Htjm Bert Hintzen

Long wavelength Ce3+ emission in Y–Si–O–N materials

Abstract An investigation of new luminescent materials, Y5(SiO4)3N:Ce, Y4Si2O7N2:Ce, YSiO2N:Ce and Y2Si3O3N4:Ce is presented. In a series of these oxynitride materials, long wavelength emission of Ce3+ is observed, the exact position determined by the nephelauxetic effect, the crystal-field splitting and the Stokes shift. It was found that the crystal-field splitting becomes larger when more N3− versus O2− coordinates to Ce3+. This is ascribed to the higher formal charge of N3− compared to O2−. Furthermore, several parameters, like increased nitrogen versus oxygen coordination, more non-bridging versus bridging nitrogen coordinated to Ce3+, as well as its coordination by free oxygen ions, contribute to the increase of the covalency, which shifts the centre of gravity of the 5d state to lower energy (nephelauxetic effect). The Stokes shift is observed to become smaller for lattices with a more extended silicon network formation when more N3− is incorporated. The smaller Stokes shift is ascribed to the increasing rigidity of the lattice. The energy difference between the lowest 5d excitation band of Tb3+ and that of Ce3+ amounts to 12–15∗103 cm−1, which is in agreement with literature.

Luminescence of a new class of UV–blue-emitting phosphors MSi2O2−δN2+2/3δ:Ce3+(M = Ca, Sr, Ba)

The luminescence properties of Ce3+,Na+-codoped MSi2O2−δN2+2/3δ (M = Ca, Sr, Ba) are reported. The undoped and Ce3+,Na+-codoped MSi2O2−δN2+2/3δ powders were prepared by a solid-state reaction at temperatures between 1300–1400 °C under N2–H2 (10%) atmosphere in the system MO–SiO2–Si3N4 (M = Ca, Sr, Ba). MSi2O2−δN2+2/3δ (M = Ca, Sr, Ba) crystallizes in the monoclinic system with different crystal structures. For excitation in the 300–360 nm range, MSi2O2−δN2+2/3δ:Ce3+ shows typical broad emission bands peaking at about 392, 473 and 396 nm for M = Ca, Sr and Ba, respectively. In particular, CaSi2O2−δN2+2/3δ:Ce3+ shows an unusual short-wavelength emission (∼392 nm) with a very small Stokes shift of 2200 cm−1; BaSi2O2N2:Ce3+ shows an interesting white-light emission in the visible range 350–600 nm for excitation at 365 nm.

Phase formation of Ca-α-sialon by reaction sintering

Abstract In this study the reaction sintering route for the formation of Ca-α-sialon with a composition on the line Si 3 N 4 CaO · 3AlN (Ca 0.8 Si 9.6 Al 2.4 O 0.8 N 15.2 , m = 1.6, n = 0.8) has been investigated. This is compared with the hot-pressing of Ca-α-sialon and the reaction sintering of Y- or lanthanide-α-sialons. The reaction follows the same sequence: first, the formation of a Ca-rich α-sialon phase ( m = 1.9) which is gradually transformed to a Ca-α-sialon with a lower Ca concentration ( m = 1.2). The gehlenite phase (Ca 2 Al 2 SiO 7 , melilite group) is observed as an intermediate product. A potential advantage of Ca-α-sialon over Ln-α-sialon (Si 3 N 4 1 3 Ln 2 O 3 · 3AlN , where Ln = Y, lanthanide) is liquid phase formation at a lower temperature, which has a positive influence on the processing temperature. Moreover, Ca is cheaper than the lanthanides. The solubility of Ca in the α-sialon is in agreement with values found in the literature.

On the Ce3+ luminescence in the melilite-type oxide nitride compound Y2Si3−xAlxO3+xN4−x

Abstract Ce3+ luminescence was studied in the system Y1.98Ce0.02Si3−xAlxO3+xN4−x with a weighed-out x of 0, 0.25, 0.5, 0.6, and 1. The lattice parameters show a nearly linear increase between x = 0 and x = 0.6, indicating an increasing substitution of Si–N by the larger Al–O in the melilite-type lattice. The solubility limit is near x = 0.6; for higher values of x, the lattice parameters remain constant. The luminescence spectra show typical Ce3+ luminescence (excitation maxima at 310 and 390 nm and emission maximum near 475 nm). No shifts in the excitation spectrum and only slight shifts in the emission for increasing x were observed, indicating that a change in the overall composition does not affect the local coordination of the Ce3+ ion. This is explained by the preferential occupation of the large Ce3+ ion on “roomier” O-rich sites, as compared with the average coordination around Y3+ in Y2Si3O3N4. As a result of the preferential Ce3+ coordination, extra O2−, introduced with the incorporation of Al–O in Y2Si3−xAlxO3+xN4−x, will substitute on N-richer sites, which preferentially coordinate with the smaller Y3+ ion. The minimal shift of the emission spectrum results in a slightly larger Stokes shift (from about 4200 to 4300 cm−1), which suggests a decreasing rigidity of the host-lattice for increasing substitution of Si–N by Al–O. This is explained by the preferential substitution of Al on Si sites near Ce3+, which counterbalances the deficit in negative charge due to extra oxygen versus nitrogen in the Ce3+ coordination.

Long wavelength Eu2+ emission in Eu-doped Y–Si–Al–O–N glasses

Abstract A number of (Eu,Y)–Si–Al–O–N glasses have been prepared and their optical properties examined. Eu was found to be present in the divalent instead of in the trivalent state due to a reaction between the Eu 3+ and the chemically incorporated N 3- during the preparation of the glasses. The luminescence characteristics were found to be negligibly influenced by the O/N and Si/Al ratio, but appear to be strongly dependent on the concentration and type of modifying cations (Eu,Y). As a function of the cationic composition the emission shifts from wavelengths below 500 nm to wavelengths as long as 640 nm, which is very unusual for Eu 2+ containing compounds. This shift to longer wavelengths is ascribed to a combination of energy transfer between the different sites and change of the Eu 2+ site distribution.

The temperature dependence of the Young's modulus of MgSiN2, AlN and Si3N4

The temperature dependence of the Youngs modulus of MgSiN2 and AlN was measured between 293 and 973 K using the impulse excitation method and compared with literature data reported for Si3N4. The data could be fitted with . The values of the fitting parameters E0 and T0 are related to the Debye temperature, and the parameter B to the harmonic character of the bond.

Luminescence properties of Eu2+-doped MAl2-xSixO4-xNx (M = Ca, Sr, Ba) conversion phosphor for white LED applications

Undoped and Eu-doped MAl 2-x Si x O 4-x N x (M = Ca, Sr, Ba) were synthesized by a solid-state reaction method at 1300-1400°C under nitrogen-hydrogen atmosphere. The solubility of (SiN) + in MAl 2 O 4 was determined. Nitrogen can be incorporated into MAl 2 O 4 by replacement of (AlO) + by (SiN) + , whose amount of solubility depends on the M cation. The solubility of (SiN) + is very low in CaAl 2 O 4 and SrAl 2 O 4 lattices (x ≈ 0.025 and 0.045, respectively), whereas a large amount of (SiN) + can be incorporated into BaAl 2 O 4 (x ≈ 0.6). Incorporation of (SiN) + hardly modifies the luminescence properties of Eu 2+ -doped MAl 2 O 4 (M = Ca, Sr) because of limited solubility of (SiN) + , showing the blue and green emission at almost constant wavelength of 440 and 515 nm, respectively. Eu 2+ -doped BaAl 2-x Si x O 4-x N x exhibits a broad green emission band with a maximum in the range of 500-526 nm, depending on the concentration of (SiN) + and Eu 2+ . In addition, both excitation and emission bands of Eu 2+ show a significant red shift as nitrogen is incorporated. BaAl 2-x Si x O 4-x N x :Eu 2+ can be efficiently excited in the range of 390-440 nm radiation, which makes this material attractive as conversion phosphor for white light-emitting diode (LED) lighting applications.

Ab initio band structure calculations of Mg3N2 and MgSiN2

Ab initio band structure calculations were performed for MgSiN2 and Mg3N2. Calculations show that both nitrides are semiconductors with direct energy gaps at . The valence bands are composed mainly of N 2p states hybridized with s and p characters of the metals. The bottom of the conduction band consists of the s characters of Mg and N for Mg3N2, as well as for MgSiN2, while the characters of Si are higher in energy. The optical diffuse spectra show an energy gap of about 2.8 eV for Mg3N2 and 4.8 eV for MgSiN2, in agreement with the calculated values.

Eu‐Doped Barium Aluminum Oxynitride with the β‐Alumina‐Type Structure as New Blue‐Emitting Phosphor

Attractive new blue-emitting phosphors for use in low-pressure mercury gas discharge lamps are synthesized by Eu-substitution in the barium aluminum oxynitride host lattice with the -alumina-type structure. The emission spectra of these phosphors for 254 nm excitation show a band at about 450 nm with a shoulder at higher wavelength. The maximum quantum efficiency of these materials is about 85–90% just like commercial BaMgAl10O17:Eu with the -alumina type structure. The nonoptimized oxynitride phosphors are more sensitive to oxidation (at 873 K) and to short-term depreciation due to 185 nm irradiation compared to commercial BaMgAl10O17:Eu. However, the maintenance of the oxynitride phosphors in single component fluorescent lamps is improved. Calculations indicate that by using these phosphors in tricolor fluorescent lamps instead of BaMgAl10O17:Eu with the -alumina type structure, the color rendering index will improve while the lumen output remains high.

Carbothermal preparation and characterisation of Ca-α-sialon

Abstract Ca-α-sialon can be synthesised by simultaneous carbothermal reduction-nitridation of a mixture of fine SiO 2 , Al 2 O 3 , CaSiO 3 and carbon black powder at 1500 °C. For the first time a single phase material is obtained in a one step process. The main intermediate product in these processes is a β-sialon with a low z-value ( z