Design of coupling structures for groove guide resonators

Three different coupling structures for a groove guide resonator operating at 10 GHz are investigated. First, propagatable modes are determined analytically. Then, the efficiency of the coupling structures is investigated by means of the method of moments and measurements. Finally, a groove guide resonator is designed and the resonance spectrum for each coupling structure is computed numerically, applying the finite-difference time-domain method. Numerical results are compared with experimental measurements. There is good agreement between the measured and simulated data.TÜBİTA

Lattice modification of ZrO2-δ and formation of rocksalt structure ZrO and Zr(O,N) after DC electrical loading of 3YSZ Author links open overlay panel

It is widely acknowledged that electrochemical reduction occurs during DC flash sintering (FS) of YSZ. In situ experiments have shown the emergence of new phases during FS that disappear again when the current is switched off. In addition, it has recently been reported that YSZ flash sintered in the absence of oxygen has a high electrical conductivity at room temperature. In this investigation electrical loading of YSZ was conducted in Ar and N2 and maintained as the furnace cooled down such that on removing the current, the sample was rapidly quenched, avoiding reoxidation or other diffusion-limited phase transformations. XRD showed that the tetragonality of the ZrO2-δ phase was diminished and the volume of the unit cell increased, the effect of the electrochemical reduction being similar to an increase in Y2O3 content. In addition, rocksalt structure Zr(O,N) with a lattice constant of 0.4580 nm was formed at the surface and retained at room temperature. This phase had a metallic lustre and electrical conductivity approaching those of metals. The results enable several recent results in the literature to be reconciled and the implications for flash sintering are briefly discussed

Electrochemical reduction‐induced phase transformation and lattice modification in cubic zirconia (8YSZ)

This study investigates the effects of electrical loading on cubic 8YSZ (8 mol% yttria‐stabilized zirconia) under different atmospheric conditions with a focus on electrical and structural changes, lattice modifications, and the formation of new phases. Upon applying an electric field during various atmospheric transitions from oxygen to air to argon or nitrogen, an increase in oxygen extraction and enhanced electrical conductivity was linked to the formation of an atmosphere‐dependent defective FCC rocksalt phase, ZrO in Ar, and Zr(O,N) in nitrogen, as confirmed by EDS, X‐ray diffraction, and Raman spectroscopy, with lattice parameters consistent at approximately 0.458 nm. The microstructure of the electrically loaded material showed elongated precipitates of this new phase, particularly along grain boundaries, which were a few tens to hundreds of nanometers in size. Furthermore, dopant segregation and microstructural instability due to this lattice modification and phase transformation were observed. Comparison to the behavior of tetragonal 3YSZ revealed significant differences between the two compositions. These findings highlight the role of electrochemical reduction in altering material properties, with potential implications for flash sintering and energy applications like solid oxide fuel and electrolyzer cells

Investigation about the electrochemical reduction in 3YSZ, related phase transition and consequences

In this research the electrochemical reduction of 3YSZ was investigated in various atmospheres with different oxygen partial pressures under an electric field of 25 V/cm at an environmental temperature of 800 {\deg}C. At a certain oxygen partial pressure insufficient incorporation of oxygen in the sample led to electrochemical reduction of YSZ which shows two clearly distinguishable states. First, greying of the material without a significant change in properties was detected which then transitioned into a second stage where a fundamental phase transition in the material happened within seconds from tetragonal 3YSZ towards FCC rocksalt ZrO or ZrON, dependent on the atmosphere. This phase transition is accompanied by blackening of the material, sudden increase in electrical conductivity, current concentration, and an obvious change in Raman spectrum.Comment: 17 pages, 5 figure

Plasma formation during flash sintering of boron carbide – part II: effect on sintering and optimisation of process variables

Field assisted sintering techniques, such as Flash Sintering (FS), allow rapid heating and densification of ceramics within seconds at significantly reduced environmental temperatures. FS of oxides has been extensively researched since 2010, whereas the potential densification of non-oxides such as carbides is rarely investigated. In the first part of this investigation the atmospheric breakdown and plasma formation under FS-conditions relevant to SiC and B4C was researched. In the present work, the FS of B4C was investigated with and without plasma formation for comparison, and the influence and optimum combination of the other main FS-parameters determined. The results showed that at a furnace temperature of 1500 °C, it is possible to densify B4C by DC FS with or without an initial plasma formation. However, though plasma formation did not itself lead to densification of the material, the plasma-induced removal of the B2O3-layer on the surface of the powder led to a significant improvement in subsequent densification during FS. The effect of the Ar-plasma generated in removing the surface oxide from B4C-particles was confirmed by SEM and XPS. Other process variables such as carbon additives, thermal insulation and hold time also significantly influenced the densification of B4C by FS. With the optimised conditions, 96 % dense B4C, with a grain size of 10 μm and a hardness of 31 GPa (HV1) was produced by pressureless FS in 5 min at a furnace temperature of 1500 °C

Plasma formation during flash sintering of boron carbide – part I: plasma characteristics

In so-called Field Assisted Sintering Techniques such as Spark Plasma Sintering (SPS) or Flash Sintering (FS), ceramic powder compacts are densified at high sample temperature in the presence of an electrical field. The formation of a plasma is often discussed without actual evidence for its appearance. In this work, which is the first of two parts, the mechanisms of plasma formation as well as its properties under FS conditions were investigated. Field induced breakdown of various atmospheres was conducted by a DC-field (up to 350 V/cm) at temperatures between 1000 and 1500 °C. Experiments were conducted both with and without a sample mounted between the electrodes. Alumina, boron carbide and silicon carbide in green or sintered form were used as the samples. The electrical and optical properties of the plasma were characterised, and the early stages were visualised by high-speed camera recordings. The results showed that the breakdown is caused by a flashover mechanism and the conductance of the plasma under the present conditions was of the same order as the conductivity of a boron or silicon carbide green body. Plasma formation did not lead to extensive sintering but the plasma was able to infiltrate the green body and cause transfer of matter from the interior to the surface and beyond. Based on the present results, it seems unlikely that a gas plasma is formed under the normal conditions of SPS or FS

Realization and analitical and numerical analysis of a groove type resonator at millimeter waves

Bir yarı-simetrik oyuklu dalga kılavuzu rezonatörü 20 – 40 GHz frekans bandında kullanılmak üzere tasarlanmış ve duraleminyum kullanılarak gerçeklenmiştir. Rezonatör, biri düz diğeri ise dairesel oyuk içeren iki plakadan oluşmuştur. Uygun hareketli bir destek sistemine monte edilen rezonatörün, gerekli ölçüm düzeni de hazırlanarak, frekans spektrumu gözlenmiş, rezonans frekansları ve kalite faktörü ölçülmüştür. Oyuklu rezonatör, hem analitik ve hem de sayısal yöntemler kullanılarak analiz edilmiştir. Kullanılan sayısal yöntem zamanda sonlu farklar (FDTD) yöntemidir. Ölçüm sonucu elde edilen rezonans frekansları, analitik ve sayısal yöntemlerden elde edilenlerle karşılaştırılmıştır. Anahtar Kelimeler: Oyuklu dalga kılavuzu, oyuklu rezonatör, FDTD, mikrodalga, transfer rezonans metodu.As an alternative to the classical waveguides, such as the H-waveguide and the rectangular waveguide, the Groove waveguide has been used at millimeter wavelengths for the last couple of decades. It has many advantages such as low-loss and higher power handling capacity. Here, in this study, a semi-symmetrical groove guide resonator based on groove waveguide structure is designed and fabricated. Instead of using the classical symmetrical shaped groove guide, a novel semi-symmetrical groove guide structure is presented for the first time. This semi-symmetrical structure was chosen because it is easier to fabricate and excite. The groove resonator which has one grooved plate and one plane plate, first analyzed analytically. Since exact solutions usually exist for idealized geometries, the closed resonator structure is taken into account first. The Green's function method is applied for different oriented line sources. Then the transverse resonance method is used for the open groove resonator. The numerical simulations are performed via the powerful time domain simulator FDTD (Finite Difference Time Domain). Both cartesian and cylindrical coordinate systems are used for FDTD calculations. Here, the purpose to use cylindrical coordinate is to calibrate cartesian coordinates for discretization. Then, a measurement setup which includes a movable support platform was established from 20 GHz to 40 GHz. The resonance frequencies and loaded quality factor are measured. Analitical and numerical results are compared with measurements and a good agreement was observed.Keywords: Groove waveguide, groove resonator, FDTD, microwave, transverse resonance method.

Geographical structure, narrow species ranges, and Cenozoic diversification in a pantropical clade of epiphyllous leafy liverworts

The evolutionary history and classification of epiphyllous cryptogams are still poorly known. Leptolejeunea is a largely epiphyllous pantropical liverwort genus with about 25 species characterized by deeply bilobed underleaves, elliptic to narrowly obovate leaf lobes, the presence of ocelli, and vegetative reproduction by cladia. Sequences of three chloroplast regions (rbcL, trnL-F, psbA) and the nuclear ribosomal ITS region were obtained for 66 accessions of Leptolejeunea and six outgroup species to explore the phylogeny, divergence times, and ancestral areas of this genus. The phylogeny was estimated using maximum-likelihood and Bayesian inference approaches, and divergence times were estimated with a Bayesian relaxed clock method. Leptolejeunea likely originated in Asia or the Neotropics within a time interval from the Early Eocene to the Late Cretaceous (67.9 Ma, 95% highest posterior density [HPD]: 47.9-93.7). Diversification of the crown group initiated in the Eocene or early Oligocene (38.4 Ma, 95% HPD: 27.2-52.6). Most species clades were established in the Miocene. Leptolejeunea epiphylla and L. schiffneri originated in Asia and colonized African islands during the Plio-Pleistocene. Accessions of supposedly pantropical species are placed in different main clades. Several monophyletic morphospecies exhibit considerable sequence variation related to a geographical pattern. The clear geographic structure of the Leptolejeunea crown group points to evolutionary processes including rare long-distance dispersal and subsequent speciation. Leptolejeunea may have benefitted from the large-scale distribution of humid tropical angiosperm forests in the Eocene

Comment on the letter of the Society of Vertebrate Paleontology (SVP) dated April 21, 2020 regarding 'Fossils from conflict zones and reproducibility of fossil‑based scientific data': Myanmar amber

Recently, the Society of Vertebrate Paleontology (SVP) has sent around a letter, dated 21st April, 2020 to more than 300 palaeontological journals, signed by the President, Vice President and a former President of the society (Rayfield et al. 2020). The signatories of this letter request significant changes to the common practices in palaeontology. With our present, multi-authored comment, we aim to argue why these suggestions will not lead to improvement of both practice and ethics of palaeontological research but, conversely, hamper its further development. Although we disagree with most contents of the SVP letter, we appreciate this initiative to discuss scientific practices and the underlying ethics. Here, we consider different aspects of the suggestions by Rayfield et al. (2020) in which we see weaknesses and dangers. It is our intent to compile views from many different fields of palaeontology, as our discipline is (and should remain) pluralistic. This contribution deals with the aspects concerning Myanmar amber. Reference is made to Haug et al. (2020a) for another comment on aspects concerning amateur palaeontologists/citizen scientists/private collectors