Kyota Uheda

Luminescence Properties of a Red Phosphor, CaAlSiN3 : Eu2 + , for White Light-Emitting Diodes

We developed a new red phosphor, CaAlSiN 3 :Eu 2+ , which has a broad excitation band extended from UV region to 590 nm. At the optimum Eu concentration of 1.6 mol %, quantum output is seven times higher than for a conventional red phosphor, La 2 O 2 S:Eu 3+ , under 405 nm excitation. The phosphor is also more efficient than CaSiN 2 :Eu 2+ or Ca 2 Si 5 N 8 :Eu 2+ at any excitation wavelength. One reason of the high room-temperature efficiency is small thermal quenching, which is probably related to a rigid network of [SiN 4 ] and [AlN 4 ] tetrahedra. The phosphor is chemically stable as well. Accordingly, it is a promising material for warm-white light-emitting diodes.

High Pressure Synthesis of New Filled Skutterudites

Filled skutterudites exhibit properties that comply with the concept of a “phonon-glass electron-crystal”, as proposed by Slack. The optimal filled skutterudite would have filler atoms that exhibit large thermal vibration amplitudes in the voids of the crystal structure. It is desirable that these loosely bound atoms give rise to strong phonon scattering without greatly affecting the essential part of the band structure of the skutterudites. This criterion is difficult to meet. Most attempts have employed charge compensation for filling fractions above 50 %. In this report we present the use of a high-pressure technique for the synthesis of new filled skutterudites. By using our high-pressure synthesis technique CoSb 3 -based skutterudites filled with group-14 elements (Ge, Sn, and Pb) have been synthesized with up to 100 % filling without charge compensation of the host lattice. The structural analysis reveals that the Sn atoms exhibit very large thermal vibration amplitude, indicative of a large “rattling” motion. The Sn-filled specimens exhibit a low thermal conductivity, lower than that of any previously reported filled skutterudite, while the favorable semiconducting nature of the host lattice is not substantially changed by Sn filling. Tin atoms may therefore be better “rattlers” in the CoSb 3 host lattice than lanthanide or actinide atoms.

Electroluminescence Cell Based on Polyoxometalates Pulsed Electric Field‐Induced Luminescence of Decatungstoeuropate Dispersion Layers

electroluminescence (EL) cells have been fabricated with the layer with 40–80 μm thickness, as the emissive medium. The spectral distribution of EL, induced by the applied negative voltage pulse, indicates the presence of three emission envelopes of nitrogen plasma, oxygen‐to‐tungsten (O → W) ligand‐to‐metal charge transfer (LMCT) emission, and Eu3+ f‐f emission for every dispersion layer of Ca2+, Sr2+, or Ba2+ salt of and pellet. The Eu3+ EL lines are similar to Eu3+ photoluminescence lines but with relatively higher intensity in the transition. Typical operating conditions are at 100–300 Hz. A high porosity of the layer which allows a voltage drop to occur in the layer, resulted in a significant contribution of the nitrogen plasma to the total emission. Comparison of transient behaviors among three emissions exhibits an accumulation of charges inside the layer which induces an internal field. Such charges once trapped are released and accelerated by external field or by internal field after the applied pulse is turned off, until enough kinetic energy is acquired to excite . The insulating Mylar film is important for the EL in that it allows accumulation charges to remain inside the layer. The EL intensity is still very low, due to the small number of internal carriers and the small amount of carrier accumulation in the vicinity of the .

Mechanism of Luminescence Enhancement in Ca4YO ( BO3 ) 3 : Eu3 + by Gd Substitution under Vacuum Ultraviolet Light Excitation

The role of Gd 3+ in enhancing Eu 3+ luminescence intensity of Ca4(Y,Gd)0(B03) 3 :Eu 3+ under excitation by vacuum UV light was investigated. A dominant excitation band peaked at 175 nm is assigned to the transitions from O 2p valence state to mixed states of B 2p and Y 4d orbitals on the basis of calculation by the relativistic DV-Xa method, which is a popular computational code of the molecular orbital calculation. The 175 nm band is related also to Gd 3+ , because its intensity increases with Gd molar fraction. Presumably excitation energy absorbed by the above transitions is transferred to Eu 3+ via the 4f excited levels of Gd 3+ , because both the charge transfer and 4f-5d transition energy of Gd 3+ are much higher than the photon energy of the 175 nm band.

Luminescence Efficiency Enhancement by Impurity Doping in SrIn2O4 : Pr3 +

A red-emitting phosphor, SrIn 2 O 4 :Pr 3+ is a candidate of Cd-free phosphor for vacuum fluorescent tubes. Luminescence of Sr x In 2 O 4 :Pr 3+ is quenched at a Sr-rich composition (x ≥ 1) by formation of defects that work as nonradiative centers. At the stoichiometric composition (x = 1.0), the luminescence efficiency is increased by addition of Gd 3+ , while decreased by La 3+ addition. Measurement of the unit cell volume showed that Pr 3+ and Gd 3+ occupy the In 3+ site, while La 3+ presumably occupies the Sr 2+ site. This site preference shifts the atomic ratio of the Sr/In site to the Sr-poor side by Gd 3+ doping and to the Sr-rich side by La 3+ doping, resulting in an increase or a decrease in efficiency at the stoichiometric composition. Under 1.4 keV electron excitation and at the dopant concentration of 7 atom % per 1 mol. SrIn 2 O 4 , Gd 3+ doping provides the maximum efficiency about twice higher than the maximum efficiency of an undoped sample. The optimized Sr fraction for La 3+ doping, x = 0.8, has led to efficiency about 2.7 times as high as efficiency of the undoped SrIn 2 O 4 :Pr 3+ .