Light-emitting Diode Encapsulation Research Articles

Synthesis of highly hygroscopic liquid desiccant for the application of organic light-emitting diodes encapsulation

Organic light-emitting diodes (OLEDs) have been widely studied and used due to their excellent photoelectric properties. However, OLEDs suffer from the relatively inferior environmental stability due to their water-susceptible organic compositions. Here, we have developed a novel liquid desiccant (LD) composed of porous CaO particles (0.1819 cm³/g) with high specific surface area (23.37 m2/g). The LD shows a fast water-absorbing rate and a large saturated water-absorbing capacity (0.4995 g/g). The LD is further applied to the OLED encapsulation and tested at 25 °C and relative humidity of 50 %. Compared to the OLED without LD, the LT90 lifetime (time for the brightness decreased to 90 % of the initial brightness) of the OLED with LD is prolonged from 72 h to 240 h under constant current drive. The results revealed that introducing the porous and high specific surface area morphology is an efficient way to improve the hygroscopic property of desiccant. The good performance of LD facilitates the application of OLED in the commercialized application.

Identification and Characterization of Mercury Contamination in Vegetables and Herbs Cultivated on a Commercial Vertical Indoor Farming System with Light-Emitting Diode Lighting: Unveiling an Unusual Food Safety Risk of Some Improperly Manufactured High-Density Agricultural Production Systems.

Artificial grow lights, such as light-emitting diodes (LEDs) and fluorescent grow lights, are commonly used in modern day indoor farming, citing advantages in energy efficiency and a higher controlled environment. However, the use of LEDs poses a risk in mercury contaminations as a result of its production process, specifically LEDs with polyurethane encapsulates that were traditionally produced using mercury resins as a catalyst. A total of 10.0 ppm of mercury was detected in a curly kale sample harvested from an indoor hydroponic vegetable farm, exceeding Singapore Food Regulation's limit of 0.05 ppm. Vegetables, farming inputs, and surface swabs from the affected farm were analyzed using wet acid digestion followed by cold vapor atomic absorption spectroscopy analysis. The investigation found high concentrations of mercury in the LED encapsulant, and the encapsulant material was identified to be polyurethane by Fourier transform infrared spectroscopy and pyrolysis-gas chromatography-mass spectrometry analysis, indicating the source of mercury contamination to be the LED polyurethane encapsulant.

Increasing circular and bio-based content of a thermosetting polyurethane for encapsulation of optoelectronic devices: A multivariate investigation

Transparent thermosetting polyurethanes (PU) with increased circular and bio-based content have been developed with an effective and efficient approach based on the Design of Experiment. Starting from a commercial PU formulation for light-emitting diode encapsulation, the fossil-based polyol blend has been partially replaced with potentially recycled bis(2-hydroxyethyl) terephthalate (BHET) diol and bio-based polyol. BHET has been directly implemented in the polyol blend without requiring any additional solvent or treatment. The multivariate approach allowed to study formulative (i.e. BHET, bio-based and fossil-based polyols) and process factors (i.e. mixing time and temperature of polyol blend and PU blend) of the polymer preparation by using optical spectroscopy, differential scanning calorimetry, and thermogravimetric analysis. The effect of each factor on the optical and thermal properties of the resulting polyurethane films have been observed and clearly quantified. Particularly, the implementation of BHET improved the thermal stability of PU but reduced the film transmittance as the concentration increases. Circular and bio-based components into the polyol blend have been increased up to 80%w/w and the formulation has been optimized by following a property-driven analysis. Polyurethane formulated with the selected polyol blend showed transparency, thermal behaviour, long-term stability, and cost comparable to the starting commercial formulation. Market-level properties and ease of preparation highlight that circular and bio-based PUs can be rapidly developed and implemented within current polyurethane production processes, promoting the shift towards Circular Economy with solid industrial symbiosis. At last, the present approach can be easily repurposed to further improve sustainability of polyurethanes, as well as to develop other classes of polymers.

Integration and Encapsulation of Light-Emitting Diode and CMOS-MEMS Chips for Fluorescence Quenching Gas Sensor

Environmental sensing units such as gas sensors, humidity sensors, pressure sensors, PM 2.5 sensors, or temperature sensors are widely used in our daily lives. In this study, CMOS-MEMS technology is exploited to fabricate and monolithically integrate the photo-sensors, temperature sensor, and mechanical structures for an optical gas sensing chip. An LED is bonded (heterogeneous integration) on the CMOS-MEMS chip as an excitation light source, and fluorescence quenching technology is employed for the presented optical gas sensor. Finally, the light emitted from the LED is reflected and redirected onto the CMOS-MEMS chip by using an encapsulated optical reflector to increase the sensitivity and reduce the power consumption for the presented sensor. In applications, the sensing materials are, respectively, mixed with C30H24Cl2N6Ru·6H2O and C16H7Na3O10S3 for O2 and CO2 detection. Moreover, the Si-based (by micromachining) and polymer-based (by 3D printing) optical reflectors are used to encapsulate the sensing chip to demonstrate the presented concept. Measurements show that the LED driving currents for gas sensors with reflectors are significantly reduced. Measurements also indicate that the sensitivities of gas sensors for sensing chips without optical reflectors are, respectively, 0.023 μA/% (O2/N2) and 0.12 μA/% (CO2/N2); for sensing chips with hemispherical shell optical reflectors are, respectively, 0.12 μA/% (O2/N2) and 0.19 μA/% (CO2/N2); and for sensing chips with flat plate optical reflectors are, respectively, 0.24 μA/% (O2/N2) and 0.32 μA/% (CO2/N2). The sensitivity of the temperature sensor is 0.07%/°C.

High-performance transparent nanocomposites based on robust organic nanoparticles for optoelectronic applications

Transparent nanocomposites, which are optically transparent and mechanically robust, have been drawn numerous interests for various applications, including adhesives, encapsulants, and other functional coatings. We demonstrated high-performance transparent nanocomposites with robust organic polystyrene nanoparticles (NPs) for optoelectronic devices. By optimizing the crosslinker concentration, the NPs improved their thermal stability and solvent resistance, allowing the NPs to be utilized in broad fields. Based on the nanoindentation results, we confirmed that the mechanical properties and elastic modulus were improved. We verified that the light-emitting diode encapsulation with this nanocomposite can enhance light-extraction efficiency by up to 10%, thereby suggesting the high optical clarity of nanocomposites with organic NPs.

Scattering Effect on Optical Performance of Quantum Dot White Light-Emitting Diodes Incorporating SiO₂ Nanoparticles

The scattering effect plays an important role in improving the optical performance of the quantum dot (QD) white light-emitting diodes (LEDs). A majority of the previous studies have focused on the planar packaging structure only with respect to total internal reflection (TIR). In this study, SiO2 nanoparticles are incorporated into QD-silicone encapsulation of LEDs with semi-spherical lens packaging (SSLP) to exploit their scattering effect. The results show that the radiant efficacy (31.35%@100 mA) and luminous efficacy (87.56 lm/W@100 mA) of QD white LEDs can be optimized using SiO2 nanoparticle concentration (0.1 wt%) and that they increase by 5.04% and 11.08%, respectively, as compared to the conventional structure. A comprehensive ray-tracing simulation validates that the nanoparticles in the SSLP structure lead to severe loss of chip light. On the contrary, the fluorescence light increases due to the enhancement of conversion by QDs. The transmission electron microscopy images and the finite-difference time-domain simulation have been introduced to investigate the surface adsorption of SiO2 nanoparticle. This study indicates that SiO2 incorporation is an effective method to improve the efficiency of QD white LEDs, and provides a better understanding on the scattering effect based on TIR, color conversion, and surface adsorption.

Synthesis of Hydrogen-Containing Methyl Phenyl Silicone Resins with a High Refractive Index for LED Encapsulation

Addition-curing is the most common method for preparing high-performance silicone materials for light-emitting diode (LED) encapsulation. The structure and the hydrogen content of the hydrogen-containing cross-linking agents used in addition reactions have a significant influence on the performance of the cured products. In this study, a series of hydrogen-containing methyl phenyl silicone resins with different hydrogen content were synthesized through hydrolysis condensation. Subsequently, the high-refractive-index hydrogen-containing silicone resins reacted with vinyl methyl phenyl silicone resins to obtain methyl phenyl silicone materials. The cured products showed high light transmittance (> 89% at 450 nm), appropriate hardness and good thermal stability. The tests of reflow soldering (265°C ± 5°C for 10 cycles) and lumen depreciation (50 mA for 165 h) further demonstrated that the prepared silicone materials possessed high thermal resistance and stability. In addition, increasing the hydrogen content enhanced the cross-link density, thereby improving the thermal stability of the silicone materials. The prepared hydrogen-containing methyl phenyl silicone resins can thus serve as a cross-linker for addition-curing in high-power LED encapsulation.

Preparation of ZrO2/silicone hybrid materials for LED encapsulation via in situ sol‐gel reaction

Organic silicones have been used as encapsulant materials for light‐emitting diodes (LEDs) for many years, while their performances need to be improved in order to satisfy the requirements of high‐power LEDs. A ZrO2/silicone hybrid resin (ZHR) was synthesized for LEDs encapsulation by in situ sol‐gel reactions. The oligosiloxane was synthesized as polymeric matrix by nonaqueous sol‐gel condensation using diphenylsilanediol (DPSD), vinyltrimethoxysilane (VTMS), and 3‐methacryloxy propyl trimethoxysilane (MPTMS) as monomers. Then zirconium propoxide was added into this polymeric matrix to be hydrolyzed to obtain the hybrid resin with a uniform dispersion of ZrO2 nanoparticles. The Si–O–Zr covalent bond was detected and benefited for excellent dispersibility of the ZrO2 nanoparticles and the well compatibility between organic and inorganic phases. The cured ZHR with 5 wt% ZrO2 content showed high light transmittance (greater than 80% in visible light range), high refractive index (=1.56), and high thermal stability (no yellowing at 150°C for 240 hours). The luminous flux of the LED chip with ZHR encapsulant was 10% higher than that of the pure silicone resin, suggesting that ZHR has great application potential in the field of LED packaging.

Synthesis of High-Refractive-Index Epoxy-Modified Vinyl Methyl Phenyl Silicone Resins for Encapsulation of LEDs

A series of high-refractive-index epoxy-modified vinyl methyl phenyl silicone resins with different content of vinyl groups have been synthesized by a combination of hydrolytic and nonhydrolytic sol–gel processes to improve their adhesion. The products were characterized by Fourier-transform infrared spectroscopy and 1H nuclear magnetic resonance spectroscopy. The results showed that epoxy and vinyl groups were successfully introduced into the products while the content of hydroxyl groups was low. Then, epoxy-modified silicone materials were manufactured by addition reaction between the epoxy-modified vinyl methyl phenyl silicone resins and hydrogen-containing phenyl silicone resin. The epoxy-modified silicone materials were characterized by thermal aging (250°C for 2 h) experiments, light transmittance measurements, thermogravimetric analysis, red dye penetration tests, reflow soldering (270°C for 10 cycles), and lumen depreciation (50 mA for 165 h) tests. The results showed that the epoxy-modified silicone materials exhibited high transmittance (> 90%) and good thermal stability. The epoxy-modified silicone materials showed better resistance to high temperature and ultraviolet (UV) radiation than commercial epoxy resin (6103). The introduced epoxy groups greatly improved the adhesion of the silicone resin. The epoxy-modified silicone material prepared using epoxy-modified vinyl methyl phenyl silicone resin with higher content of vinyl groups showed better mechanical performance (50 Shore D). The epoxy-modified silicone material EVMPS-3′ is expected to be applicable in packaging of high-power light-emitting diodes (LEDs).

High-gravity-assisted scalable synthesis of zirconia nanodispersion for light emitting diodes encapsulation with enhanced light extraction efficiency

Zirconia-based hybrid polymers has become one of the ideal encapsulation material for light emitting diodes (LEDs) with high extraction efficiency due to their proper refractive index to those of semiconductor chips. However, the controlling of the dispersion for zirconia nanoparticles in polymeric hosts to obtain optically transparent hybrid nanocomposites has been one of the major challenges. Herein, we reported the synthesis of zirconia nanodispersion via high-gravity-assisted homogeneous precipitation in an internal circulation rotating packed bed (RPB) reactor followed by two-step modification. An alternative to conventional precipitation in batch stirred tank reactors (STR), the process intensification by high-gravity RPB results in homogeneous micromixing during the nucleation and growth of zirconia particles, which are benefit for continuous and reproducible production of ultrasmall zirconia nanoparticles. The obtained zirconia nanoparticles are purely cubic phase with narrow size-distribution in the range of 3–5 nm according to the X-ray diffraction and transmission electron microscopy characterization, which were similarity to that by the conventional methods. Nevertheless, the average hydrodynamic diameters of zirconia nanoparticles in aqueous solutions obtained by RPB methods were much smaller than those prepared in conventional STR, which enabled easy control and surface modification for highly dispersed nanodispersion in organic solvents and/or polymeric hosts. After two-step surface modification, zirconia nanoparticles with goal-directed structures were obtained, with highly dispersity in various organic solvents (e.g. toluene, trichloromethane, tetrahydrofuran, etc.) and aliphatic epoxy resin, forming transparent hybrid films with tunable refractive indexes. The preliminary applications of these zirconia nanodispersions for LEDs encapsulation were demonstrated. The light extraction efficiency of the LEDs devices packaged with zirconia/epoxy hybrid material increased by 10 percent when the doping content of zirconia nanoparticles was 0.2, compared to the devices encapsulated by original epoxy.

High refractive index adamantane‐based silicone resins for the encapsulation of light‐emitting diodes

The encapsulation of high power light emitting diode (LED) needs the silicone resins to have relative high refractive index and thermal‐aging properties. Herein, high refractive index adamantane‐based phenyl epoxy‐silicone (APES) resins for LED encapsulation were synthesized by the sol‐gel condensation of 1‐adamantane methanol propyltrimethoxysilane‐3‐urethane, γ‐(2,3‐epoxypropoxy)propytrimethoxysilane and diphenylsilanediol. These adamantane‐based silicone resins have multifunctional groups including adamantyl group, phenyl group, and epoxy group in order to meet the various requirements for LED encapsulation. Importantly, the adamantane group in the silicone resins benefits for high refractive index and anti‐thermal properties. These APES resins were characterized by proton nuclear magnetic resonance and Fourier transform infrared spectroscopy. When APES resins were cured by methylhexahydrophthalic anhydride, they showed relatively high refractive index of 1.56, high hardness, and high thermal resistance. The encapsulated LED demonstrated high adhesion properties by red‐ink tests. These merits make adamantane‐based silicone resins promising candidates as LED encapsulation materials.

Preparation and origin of thermally resistant biobased epoxy resin with low internal stress and good UV resistance based on SiO2 hybridized cellulose for light emitting diode encapsulation

Big internal stress and poor UV resistance are two bottlenecks of epoxy resins for light emitting diode (LED) encapsulation, and how to solve them with a green method is still a great challenge. Herein, a unique hybrid was synthesized by forming SiO2 particles on surface of cellulose, the hybridized cellulose (mCNC@SiO2) was then used to develop new high performance epoxy resin (mCNC@SiO2/EP) for LED encapsulation. The effect of the content of mCNC@SiO2 on the comprehensive performances of mCNC@SiO2/EP resins was studied. Results show that the presence of mCNC@SiO2 not only greatly reduces the internal stress and improves UV resistance, but also increases moisture resistance while maintaining high heat resistance. When the content of mCNC@SiO2 is only 2.0 wt%, the internal stress is reduced by 62%, thanking to the significant decrease in thermal expansion coefficient, and curing stress derived from the flexibility of Si-O-Si segment in mCNC@SiO2. In addition, mCNC@SiO2/EP resin has good transmittance, proving that the presence of mCNC@SiO2 can effectively alleviate yellowing and improve UV resistance. These excellent comprehensive properties of mCNC@SiO2/EP resin demonstrate that mCNC@SiO2 is a green and multifunctional modifier of epoxy resin for high performance LED encapsulation.

Oligo-fluoropolymer Modified Cycloaliphatic Epoxy Resins with Excellent Compatibility, Waterproof and Mechanical Properties for LED Encapsulation

An oligo-fluoropolymer(PFM) with functional cycloaliphatic epoxy and fluorinated groups was obtained via free radical polymerization and applied to the modification of cycloaliphatic epoxy resins(CE). The chemical structure of PFM was characterized by Fourier transform infrared(FTIR) spectroscopy, gel permeation chromatogra-phy(GPC) and nuclear magnetic resonance(NMR) spectroscopy, and the effects of different PFM concentra-tions(0.5%—6%, mass fraction) on the thermal resistance, mechanical properties, surface dewettability, light trans-mission, refractive index and various cured polymer properties were studied in detail. The DSC and TGA results demonstrate that the modified epoxy resins possess a higher thermal resistance than the neat epoxy resin. The im-provements in the surface dewettability and water resistance are caused by the high crosslinking density and the enrichment of the oligo-fluorinated random copolymers dispersed in the matrix. The fracture surface morphologies of the thermosets were investigated by scanning electron microscopy(SEM) and transmission electron microsco-py(TEM). It was observed that the optical transmittance of the composites was maintained even though microphase separation occurred during the curing process. With respect to the corresponding properties of the neat epoxy resins, the 2 phr(parts per hundreds of resin) PFM thermoset exhibited relatively better comprehensive properties, making the cured material a good candidate for light-emitting diode(LED) encapsulation.

Effect of sintering temperature on the photoluminescence properties of red-emitting color conversion glass

The red-emitting color conversion glasses were prepared by cosintering borosilicate glass frits and CaAlSiN3:Eu2+ phosphors at various temperatures in the 600–800 °C range. The effect of sintering temperature on the photoluminescence (PL) and excitation (PLE) of the color conversion glasses was investigated. Characteristics of color conversion glasses, with respect to excitation and emission intensity, quantum yield (QY) and CIE (Commission International de I’Eclairage) chromaticity were analyzed based on excitation and emission spectra. The glass, sintered at 600 °C, showed the best luminescence properties. Then, the intensity of excitation and emission peak decreased with the sintering temperature increasing. All excitation and emission peaks disappeared when the sintering temperature increased beyond 800 °C. The luminescence properties with the higher sintering temperature may be destroyed because of the lattice disturbance of CaAlSiN3 and oxidation of Eu2+, which is caused by reactions between glass matrices and CaAlSiN3:Eu2+ phosphors. The present study may promote the application of the red-emitting color conversion glasses in light emitting diodes encapsulation.

Performance of methylphenyl hydrogen-containing silicone oils for LED encapsulation

ABSTRACTThree-dimensional light-emitting diodes (LEDs) are the preferred light source for bulb lamps due to their 360° illumination. Addition curing silicone resins have usually been used to encapsulate LEDs. LED-filament encapsulation needs silicone resins having high thermal performance and thixotropy. Herein, a low refractive index and highly anti-thermal aging silicone oil was synthesized by hydrolysis and co-condensation method, and used to prepare an encapsulation material for high-power LED-filament. The cured silicone materials were characterized by thermogravimetric analysis (TGA) and thermal aging test under 180°C and 250°C. The results demonstrated that the thermal stability of the cured silicone resins with short-chain phenyl silicone as a crosslinker was higher than that with long-chain methyl silicone oil crosslinker. Owing to the excellent thermal stability, appropriate hardness, high transparency and photoelectric efficiency, this LED encapsulation material is a promising candidate for high-power LED package.

Impact of ZnO Nanoparticles on Dielectric and Optical Properties of Poly (3-hydroxybutyrate) for Electronics Applications

ABSTRACTThe dielectric and optical properties of pure poly (3-hydroxybutyrate) and ZnO nanoparticle (15–30%)-reinforced poly (3-hydroxybutyrate) composite films were studied. Nanocomposites exhibit improved relative permittivity (ε′ = 4.6720 @ 1 GHz) with acceptable loss tangent (tan δ < 0.02) which is greater than that of FR4 substrate (ε′ = 4.5). High absorption in ultraviolet region (200–400 nm) was observed for the nanocomposites. From the observation, the prepared biobased composites can be used as a dielectric substrate for microwave applications and coating material for light-emitting diode encapsulation to block ultraviolet rays and to develop ultraviolet-resistive fabrics for textile industry.

Self‐adhesive epoxy modified silicone materials for light emitting diode encapsulation

In this paper, we prepared the light emitting diode (LED) encapsulant with self‐adhesion and high refractive index. In order to improve adhesion properties, we synthesized a series of multifunctional polysiloxanes with different contents of epoxy groups via the sol–gel condensation of methylvinyldimethoxysilane, diphenylsilanediol and 3‐glycidoxypropyldimethoxymethylsilane. The structures of epoxyphenylvinyl silicone (EPVS) resins were confirmed by proton nuclear magnetic resonance and Fourier‐transform infrared. The effect of epoxy group content on the adhesion property of EPVS resins was fully studied. The performances of the LED encapsulation materials based on EPVS resins were investigated in detail. These self‐adhesive encapsulating materials showed excellent thermal stability, a high refractive index of 1.55 and good adhesive property. These EPVSs can be used as an adhesion promoter for LED encapsulation materials. Copyright © 2017 John Wiley &amp; Sons, Ltd.

Preparation and Performance of Poly(butyl fumarate)-Based Material for Potential Application in LED Encapsulation.

A UV-curable poly(butyl fumarate) (PBF)/poly(propylene fumarate)-diacrylate (PPF-DA) hybrid material with good performance for LED encapsulation is introduced in the paper. They have been prepared by radical polymerization using PBF and PPF-DA macromers with a UV curing system. PBF and PPF-DA were characterized by Fourier-transform infrared (FT-IR) and H-nuclear magnetic resonance (1H NMR). The thermal behavior, optical and mechanical properties of the material were examined by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), ultraviolet-visible spectroscopy (UV–vis), and a material testing system mechanical testing machine, respectively. The results indicated that the hybrid material has a suitable refractive index (n = 1.537) and high transmittance (99.64% in visible range) before/after thermal aging. With the increasing of the double bond ratio from 0.5 to 2, the water absorption ratios of the prepared encapsulation material were 1.22%, 1.87% and 2.88%, respectively. The mechanical property experiments showed that bonding strength was in the range of 1.86–3.40 MPa, tensile-shear strength ranged from 0.84 MPa to 1.57 MPa, and compression strength was in the range of 5.10–27.65 MPa. The cured PBF/PPF-DA hybrid material can be used as a light-emitting diode (LED) encapsulant, owing to its suitable refractive index, high transparency, excellent thermal stability, lower water absorption, and good mechanical properties.

Multifunctionalgraphenefilled silicone encapsulant for high-performance light-emitting diodes

Graphene is well-dispersed into a silicone encapsulant of a light-emitting diode (LED) through a simple solvent-exchange method. The embedded graphene sheets play a multifunctional role in producing a high-performance encapsulant. Compared with pure silicone encapsulant, the tensile strength of the graphene-based materials significantly increased by 46% and elongation at break decreased only by 7% with the incorporation of 0.1wt% graphene. Furthermore, the presence of graphene can absorb twice as much UV light as pure silicone, which can protect chip and encapsulant. High transmittance values (>90%) were achieved over 400–800nm for 5mm-thick encapsulant film containing 0.01wt% graphene. The refractive index, thermal stability, and dimensional stability also dramatically increased. Owing to their multifunctionality, these hybrid materials are feasible candidates for LED encapsulation.

Nano- and micro-structured random copolymer modified cycloaliphatic epoxy resins for use as light-emitting diode encapsulation

ABSTRACTPoly(2,2,3,4,4,4-Hexafluorobutylmethacrylate–random–glycidolmethacrylate) random copolymer (P(HFBMA-r-GMA)) was synthesized via free radical polymerization. The novel reactive random copolymer was incorporated to modify cycloaliphatic epoxy resins and obtain the nano- or micro- structured composites. The chemical structures of P(HFBMA-r-GMA) were confirmed by Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR). The morphology and light transmittance of the cured epoxy resins were observed by scanning electron microscopy (SEM), transmission electron microscope (TEM) and ultraviolet-visible spectrophotometry (UV-vis), respectively. It is indicated that the optical transmittance of composites were basically kept although the microphase separation occurred in the curing process, which has a profound influence on the mechanical properties and refractive indexes. The thermal properties, surface dewettability and water absorbency of the cured epoxy resins were examined by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), contact angle measurement and immersion test respectively. The experimental results revealed that the values of glass transition temperatures (Tg), surface dewettability and water resistance were effectively improved by the high cross-linking density and the enrichment of the fluorinated random copolymer dispersing in the composites. With respect to the corresponding properties of the neat epoxy resin, P (HFBMA-r-GMA)-0.25 hybrimer embraced the relatively good comprehensive properties, making the modified epoxy resins as good candidates for LED encapsulation.