Bidirectional light-emitting diode as a visible light source driven by alternating current.

Gallium nitride-based light-emitting diodes have revolutionized the lighting market by becoming the most energy-efficient light sources. However, the power grid, in example electricity delivery system, is built based on alternating current, which raises problems for directly driving light emitting diodes that require direct current to operate effectively. In this paper, we demonstrate a proof-of-concept device that addresses this fundamental issue - a gallium nitride-based bidirectional light-emitting diode. Its structure is symmetrical with respect to the active region, which, depending on the positive or negative bias, allows for the injection of either electrons or holes from each side. It is composed of two tunnel junctions that surround the active region. In this work, the optical and electrical properties of bidirectional light emitting diodes are investigated under direct and alternating current conditions. We find that the light is emitted in both directions of the supplied current, contrary to conventional light emitting diodes; hence, bidirectional light-emitting diodes can be considered a semiconductor light source powered directly with alternating current. In addition, we show that bidirectional light-emitting diodes can be stacked vertically to multiply the optical power achieved from a single device.

Investigation of MXenes Oxidation Process during SPS Method Annealing.

This paper discusses the effects of the environment and temperature of the Ti3C2 (MXene) oxidation process. The MXene powders were annealed at temperatures of 1000, 1200, 1400, 1600, and 1800 °C in argon and vacuum using a Spark Plasma Sintering (SPS) furnace. The purpose of the applied annealing method was to determine the influence of a high heating rate on the MXene degradation scheme. Additionally, to determine the thermal stability of MXene during the sintering of SiC matrix composites, SiC-C-B-Ti3C2 powder mixtures were also annealed. The process parameters were as follows: Temperatures of 1400 and 1600 °C, and pressure of 30 MPa in a vacuum. Observations of the microstructure showed that, due to annealing of the SiC-C-B-Ti3C2 powder mixtures, porous particles are formed consisting of TiC, Ti3C2sym, and amorphous carbon. The formation of porous particles is a transitional stage in the formation of disordered carbon structures.

Role of Metal Vacancies in the Mechanism of Thermal Degradation of InGaN Quantum Wells.

In this work, we study the thermal degradation of In-rich InxGa1-xN quantum wells (QWs) and propose explanation of its origin based on the diffusion of metal vacancies. The structural transformation of the InxGa1-xN QWs is initiated by the formation of small initial voids created due to agglomeration of metal vacancies diffusing from the layers beneath the QW. The presence of voids in the QW relaxes the mismatch stress in the vicinity of the void and drives In atoms to diffuse to the relaxed void surroundings. The void walls enriched in In atoms are prone for thermal decomposition, what leads to a subsequent disintegration of the surrounding lattice. The phases observed in the degraded areas of QWs contain voids partly filled with crystalline In and amorphous material, surrounded by the rim of high In-content InxGa1-xN or pure InN; the remaining QW between the voids contains residual amount of In. In the case of the InxGa1-xN QWs deposited on the GaN layer doped to n-type or on unintentionally doped GaN, we observe a preferential degradation of the first grown QW, while doping of the underlying GaN layer with Mg prevents the degradation of the closest InxGa1-xN QW. The reduction in the metal vacancy concentration in the InxGa1-xN QWs and their surroundings is crucial for making them more resistant to thermal degradation.

Influence of Ti3C2Tx MXene and Surface-Modified Ti3C2Tx MXene Addition on Microstructure and Mechanical Properties of Silicon Carbide Composites Sintered via Spark Plasma Sintering Method.

This article presents new findings related to the problem of the introduction of MXene phases into the silicon carbide matrix. The addition of MXene phases, as shown by the latest research, can significantly improve the mechanical properties of silicon carbide, including fracture toughness. Low fracture toughness is one of the main disadvantages that significantly limit its use. As a part of the experiment, two series of composites were produced with the addition of 2D-Ti3C2Tx MXene and 2D-Ti3C2Tx surface-modified MXene with the use of the sol-gel method with a mixture of Y2O3/Al2O3 oxides. The composites were obtained with the powder metallurgy technique and sintered with the Spark Plasma Sintering method at 1900 °C. The effect adding MXene phases had on the mechanical properties and microstructure of the produced sinters was investigated. Moreover, the influence of the performed surface modification on changes in the properties of the produced composites was determined. The analysis of the obtained results showed that during sintering, the MXene phases oxidize with the formation of carbon flakes playing the role of reinforcement. The influence of the Y2O3/Al2O3 layer on the structure of carbon flakes and the higher quality of the interface was also demonstrated. This was reflected in the higher mechanical properties of composites with the addition of modified Ti3C2Tx. Composites with 1 wt.% addition of Ti3C2Tx M are characterized with a fracture toughness of 5 MPa × m0.5, which is over 50% higher than in the case of the reference sample and over 15% higher than for the composite with 2.5 wt.% addition of Ti3C2Tx, which showed the highest fracture toughness in this series.

The impact of point defects in n-type GaN layers on thermal decomposition of InGaN/GaN QWs.

The aim of this paper is to give an experimental evidence that point defects (most probably gallium vacancies) induce decomposition of InGaN quantum wells (QWs) at high temperatures. In the experiment performed, we implanted GaN:Si/sapphire substrates with helium ions in order to introduce a high density of point defects. Then, we grew InGaN QWs on such substrates at temperature of 730 °C, what caused elimination of most (but not all) of the implantation-induced point defects expanding the crystal lattice. The InGaN QWs were almost identical to those grown on unimplanted GaN substrates. In the next step of the experiment, we annealed samples grown on unimplanted and implanted GaN at temperatures of 900 °C, 920 °C and 940 °C for half an hour. The samples were examined using Photoluminescence, X-ray Diffraction and Transmission Electron Microscopy. We found out that the decomposition of InGaN QWs started at lower temperatures for the samples grown on the implanted GaN substrates what provides a strong experimental support that point defects play important role in InGaN decomposition at high temperatures.

Microstructure and Mechanical Properties of Alumina Composites with Addition of Structurally Modified 2D Ti3C2 (MXene) Phase.

This study presents new findings related to the incorporation of MXene phases into ceramic. Aluminium oxide and synthesised Ti3C2 were utilised as starting materials. Knowing the tendency of MXenes to oxidation and degradation, particularly at higher temperatures, structural modifications were proposed. They consisted of creating the metallic layer on the Ti3C2, by sputtering the titanium or molybdenum. To prepare the composites, powder metallurgy and spark plasma sintering (SPS) techniques were adopted. In order to evaluate the effectiveness of the applied modifications, the emphasis of the research was placed on microstructural analysis. In addition, the mechanical properties of the obtained sinters were examined. Observations revealed significant changes in the MXenes degradation process, from porous areas with TiC particles (for unmodified Ti3C2), to in situ creation of graphitic carbon (in the case of Ti3C2-Ti/Mo). Moreover, the fracture changed from purely intergranular to cracking with high participation of transgranular mode, analogously. In addition, the results obtained showed an improvement in the mechanical properties for composites with Ti/Mo modifications (an increase of 10% and 15% in hardness and fracture toughness respectively, for specimens with 0.5 wt.% Ti3C2-Mo). For unmodified Ti3C2, enormously cracked areas with spatters emerged during tests, making the measurements impossible to perform.

Influence of MXene (Ti3C2) Phase Addition on the Microstructure and Mechanical Properties of Silicon Nitride Ceramics.

This paper discusses the influence of Ti3C2 (MXene) addition on silicon nitride and its impact on the microstructure and mechanical properties of the latter. Composites were prepared through powder processing and sintered using the spark plasma sintering (SPS) technic. Relative density, hardness and fracture toughness, were analyzed. The highest fracture toughness at 5.3 MPa·m1/2 and the highest hardness at HV5 2217 were achieved for 0.7 and 2 wt.% Ti3C2, respectively. Moreover, the formation of the Si2N2O phase was observed as a result of both the MXene addition and the preservation of the α-Si3N4→ß-Si3N4 phase transformation during the sintering process.