Extrusion and characterization of aluminum/graphene composites

Since its first synthesis in 2004 graphene was characterized intensively and exceptional properties in terms of e.g. mechanical strength, stiffness and electrical as well as thermal conductivity were revealed. These properties make graphene very attractive to be applied as additive in composite materials e.g. to increase strength and conductivity compared to the pure matrix material. In this study graphene nano platelets (GNP) in contents of 0. 5%, 1.0 % and 1.5 % were added to pure (99.7 %) aluminum powder and dispersed via EIRICH mixer method. This method is very appealing since homogenous mixtures can be achieved in significantly lower time when compared to e.g. the ball milling process. After subsequent cold compaction the composite materials were extruded with three different extrusion ratios. The influence of GNP content and extrusion ratio on the specific extrusion pressure is characterized as well the resulting rod surface quality, respectively. The effects of GNP content and extrusion ratio on homogeneity of graphene dispersion in the aluminum matrix, the relative density of the composite as well as hardness were also investigated.

Pitting Corrosion of Type 316L Stainless Steel Elaborated by the Selective Laser Melting Method: Influence of Microstructure

The microstructure of two sets of 316L alloys (SLM and wrought structure) is determined using SPECTROMAXx stationary metal analyzer, FE-SEM/EDS and XRD. The physical–chemical properties of the passive films are also investigated by means of XPS, Auger after sputtering and electrochemical impedance spectroscopy measurements. Differences and similarities between the two sets of alloys are then identified. The corrosion behavior of alloys is investigated in NaCl solution at the macro- and microscale (microcapillary technique). It was found that the inclusion/particles cleanliness is the first-order parameter explaining differences between 316L(WS) and 316L(SLM). In the absence of particles (pure matrix), the two sets of alloys have the same corrosion behavior. Residual stresses, the average grain size, the PREN and the passive films properties are of second order.

Optimization of Mechanical Properties and Damage Tolerance in Polymer-Mineral Multilayer Composites

Talcum reinforced polypropylene was enhanced with a soft type of polypropylene in order to increase the impact strength and damage tolerance of the material. The soft phase was incorporated in the form of continuous interlayers, where the numbers of layers ranged from 64 to 2048. A blend with the same material composition (based on wt% of the used materials) and the pure matrix material were investigated for comparison. A plateau in impact strength was reached by layered architectures, where the matrix layer thickness was as small or smaller than the largest talcum particles. The most promising layered architecture, namely, 512 layers, was subsequently investigated more thoroughly using instrumented Charpy experiments and tensile testing. In these tests, normalised parameters for stiffness and strength were obtained in addition to the impact strength. The multilayered material showed remarkable impact strength, fracture energy and damage tolerance. However, stiffness and strength were reduced due to the addition of the soft phase. It could be shown that specimens under bending loads are very compliant due to a stress-decoupling effect between layers that specifically reduces bending stiffness. This drawback could be avoided under tensile loading, while the increase in toughness remained high.

Fly-Ash-Based Geopolymers Reinforced by Melamine Fibers

This paper presents the results of research on geopolymer composites based on fly ash with the addition of melamine fibers in amounts of 0.5%, 1% and 2% by weight and, for comparison, without the addition of fibers. The melamine fibers used in the tests retain their melamine resin properties by 100% and are characterized by excellent acoustic and thermal insulation as well as excellent filtration. In addition, these fibers are nonflammable, resistant to chemicals, resistant to UV radiation, characterized by high temperature resistance and, most importantly, do not show thermal-related shrinking, melting and dripping. This paper presents the results of density measurements, compressive and flexural strength as well as the results of the measurement of thermal radiation changes in samples subjected to a temperature of 600 °C. The results indicate that melamine fibers can be used as geopolymer reinforcement. The best result was achieved for 0.5% by weight amount of reinforcement, approximately 53 MPa, compared to 41 MPa for a pure matrix. In the case of flexural strength, the best results were obtained for the samples made of unreinforced geopolymer and samples with the addition of 0.5% by weight of melamine fibers, which were characterized by bending strength values above 9 MPa, amounting to 10.7 MPa and 9.3 MPa, respectively. The thermal radiation measurements and fire-jet test did not confirm the increasing thermal and fire resistance of the composites reinforced by melamine fiber.

Polypropylene/Basalt Fabric Laminates: Flexural Properties and Impact Damage Behavior

Recently, the growing interests into the environmental matter are driving the research interest to the development of new eco-sustainable composite materials toward the replacement of synthetic reinforcing fibers with natural ones and exploiting the intrinsic recyclability of thermoplastic resins even for uses in which thermosetting matrices are well consolidated (e.g., naval and aeronautical fields). In this work, polypropylene/basalt fabric composite samples were prepared by film stacking and compression molding procedures. They have been studied in terms of flexural and low-velocity impact behavior. The influence related to the matrix modification with a pre-optimized amount of maleic anhydride grafted PP as coupling agent was studied. The mechanical performances of the composite systems were compared with those of laminates consisting of the pure matrix and obtained by hot-pressing of PP pellets and PP films used in the stacking procedure. Results, on one side, demonstrated a slight reduction of both static and dynamic parameters at the break for specimens from superimposed films to ones prepared from PP pellets. Moreover, an outstanding improvement of mechanical performances was shown in the presence of basalt layers, especially for compatibilized samples.

On Jacob's construction of the rational canonical form of a matrix

H.G. Jacob's elegant approach to the rational canonical, or Frobenius normal form of a linear map is presented here in pure matrix language, thereby avoiding the abstract machinery and prerequisites in the original paper. Related algorithmic aspects and an efficient implementation in the computer algebra system GAP are also discussed.

Symmetries of Relativistic Hydrogen Atom

The Dirac equation in the external Coulomb field is proved to possess the symmetry determined by 31 operators, which form the 31-dimensional algebra. Two different fermionic realizations of the SO(1,3) algebra of the Lorentz group are found. Two different bosonic realizations of this algebra are found as well. All generators of the above-mentioned algebras commute with the operator of the Dirac equation in an external Coulomb field and, therefore, determine the algebras of invariance of such Dirac equation. Hence, the spin s = (1, 0) Bose symmetry of the Dirac equation for the free spinor field, proved recently in our papers, is extended here for the Dirac equation interacting with an external Coulomb field. A relativistic hydrogen atom is modeled by such Dirac equation. We are able to prove for the relativistic hydrogen atom both the fermionic and bosonic symmetries known from our papers in the case of a non-interacting spinor field. New symmetry operators are found on the basis of new gamma matrix representations of the Clifford and SO(8) algebras, which are known from our recent papers as well. Hidden symmetries were found both in the canonical Foldy–Wouthuysen and covariant Dirac representations. The found symmetry operators, which are pure matrix ones in the Foldy–Wouthuysen representation, become non-local in the Dirac model.

Incorporating side information into Robust Matrix Factorization with Quantile Random Forest under Bayesian framework (preprint)

Matrix Factorization is a widely used technique for modeling pairwise and matrix-like data. It is frequently used in pattern recognition, topic analysis and other areas. Side information is often available, however utilization of this additional information is problematic in the pure matrix factorization framework. This article proposes a novel method of utilizing side information by combining arbitrary nonlinear Quantile Regression model and Matrix Factorization under Bayesian framework. Gradient-free optimization procedure with the novel Surrogate Function is used to solve the resulting MAP estimator. The model performance has been evaluated on real data-sets.

A Case of Pure Matrix Ureteral Stone: A rare type of urinary calculi that may be overlooked

Matrix stones are a rare form of urinary calculi with a low mineral content. We report a 63-year-old female patient who presented to the Sohar Hospital, Sohar, Oman, in 2018 with unexplained left flank pain and constipation. She had a history of chronic renal failure (CRF) and had previously undergone haemodialysis (HD). Non-contrast computed tomography (CT) did not show any renal or urinary stones. However, a left-sided ureteroscopy revealed a yellow-coloured stone that was soft in consistency occupying the proximal 5 cm section of the ureter. Many attempts at forceps extraction were required for complete clearance. A global quantitative composition analysis revealed the extracted stone to be composed entirely of protein-matrix material. Matrix stones require a high index of suspicion as they are sometimes radiolucent and cannot be visualised on CT scans. Risk factors include being female and a history of urinary tract infections, CRF and HD.Keywords: Urolithiasis; Ureteral Calculi; Diagnostic Imaging; Ureteroscopy; Case Report; Oman.

Preparation and Properties of Si3N4-BaTiO3 Composite Ceramics

Si3N4-BaTiO3 composite ceramics, a type of high dielectric materials applied in the multifunction radome, were prepared by gas pressure-sintering method. The influences of BaTiO3 power content on the mechanical performances, dielectric properties and microstructure of Si3N4-BaTiO3 composite ceramics were investigated. The results showed that the sintering density, the elastic modulus and the flexural strength of Si3N4-BaTiO3 composite ceramics all firstly increased and then decreased along with the increase of BaTiO3 power in sample. Meanwhile, Relationship between BaTiO3 power content and dielectric properties of Si3N4-BaTiO3 composite ceramics within the frequency range of 8.2–12.4 GHz (X-band) was studied. The average of dielectric constant e of Si3N4-BaTiO3 composite ceramics increased from 7.33 to 9.98 and the average of dielectric loss tand increased from 2.8 ´ 10-3 to 0.0168 when the content of BaTiO3 power increased from 0 to 25 wt.%. The increase of the dielectric properties of Si3N4–BaTiO3 composite ceramics were attributed to the electronic and molecular polarization at interface between Si3N4 and BaTiO3, compared with the pure matrix Si3N4.