A conformal encapsulation material for use in high-power, thermally stable ultraviolet (UV) light-emitting diodes was successfully developed. For silicone, thermal degradation started at ∼200°C, and the transmittance was 85.5% at 365 nm. The transmittance decreased by 55% after thermal aging at 250°C for 72 h and it decreased further by 2.5%, even at room temperature, under continuous exposure to UV light at 365 nm for 72 h. By contrast, for the sol–gel material, thermal degradation started at ∼300°C, and the transmittance was 90% at 365 nm. The transmittance decreased negligibly after thermal aging at 250°C for 72 h and it did not decrease further even at 75°C under continuous exposure to UV light at 365 nm for 72 h.
We investigate a new type of high-power light emission diode module with glass encapsulation instead of conventional polymer. To the best of our knowledge, this work could be the first very early attempt to make a glass encapsulation light-emitting diode module into a prototype.
The lumen degradation and chromaticity shift in glass and silicone based high-power phosphor-converted
white-emitting diodes (PC-WLEDs) under accelerated thermal tests at 150°C, 200°C, and 250°C are presented and
compared. The glass based PC-WLEDs exhibited better thermal stability than the silicone by 4.8 time reductions
in lumen loss 6.8 time reductions in chromaticity shift at 250°C, respectively. The mean-time-to-failure (MTTF)
evaluation of glass and silicone based high-power PC-WLEDs in accelerated thermal tests is also presented and
compared. The results showed that the glass based PC-WLEDs exhibited higher MTTF than the silicone by 7.53
times in lumen loss and 14.4 times in chromaticity shift at 250°C, respectively. The thermal performance of lumen,
chromaticity, and MTTF investigations demonstrated that the thermal stability of the glass based PC-WLEDs
were better than the silicone. A better thermal stability phosphor layer of glass as encapsulation material may be
beneficial to the many applications where the LED modules with high power and high reliability are demanded.
Various oxidation states and location sites of chromium (Cr) in doped crystal have been known. Since the high sensitivity
of ligand field on chromium ions, these are attributable to a broadband emission. The wide broadband emission can
potentially be exploited in many ways. For instance, a Cr-doped broadband amplified spontaneous emission can improve
spatial resolution many times in optical coherence tomography (OCT) diagnostic instrumentation.
It is difficult to further extend the emission bandwidth of Cr in a single crystal, although a single crystal is generally
considered as the best host. Amorphous as a host, on the other hand, may potentially expand the bandwidth of emission
and accommodate higher doping concentration. However, a single crystal is beneficial to a specific valance state through
charge compensation technique.
To investigate chromium's full potential, both Cr-doped nano-crystalline embedded in an amorphous host and Cr-doped
glasses are proposed. The ultimate aims are capable of tailoring oxidation states, site symmetry and concentrations of Cr
doping. In the first stage, Cr:YAG-doped silica fiber will be studied as a part of our research scope.
In this paper, we would like to report the following two subjects:(1) Thermal
decay mechanisms of phosphor-doped silicone in high power phosphor-converted
white light emitting diode (PC-WLED) module and (2) Thermal aging variations of
light profile and output power of blue LED modules having a polycarbonate lens and
silicone as an encapsulant.
Although silicone degradation attributed to the final thermal degradation, it is not
a dominant factor until a much thicker silicone is employed in PC-LEDs. The major
degradation mechanism of the PC-LEDs results from the higher doping concentration
of Ce:YAG in silicone. However, the negligible difference of fluorescent lifetimes
among the test samples before and after thermal aging (at 150°C for 500hrs)
eliminated any significant quenching processes that existed in our aged samples. The
emission spectra suggest that a higher doping concentration in silicone causes a higher
degree of loss at the emission wavelength of Ce:YAG, namely 570nm. Therefore,
minimizing any mismatch of the refractive index, thermal expansion , and chemistry
between the phosphor and the silicone is a new sign of improving thermal reliability
for high power PC-LEDs.
Thermal aging variations of light profile and output power of LED modules
fabricated by three manufacturers (namely, Type I, II, and III) were investigated
experimentally and numerically. Both experimental results and simulation results
suggested that improving the lens/ encapsulant materials and packaging designs are
essential to not only greatly extend the product lifetime but also enhance the light quality of LED modules as illumination sources.
Index Terms -High-powered phosphor-converted white-light-emitting diodes
(PC-WLEDs), lumen loss, chromaticity shift, silicone, polycarbonate, thermal aging,
etc.
Although the success of demonstrating tellurite glass as a waveguide material in many applications, including tellurite fiberization, Nd3+-doped tellurite fiber laser, and 1.5 μm ultra-broad band Er3+-doped optical amplifier, the advance of tellurite is still necessary in the areas of improving the quality of waveguide and understanding the correlations among processing, structure, and desired property, such as nonlinearity, rare-earth spectroscopy, nanocrystalline doping, and microstructured holey fiber. In the paper, we report some initial experimental results on fiberization processing of KNbO3-Na2O-ZnO-TeO2 and Er2O3-WO3-TeO2 systems. The report, in particular, focuses on the thermal characteristics of these glasses.
There is strong need for low cost, optically active materials whose high electro-optic (EO) and second harmonic generation (SHG) properties can be engineered flexibly, in bulk and fiber forms. Therefore, we have fabricated transparent ferroelectric composites consisting of strontium barium niobate crystallites in a refractive index compatible tellurium oxide (TeO2) glass matrix. Several glass compositions, in the series x SrO-(10-x) BaO-y (Nb2O5)-(90-y) TeO2 (where x=2.5, 5 and 7.5 and y = 10, 15, 20 and 25), have been prepared by a conventional melt quenching technique. The compositions have been selected on the basis of thermal stability data obtained from differential thermal analysis (DTA). X-ray diffraction studies indicate ferroelectric phase formation in the controlled crystallized glasses. The non-centrosymmetric nature of the crystallized regions has been monitored via observing the second harmonic signal.
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