High-temperature strength behavior of tantalum diboride to 2000°C

This study investigated the high-temperature strength of spark plasma sintered tantalum diboride (TaB₂) for the first time. TaB₂ exhibited a unique elastic fracture behavior below 1900°C, unlike other transition metal diborides. The consolidation process involved spark plasma sintering at 2000–2200°C yielding dense TaB₂ samples. The flexural strength was measured at elevated temperatures up to 2000°C, showing a quite high flexural strength of 400 ± 20 MPa at 1900°C. These findings provide valuable insights into the high-temperature behavior of TaB₂, highlighting its potential for advanced applications.     

Effect of terbium gallium garnet crystal orientation on the isolation ratio of a Faraday isolator at high average power

We present a comprehensive and systematic investigation of the fundamental physical limitations of Faraday isolation performance at high average powers that are due to thermally induced birefringence. First, the operation of various Faraday isolator designs by use of arbitrary orientation of cubic magneto-optic crystals is studied theoretically. It is shown that, for different Faraday isolator designs, different crystal orientations can optimize the isolation ratio. Second, thermo-optic and photoelastic constants for terbium gallium garnet crystals grown by different manufacturers were measured. Measurements of self-induced depolarization are made for various orientations of crystallographic axes. The measurements are in good agreement with our theoretical predictions. Based on our results, it is possible to select a crystal orientation that optimizes isolation performance at high average powers, resulting in a 5-dB enhancement over nonoptimized orientations.     

Peculiarities in thermal evolution of precipitated amorphous calcium phosphates with an initial Ca/P ratio of 1:1

Thermal evolution of amorphous calcium phosphate (ACP) powder from a fast nitrate synthesis with a Ca/P ratio of 1:1 were studied in the range of 20–980?°C. The powder consisted of amorphous dicalcium phosphate anhydrate (CaHPO₄) after heating to 200?°C. CaHPO₄ gradually condensed to amorphous calcium pyrophosphate Ca₂P₂O₇ (CPP) between 200 to 620?°C. Amorphous CPP crystallized at 620–740?°C to a metastable polymorph α′-CPP of the high-temperature phase α-CPP and β-CPP. The α′-CPP/ β-CPP phase ratio reached a maximum at 800?°C (60?wt% α′-CPP/40?wt% β-CPP), and α′-CPP gradually transformed to β-CPP at a higher temperature. Some β-TCP occurred at 900?°C, so that a three-phasic mixture was obtained in the powder heated to 980?°C. The occurrence of metastable α′-CPP is attributed to Ostwald’s step rule, and a mechanism for β-TCP formation is proposed. The advantages of prospective biomaterials from these powders are discussed.     

Formation of a Network Structure in the Gaseous Reduction of Magnetite Doped with Alumina

Reduction of un-doped magnetite is developed topochemically with the formation of a dense iron shell. However, the reduction of alumina-doped magnetite to wüstite proceeds with the formation of a network-like structure which consists of criss-crossed horizontal and vertical plates of wüstite. Reduction of magnetite includes the conversion of Fe³⁺ to Fe²⁺ and the movement of iron cations from the tetrahedral sites on the {400} and {220} planes of magnetite to the octahedral sites on the {200} planes of wüstite. Alumina has a negligibly small solubility in wüstite. In the reduction of magnetite doped with Al₂O₃, rejected Al³⁺ cations from wüstite diffuse to the magnetite–hercynite solid solution. Enrichment of the Fe₃O₄–FeAl₂O₄ solution with alumina in the vicinity of the reduction interface restricts the growth of {220} planes of wüstite and nucleation of {220} planes adjusted to the existing planes, preventing the merging of wüstite plates during the reduction process. Reduction of magnetite from the magnetite–hercynite solid solution practically stops when the Al³⁺ content at the interface approaches the solubility limit. Wüstite in the separated plates is reduced further to iron.     

Diffusion of clusters down (1 1 1) aluminum islands

The key factor determining nucleation processes and faceting in homoepitaxial growth as well as texture competition is the mobility of adatoms and small clusters across step edges and facets. Using a combination of molecular dynamics and ab initio calculations, we investigate the mechanisms of small clusters (dimer and trimer) diffusion down the aluminum (1 1 1) surface. In this paper we report results of molecular dynamics studies. Our study shows that the clusters dissociate at the step-edge of compact islands. As a result, the clusters diffuse down the step by an exchange mechanism with a small or medium Schwoebel barrier. The mechanism of this down-diffusion/dissociation is discussed and the corresponding energetics are calculated using the molecular statics method. We find a large anisotropy between the barriers at the two types of 1 1 0 oriented steps.     

Compact Modules for Wireless Communication Systems in the E-Band (71–76 GHz)

The millimeter-wave spectrum above 70 GHz provides a cost-effective solution to increase the wireless communications data rates by increasing the carrier wave frequencies. We report on the development of two key components of a wireless transmission system, a high-speed photodiode (HS-PD) and a Schottky Barrier Diode (SBD). Both components operate uncooled, a key issue in the development of compact modules. On the transmitter side, an improved design of the HS-PD allows it to deliver an output RF power exceeding 0 dBm (1 mW). On the receiver side, we present the design process and achieved results on the development of a compact direct envelope detection receiver based on a quasi-optical SDB module. Different resonant (meander dipole) and broadband (Log-Spiral and Log-Periodic) planar antenna solutions are designed, matching the antenna and Schottky diode impedances at high frequency. Impedance matching at baseband is also provided by means of an impedance transition to a 50 Ohm output. From this comparison, we demonstrate the excellent performance of the broadband antennas over the entire E-band by setting up a short-range wireless link transmitting a 1 Gbps data signal.     

The Meissner-Ochsenfeld effect as a possible tool to control anisotropy of thermal conductivity and pinning strength of type II superconductors

This paper proposes the Meissner-Ochsenfeld effect as a possible tool for quality control of type II superconductors (SC). Visual or optical inspection of the levitation process allows rough determination of the anisotropy of thermal conductivity and of the pinning strength of type II SC material, in a materials quality control. An assembly of permanent (e.g. NdFeB) or electromagnets and a flat cryostat allowing visual or optical inspection would be required. The method is demonstrated by numerically simulating the field cooling process of a superconducting cylindrical pellet and of a coated conductor.     

Shape optimization for drag reduction in linked bodies using evolution strategies

We present results from the shape optimization of linked bodies for drag reduction in simulations of incompressible flow at moderate Reynolds numbers. The optimization relies on the covariance matrix adaptation evolution strategy (CMA-ES) and the flow simulations use vortex methods with the Brinkman penalization to enforce boundary conditions in complex bodies. We exploit the inherent parallelism of CMA-ES, by implementing a multi-host framework which allows for the distribution of the expensive cost function evaluations across parallel architectures, without being limited to one computing facility. This study repeats in silico for the first time Ingo Rechenberg’s pioneering wind tunnel experiments for drag reduction that led to the inception of evolution strategies. The simulations confirm that the results of these experimental studies indicate a flat plate is not the optimal solution for drag reduction in linked bodies. We present the vorticity field of the flow and identify the governing mechanisms for this drag reduction by the slightly corrugated linked plate configuration.     

Surface characterisation of DC plasma electrolytic oxidation treated 6082 aluminium alloy: Effect of current density and electrolyte concentration

Plasma electrolytic oxidation (PEO) is a specialised but well-developed process which has found applications in aerospace, oil/gas, textile, chemical, electrical and biomedical sectors. A novel range of coatings having technologically attractive physical and chemical properties (e.g. wear- and corrosion-resistance) can be produced by suitable control of the electrolyte as well as electrical parameters of the PEO process. Oxide ceramic films, 3 to 40 μm thick, were produced on 6082 aluminium alloy by DC PEO using 5 to 20 A/dm² current density in KOH electrolyte with varied concentration (0.5 to 2.0 g/l). Phase analysis (composition and crystallite size) was carried out using X-ray diffraction and TEM techniques. Residual stresses associated with the crystalline coating phase (α-Al₂O₃) were evaluated using the X-ray diffraction Sin²ψ method. Nanoindentation studies were conducted to evaluate the hardness and elastic modulus. SEM, SPM and TEM techniques were utilised to study surface as well as cross-sectional morphology and nano features of the PEO coatings. Correlations between internal stress and coating thickness, surface morphology and phase composition are discussed. It was found that, depending on the current density and electrolyte concentration used, internal direct and shear stresses in DC PEO alumina coatings ranged from − 302 ± 19 MPa to − 714 ± 22 MPa and − 25 ± 12 MPa to − 345 ± 27 MPa, respectively. Regimes of PEO treatment favourable for the production of thicker coatings with minimal stress level, dense morphology and relatively high content of α-Al₂O₃ phase are identified.