Highly Efficient Persistent Room-Temperature Phosphorescence from Heavy Atom-Free Molecules Triggered by Hidden Long Phosphorescent Antenna

Persistent (lifetime > 100 ms) room-temperature phosphorescence (pRTP) is important for state-of-the-art security and bioimaging applications. An unclear relationship between chromophores and physical parameters relating to pRTP has prevented obtaining an RTP yield of over 50% and a lifetime over 1 s. Here highly efficient pRTP is reported under ambient conditions from heavy atom-free chromophores. A heavy atom-free aromatic core substituted with a long-conjugated amino group considerably accelerates the phosphorescence rate independent of the intramolecular vibration-based nonradiative rate from the lowest excited triplet state. One of the designed heavy atom-free dopant chromophores presents an RTP yield of 50% with a lifetime of 1 s under ambient conditions. The afterglow brightness under strong excitation is at least 10⁴ times stronger than that of conventional long-persistent luminescence emitters. Here it is shown that highly efficient pRTP materials allow for high-resolution gated emission with a size of the diffraction limit using small-scale and low-cost photodetectors.     

Joining of silicon nitride ceramics by hot pressing

The joining of hot-pressed silicon nitride ceramics, containing Al₂O₃ and Y₂O₃ as sintering aids, has been carried out in a nitrogen atmosphere. Uniaxial pressure was applied at high temperature during the joining process. Polyethylene was used as a joining agent. Joining strength was measured by four-point bending tests. The effects of joining conditions such as temperature (from 1400 to 1600°C), joining pressure (from 0.1 to 40 MPa), holding time (from 0.5 to 8 h) and surface roughness (R _max) of the joining couple (about 0.12, 0.22 and 1.2m) on the joining strength were examined. The joining strength was increased with increases in joining temperature, joining pressure and holding time. Larger surface roughness caused lower joining strength. The higher joining strength was attributed to a larger true contact area. The area was increased through plastic deformation of the joined couple at elevated temperatures. The highest joining strength attained was 567 MPa at room temperature, which was about half the value of the average flexural strength of the original body. The high temperature strength measured at 1200° C did not differ very much from the room-temperature value.     

Processing of a Novel Multilayered Silicon Nitride

A new type of silicon nitride with a layered structure of alternating dense and porous layers was obtained by addition of β-Si₃N₄ whiskers to the porous layers. The materials consisted of dense layers 60 μm thick and porous layers 40 μm thick with a final porosity of about 30%. Highly anisotropic shrinkage behavior was observed during sintering. A large addition of whiskers to the porous layers resulted in layers with well-oriented and tightly tangled elongated grains, where porosity is represented by anisotropic shaped pores.     

High-Performance Boron Nitride-Containing Composites by Reaction Synthesis for the Applications in the Steel Industry

In this study boron nitride-containing composites (BNCC) with high strength, low Young's modulus and highly improved strain tolerance have been developed by reaction synthesis technology. The in situ formed BN particles with fine (nanosized) particle size were distributed homogeneously and isotropically in the matrixes, which decreased the negative effect of the BN phase on the composite properties. In order to be relevant to the wide applications in the steel industry such as break rings for horizontal casting of steel, the BNCC (i.e., AlON–BN composites) were evaluated in terms of the corrosion resistance to molten steel, wear and thermal shock resistance. The results demonstrate that the AlON–BN composites outperform those of the currently employed materials especially with respect to corrosion resistance.     

Synthesis and Properties of Porous Single-Phase β'-SiAlON Ceramics

Single-phase β'-SiAlON (Si_{6− z} Al _z O _z N_{8− z} , z = 0–4.2) ceramics with porous structure have been prepared by pressureless sintering of powder mixtures of á-Si₃N₄, AlN, and Al₂O₃ of the SiAlON compositions. A solution of AlN and Al₂O₃ into Si₃N₄ resulted in the β'-SiAlON, and full densification was prohibited because no other sintering additives were used. Relative densities ranging from 50%–90% were adjusted with the z -value and sintering temperature. The results of X-ray diffraction, scanning electron microscopy, and transmission electron microscopy analyses indicated that single-phase β'-SiAlON free from a grain boundary glassy phase could be obtained. Both grain and pore sizes increased with increasing z -value. Low z -value resulted in a relatively high flexural strength.     

Strain Tolerant Porous Silicon Nitride

An approach to material strain tolerance, which basically makes it possible to lower the elastic modulus while retaining strength, was experimentally confirmed using as an example a porous silicon nitride composed of oriented anisotropic grains and pores. The porous structure consisting of tightly tangled rodlike grains and anisotropic pores was obtained by using β-Si₃N₄ whiskers. This material exhibited a low Youngs modulus while retaining a relatively high fracture stress, even though it contained 14.4% porosity. Consequently, the strain to failure of silicon nitride was appreciably increased.     

Reaction Synthesis of Magnesium Silicon Nitride Powder

The synthesis of magnesium silicon nitride (MgSiN₂) by direct nitridation of a Si/Mg₂Si/Mg/Si₃N₄ powder mixture is described. A nucleation period at 550°C and stepwise heat-treatment schedule up to 1350°C was adopted for the synthesis of MgSiN₂ powder, based on TG-DTA measurements. The influence of the ratio of constituents on the final phase composition also has been studied. The content of magnesium and silicon in the starting powder should fulfill the conditions Mg₂Si/Mg ≥ 3 and Si₃N₄/Si_tot≥ 0.5 to obtain single-phase MgSiN₂. The silicon particle size of <0.5 μm is preferable to decrease the time of nitridation. The oxygen content of as-synthesized powders is in the range 0.9–1.2 wt%. However, the oxygen content of MgSiN₂ powder decreases further by the addition of 2 wt% CaF₂ or 0.75 wt% carbon and reaching the lowest value of 0.45 wt% oxygen after carbothermal reduction in an alumina-tube furnace.