Newly emerged phosphor ceramics play key roles in high-brightness solid-state laser lighting technologies. Recently, we developed a translucent CaAlSiN3:Eu2+ red-emitting ceramic, with high quantum efficiency (up to 60%) and thermal robustness (15% higher than the powders). In the current work, detailed structural analyses are carried out by advanced analytical techniques, such as TEM and EPMA, in tandem with the CL, to reveal the correlation between the optical properties and the microstructure, especially the contribution of the interface. It was found that the ceramics consist of many core–shell structured phosphor particles. The shell is coated by a highly crystallized Ca-α-Sialon thin layer to reduce the surface defects. Such core–shell particles are embedded in a non-luminescent matrix of nano-sized Ca-α-Sialon particles tightly surrounded by an amorphous phase, both of which have little influence on the light propagation. Thanks to the unique core–shell structure of phosphor particles and the tight interfaces in the matrix, the bulk CaAlSiN3:Eu2+ ceramic shows record high performance under blue laser irradiation. The output luminous flux increases linearly as the incident power density increases from 20 to 150 W cm−2 with a constant high luminous efficacy of 42.2 lm W−1, demonstrating its promising application in solid-state laser lighting.

Thermal shock behaviors of SiC ceramic with artificial defects were studied. Artificial surface cracks were simulated by Vickers and Knoop indentations, and artificial inner cracks were obtained using novel method of introducing plastic fibers. The residual flexural strength and microstructure were analyzed. The flexural strength of artificial defect free SiC ceramic maintains minimum 87% of the original strength after thermal shock ΔT no more than 400 ℃, and the residual strength of those with artificial crack, no matter surface type or inner type, can still be improved by the joint effect of thermal treatment, thermal shock and the stress relaxation by artificial crack if ΔT no more than 400 ℃. It is believed that inner and surface artificial defects ease the crack propagation during the thermal shock process.

Silicon nitride (Si3N4) ceramics were pressureless sintered with 0.2 wt% FeSi2 at 1780 °C for 2 h in nitrogen with Al2O3 and Y2O3 as sintering aids and found to have high toughness and strength. During the sintering process, β-Si3N4 crystal seeds and Fe5Si3 reinforcing particles were in situ generated by FeSi2. Abnormal growth of Si3N4 grains was promoted by β-Si3N4 crystal seeds. High thermal expansion coefficient of Fe5Si3 particles induced residual stresses in the ceramics matrix, which deflected the crack. Ultimately, the fracture toughness and flexural strength of Si3N4 ceramics reached 9.8 ± 0.5 MPa·m1/2 and 1086 ± 48 MPa respectively.Download high-res image (299KB)Download full-size image

Polycrystalline lutetium aluminum garnet (Lu3Al5O12) powders were prepared by a simple sol–gel combustion method using aluminum nitrate, lutetium oxide and citric acid as the starting materials. The XRD results showed that the amorphous precursor converted directly to pure LuAG at 900 °C. The TEM investigations revealed that the synthesized LuAG powders are nano-sized with an average particle size 20–30 nm.

Eu-doped lutetia (Lu2O3:Eu) nano-phosphors were synthesized by the sol–gel combustion process from a mixed aqueous solution of europium and lutetium nitrates, using organic glycine as the fuel. Powder X-ray diffraction shows that cubic Lu2O3:Eu crystallites are directly obtained by the sol–gel combustion process without further calcination. Electron microscopy reveals that the as-prepared phosphors are agglomerated and have a fluffy, fine, and porous morphology, consisting of primary particle size of 8–10 nm. The excitation spectrum is characterized by three dominant bands centered at 395, 466, and 534 nm, respectively. Both the photoluminescent and radioluminescent spectra are very similar and exhibit intense emission peaks centered at 612 nm due to 5D0→7F2 transition of Eu3+ ions. The energy transfer from Lu2O3 host to Eu3+ activator is more efficient in the case of calcined phosphors than for the as-prepared phosphors due to their improved lattice perfection.

Nano-sized cerium-doped lutetium aluminum garnet (LuAG:Ce) phosphors were prepared via a sol–gel combustion process from a mixed aqueous solution of metal nitrates, using glycine as a fuel. The prepared LuAG:Ce phosphors were characterized by XRD, EPMA, and TEM, respectively. The spectroscopic properties of the phosphors were investigated. The as-prepared phosphors are agglomerated with a primary particle size of about 30 nm and have a foamy-like morphology. The pure crystalline LuAG:Ce with uniform size of 40 nm was obtained after calcined at 1000 °C for 2 h. The excitation spectrum shows two bands localized at 350 and 450 nm due to transitions from the 4f ground state to the excited 5d band. Both the photoluminescence excited by UV and the radioluminescence excited by X-ray show the same two emission bands, corresponding to transitions from the lowest 5d excited state (2D) to the 4f ground state of Ce3+ (2F5/2,2F7/2).

Nanosized LuAG:Ce phosphors with different Ce concentrations were prepared by the sol–gel combustion method. Phase evolution, morphology and luminescent properties of the obtained materials were characterized by XRD, TEM, photoluminescent and radioluminescent spectra excited by blue light and X-ray, respectively. The purified crystalline phase of LuAG:Ce was obtained at 820 °C by directly crystallizing from amorphous materials. Both the photoluminescence and radioluminescence are the well-known Ce emissions located in the 470–600 nm consisting of two emission bands due to the transitions from the lowest 5d excited state (2D) to the 4f ground state of Ce3+, which matches well with the sensitivity curve of the Si-photodiode. The luminescent intensity of LuAG:Ce phosphors varies with the Ce contents and reaches the maximum at 0.5 at.% doped. There is a little red shift for the main emission component from blue light-excited emission spectra to X-ray-excited ones. The luminescent intensity of LuAG:Ce phosphors increases with increasing the calcining temperatures due to the improved crystallization.

To achieve amorphous silicon nitride (a-SiNx) thin films with minimal incorporation of impurities such as carbon and hydrogen, a novel liquid source precursor, tris(diethylamino)chlorosilane (TDEACS), was developed. TDEACS and ammonia (NH3) were used to produce a-SiNx films by low pressure chemical vapor deposition in a hot wall tubular reactor. The growth kinetics was investigated as a function of total pressure, NH3/TDEACS flow ratio, and deposition temperature. The film compositions and topography were characterized by X-ray photoelectron spectroscopy, Auger depth profile, Fourier transform infrared spectroscopy, elastic recoil detection, and atomic force microscopy, respectively. The growth rate of the films follows an Arrhenius behavior with apparent activation energy of 182.6 kJ·mol−1 between 600 and 750 °C. At NH3/TDEACS flow rate ratios below 4, carbon-containing a-SiNx films were obtained while all films were stoichiometric with a N/Si atomic ratio 1.30–1.32 as the ratios beyond 6. Both carbon and hydrogen contents of the prepared a-SiNx films were markedly lower than those prepared from other organic precursors previously reported. The surface topography of the films is smooth and uniform with a root mean square roughness value of 0.53 nm.

β-Sialon powder was fabricated by carbothermal reduction–nitridation of natural Chinese Yixing kaolin at 1300–1550 °C in nitrogen atmosphere, using carbon black as a reducing agent. The effect of reaction temperature, holding time, nitrogen flow rate and the amount of carbon black on the synthesis of β-sialon powder and its solid solution degree (z value) have been investigated systematically. The phases of the reaction products are identified by XRD. The results indicate that the products of reaction are more sensitive to the reaction temperature than holding time. Nearly all β-sialon is produced at a temperature of 1450 °C for 6 h. Higher temperatures lead to the presence of SiC phase. A slight amount of carbon favors the carbothermal reduction of kaolin and excess carbon results in the conversion of β-sialon to SiC. The z value of β-sialon (Si6−zAlzOzN8−z) rises with the duration of holding time and varies from 2 to 2.8 as the holding time varies from 2 to 6 h at 1450 °C.

Solid state laser lighting is a newly emerging technology that combines a blue laser diode (LD) with a yellow-emitting phosphor converter to generate high-brightness white light. Due to high flux irradiation as well as thermal attack from the incident laser, this technology draws a demanding requirement on the thermal performance of the color converter. In this work, we design a Al2O3–YAG:Ce phosphor ceramic with a unique composite structure, where yellow-emitting YAG:Ce particles are embedded in a non-luminescent Al2O3 matrix having high thermal conductivity. The large YAG:Ce particles (5–20 μm) show good crystallinity and a high external quantum efficiency of 76% (upon 460 nm excitation) while the fine Al2O3 grains (0.5–2 μm) contribute to the superior in-line transmittance of 55% at 800 nm by minimizing the birefringence related scattering. The phosphor ceramic exhibits a high thermal conductivity of 18.5 W m−1 K−1 and a remarkable improvement in thermal stability (only an 8% reduction at 200 °C). When irradiated under 445 nm blue laser diodes, the phosphor ceramic shows no luminescence saturation even under a high power density of 50 W mm−2, validating its suitability for high-power solid state laser lighting.

Newly emerged phosphor ceramics play key roles in high-brightness solid-state laser lighting technologies. Recently, we developed a translucent CaAlSiN3:Eu2+ red-emitting ceramic, with high quantum efficiency (up to 60%) and thermal robustness (15% higher than the powders). In the current work, detailed structural analyses are carried out by advanced analytical techniques, such as TEM and EPMA, in tandem with the CL, to reveal the correlation between the optical properties and the microstructure, especially the contribution of the interface. It was found that the ceramics consist of many core–shell structured phosphor particles. The shell is coated by a highly crystallized Ca-α-Sialon thin layer to reduce the surface defects. Such core–shell particles are embedded in a non-luminescent matrix of nano-sized Ca-α-Sialon particles tightly surrounded by an amorphous phase, both of which have little influence on the light propagation. Thanks to the unique core–shell structure of phosphor particles and the tight interfaces in the matrix, the bulk CaAlSiN3:Eu2+ ceramic shows record high performance under blue laser irradiation. The output luminous flux increases linearly as the incident power density increases from 20 to 150 W cm−2 with a constant high luminous efficacy of 42.2 lm W−1, demonstrating its promising application in solid-state laser lighting.

CaAlSiN3:Eu2+ is a very promising red phosphor for high color rendering white light-emitting diodes (wLEDs), but a large part of its emission covers wavelengths longer than 700 nm, which is not detectable by the human eye and thus wasted. Optimizing its luminescence properties will allow us to further enhance the overall optical properties of wLEDs. In this contribution, the luminescence spectra of Ce3+ or Eu2+-activated CaAlSiN3 were tuned and tailored by introducing isostructural Si2N2O to form solid solutions of Ca1−xAl1−xSi1+xN3−xOx (x = 0–0.22). The structural evolutions, microstructure, luminescence, thermal quenching and quantum efficiency of Ca1−xAl1−xSi1+xN3−xOx:RE (RE = Ce3+, Eu2+) were investigated and clarified by using various analytic techniques. Upon introducing Si2N2O into CaAlSiN3, the Ce3+-doped CaAl1−xSi1+xN3−xOx shifted its peak emission from 606 to 575 nm and retained the band width of 145 nm, whereas the Eu2+-doped one blue-shifted its emission from 650 to 638 nm and broadened its band width from 90 to 122 nm, owing to the structural distortion and disorder as well as the band gap broadening associated with the solid solution formation. By combining these phosphors with a blue LED, high color rendering (Ra) and luminous efficacy (η) white LEDs were realized (Ra = 82.5 and η = 104 lm W−1 for the Ce3+-doped phosphors, and Ra = 97 and η = 101 lm W−1 for the Eu2+-doped phosphors).

As an excellent red phosphor, CaAlSiN3:Eu2+ plays a major role in high color rendition or wide color gamut white light-emitting diodes, but it can hardly be used in high-power laser displays and lighting due to the intrinsic low thermal performance of the phosphor/silicone resin mixture. To apply CaAlSiN3:Eu2+ in laser lighting devices, a bulk ceramic form is thus required to survive thermal attack and high flux density irradiation. However, it remains an unsolved great challenge to fabricate fully densified CaAlSiN3:Eu2+ ceramics. Here, for the first time, translucent CaAlSiN3:Eu2+ ceramics with an interesting composite microstructure, where red-emitting phosphor particles with a core–shell structure are uniformly embedded in a non-luminescent α-Sialon matrix, are successfully synthesized using Si3N4 and SiO2 as sintering additives. The luminescence ceramic is superior to the corresponding powder phosphor in terms of its enhanced thermal stability (15% increase) and thermal conductivity (4 W m−1 K−1). It has a high external quantum efficiency of 60% (87% that of the powder) upon 450 nm excitation, and a luminous efficacy of 10.6 lm W−1 when irradiated under a blue laser flux density of 0.75 W mm−2. The translucent CaAlSiN3:Eu2+ ceramic is thus supposed to be a potential color converter used in emerging laser lighting and display technologies.