover, the experimental results have been rationalized with DFT calculations. Covalently functionalized CNO derivatives were synthesized by using phenyl iodide and n-hexyl iodide as electrophiles in model nucleophilic substitution reactions. The functionalized CNOs were exhaustively characterized by statistical Raman spectroscopy, thermogravimetric analysis coupled with gas chromatography and mass spectrometry, dynamic light scattering, UV–vis, and ATR-FTIR spectroscopies. This work provides important insights into the understanding of the basic principles of reductive CNOs functionalization and will pave the way for the use of CNOs in a wide range of potential applications, such as energy storage, photovoltaics, or molecular electronics.
PDF
