High Power LED Research Articles

High-performance Al2O3‒YAG:Ce composite ceramic phosphors for miniaturization of high-brightness white light-emitting diodes

The miniaturization of high-brightness white light-emitting diodes (WLEDs) is limited by the low thermal performance of phosphors. In this study, the microstructure, optical properties, and thermal performance of Al2O3–Y3Al5O12:Ce3+ (Al2O3–YAG:Ce) composite ceramics fabricated by hot pressing were investigated. By promoting the growth of Al2O3 grains while maintaining a high composite density, thermal performance of the composite ceramics was significantly increased. The thermal conductivity of a Al2O3–40-vol% YAG:Ce ceramic reached 21.8 W/m/K, which is close to the theoretical value. In addition, this composite ceramic exhibited the highest energy efficiency. After packaging with a high-power LED chip with dimensions of 1 mm × 1 mm, a high luminous flux of 639 lm was generated, while the reduction in output power at 250 °C was as low as 6%. This indicated excellent high-temperature stability and potential for applications in solid-state lighting.

(Invited) Basic Ammonothermal Growth of Bulk GaN Single Crystal Using Sodium Mineralizers

GaN is a promising semiconductor of wide band gap of 3.4 eV at room temperature and can be used as a substrate material for blue/green/UV light-emitting diode (LED) and laser diode (LD) [1]. Sapphire has been the most widely used substrate for manufacturing GaN-based LEDs. However, GaN films grown on sapphire substrates include many critical defects owing to the large lattice mismatch (~15%) and large difference of thermal expansion coefficients (~25%) between GaN and sapphire. Thus, GaN substrates are needed for fabrication of the high power LEDs and LDs. Two typical routes, ammono-basic and ammono-acidic, have been studied in the ammonothermal growth of GaN crystals. Addition of alkali (Li, Na, and K) elements to ammonia typically results in a basic ammonia solution, whereas addition of halogens (F, Cl, Br, and I) to ammonia typically results in an acidic ammonia solution. The acidic ammonothermal method has a fast crystal growth rate but requires installation of Pt liner due to the corrosive environment. Thus, the basic ammonothermal method is regarded as more appropriate for mass production of GaN crystals [2]. Only a few types of basic mineralizer including NaNH2 [3] and KNH2 [4] have been used previously for the crystal growth of GaN. The reports of comparison with other sodium-based mineralizers for ammonothermally grown GaN crystals have not to be achieved. Potassium mineralizers were not included in this study because they are more sensitive to oxygen and moisture and corrosive to autoclave compared to sodium mineralizers. In this study, we demonstrate the growth rate, crystal quality, and surface morphology of the as-grown GaN single crystals according to sodium amide, sodium azide, and sodium metal as mineralizer. Also, we describe the morphology and properties of approximately 2 inch GaN crystals grown by ammonothermal method using sodium metal mineralizer. Reference [1] S. Nakamura, G. Fosol, The Blue Laser Diode, Springer, Berlin, 1997, chap. 1. [2] R. Dwilinski, R. Doradzinski, J. Garczynski, L. P. Sierzputowski, A. Puchalski, Y. Kanbara, K. Yagi, H. Minakuchi, H. Hayashi, J. Cryst. Growth 311 (2009) 3015. [3] T. Hashimoto, F. Wu, J. S. Speck, S. Nakamura, J. Cryst. Growth 310 (2008) 3907. [4] R. Dwilinski, R. Doradzinski, J. Garczynski, L. P. Sierzputowski, A. Puchalski, Y. Kanbara, K. Yagi, H. Minakuchi, H. Hayashi, J. Cryst. Growth 310 (2008) 3911.

Photometric Long-Range Positioning of LED Targets for Cooperative Navigation in UAVs

Autonomous flight with unmanned aerial vehicles (UAVs) nowadays depends on the availability and reliability of Global Navigation Satellites Systems (GNSS). In cluttered outdoor scenarios, such as narrow gorges, or near tall artificial structures, such as bridges or dams, reduced sky visibility and multipath effects compromise the quality and the trustworthiness of the GNSS position fixes, making autonomous, or even manual, flight difficult and dangerous. To overcome this problem, cooperative navigation has been proposed: a second UAV flies away from any occluding objects and in line of sight from the first and provides the latter with positioning information, removing the need for full and reliable GNSS coverage in the area of interest. In this work we use high-power light-emitting diodes (LEDs) to signalize the second drone and we present a computer vision pipeline that allows to track the second drone in real-time from a distance up to 100 m and to compute its relative position with decimeter accuracy. This is based on an extension to the classical iterative algorithm for the Perspective-n-Points problem in which the photometric error is minimized according to a image formation model. This extension allow to substantially increase the accuracy of point-feature measurements in image space (up to 0.05 pixels), which directly translates into higher positioning accuracy with respect to conventional methods.

Manetoresistance of Ultralow-Hole-Density Monolayer Epitaxial Graphene Grown on SiC.

Silicon carbide (SiC) has already found useful applications in high-power electronic devices and light-emitting diodes (LEDs). Interestingly, SiC is a suitable substrate for growing monolayer epitaxial graphene and GaN-based devices. Therefore, it provides the opportunity for integration of high-power devices, LEDs, atomically thin electronics, and high-frequency devices, all of which can be prepared on the same SiC substrate. In this paper, we concentrate on detailed measurements on ultralow-density p-type monolayer epitaxial graphene, which has yet to be extensively studied. The measured resistivity ρxx shows insulating behavior in the sense that ρxx decreases with increasing temperature T over a wide range of T (1.5 K ≤ T ≤ 300 K). The crossover from negative magnetoresistivity (MR) to positive magnetoresistivity at T = 40 K in the low-field regime is ascribed to a transition from low-T quantum transport to high-T classical transport. For T ≥ 120 K, the measured positive MR ratio [ρxx(B) − ρxx(B = 0)]/ρxx(B = 0) at B = 2 T decreases with increasing T, but the positive MR persists up to room temperature. Our experimental results suggest that the large MR ratio (~100% at B = 9 T) is an intrinsic property of ultralow-charge-density graphene, regardless of the carrier type. This effect may find applications in magnetic sensors and magnetoresistance devices.

High-Power GaN-Based Vertical Light-Emitting Diodes on 4-Inch Silicon Substrate.

We demonstrate high-power GaN-based vertical light-emitting diodes (LEDs) (VLEDs) on a 4-inch silicon substrate and flip-chip LEDs on a sapphire substrate. The GaN-based VLEDs were transferred onto the silicon substrate by using the Au–In eutectic bonding technique in combination with the laser lift-off (LLO) process. The silicon substrate with high thermal conductivity can provide a satisfactory path for heat dissipation of VLEDs. The nitrogen polar n-GaN surface was textured by KOH solution, which not only improved light extract efficiency (LEE) but also broke down Fabry–Pérot interference in VLEDs. As a result, a near Lambertian emission pattern was obtained in a VLED. To improve current spreading, the ring-shaped n-electrode was uniformly distributed over the entire VLED. Our combined numerical and experimental results revealed that the VLED exhibited superior heat dissipation and current spreading performance over a flip-chip LED (FCLED). As a result, under 350 mA injection current, the forward voltage of the VLED was 0.36 V lower than that of the FCLED, while the light output power (LOP) of the VLED was 3.7% higher than that of the FCLED. The LOP of the FCLED saturated at 1280 mA, but the light output saturation did not appear in the VLED.

Application of Graphene Silicone Grease in heat dissipation for the Intel Core i5 Processor

Graphene was known as the material that owning many superiority properties and high thermal conductivity. Thermal conductivity of single-layer graphene was up to 5200 W/mK (compared to the thermal conductivity of Carbon nanotubes 2000 W/mK and Silver 410 W/mK). This had suggested that graphene is the most potential material for heat dissipation applications for electronic devices, such as a computer microprocessor, high power LED... To enhance the dispersion of the GNPs silicone matrix, we were functionalized graphene nanoplatelets (GNPs) with carboxyl (-COOH) groups. The silicone thermal greases containing GNPs were prepared by High- Energy Ball Milling method (8000D Mixer /Mill). The results of SEM, FTIR, Raman showed the presence of the carboxyl groups in GNPs and GNPs uniform dispersion dispersed in grease. The results of thermal conductivity from Transient Hot Bridge THB-100 showed that thermal conductivity enhancement was up to 234 % with Gr-COOH 1.0 vol.%. Thermal grease is used as a thermal interface material to coolants for Intel Core i5 processor. The results of thermal dissipation efficiency shown the saturation temperature of the processor using thermal grease containing 1.0 vol.% Gr-COOH decreased 4℃, compared to the silicone grease.

The effect of curing mode of a high-power LED unit on bond strengths of dualcure resin cements to dentin and CAD/CAM resin blocks.

The purpose of this study was to investigate the effects of curing mode with a high-power LED curing unit (VALO) in terms of microtensile bond strength (μTBS) to dentin and microhardness of two dual-cure resin cements. Panavia V5 (V5) and Rely X Ultimate (RXU) were polymerized using one of three curing modes with VALO or self-cure mode to bond a CAD/CAM resin block to a flat dentin surface. Specimens were sectioned and subjected to μTBS test. Vickers hardness values of V5 and RXU were also measured. Two-way ANOVA indicated curing mode and resin cement affected μTBS. For self-cure mode, V5 had significantly higher μTBS than RXU (p<0.05). Higher irradiance did not always provide higher μTBS of dual-cure resin cement to dentin. One-way ANOVA indicated the curing modes affected microhardness of each cement. As for microhardness of RXU, there were significant differences between selfcure mode and light cure modes (p<0.05).

Synthesis of High Refractive Index Epoxy Modified Methyl Phenyl Silicone Resin and Amine Phenyl Silicone Resin for LEDs Packaging

A series of epoxy modified methyl phenyl silicone resins with different epoxy values were prepared by an approach of combination of hydrolytic sol-gel and non-hydrolytic sol-gel process. Then, in order to solve poor compatibility between epoxy modified silicone resins and commercial amine curing agents, amine phenyl silicone resin curing agent was synthesized by non-hydrolytic sol-gel method. The test results exhibited that the process of combination of hydrolytic sol-gel and non-hydrolytic sol-gel process had a significant effect on the elimination of hydroxyl groups. A lot of experiments and mechanism analysis exhibited that the activity and content of hydroxyl groups are two important factors for ring-opening of epoxy groups and gelation of product. So, epoxy groups can be stably and efficiently introduced in the terminal of product in the course of non-hydrolytic sol-gel process, which hugely avoided ring-opening of epoxy groups. Subsequently, epoxy modified silicone materials were obtained by thermal curing reaction between epoxy modified silicone resins with different epoxy values and amine phenyl silicone resin. And the curing reaction can be completed at 80 °C for 2 h. Transmittance of light, thermogravimetric (TG) analysis, differential scanning calorimetry (DSC) and reliability test were used to analyze performance of epoxy modified silicone materials. The results exhibited that prepared silicone materials had a high transmittance (>90%), strong adhesive strength, good thermal stability and mechanical performance. Therefore, epoxy modified silicone material can be applied in high-power LEDs packaging.

An Archimedean Copula Function-Based Prediction Method for High-Power White LED Considering Multi-Performance

Lumen, chromaticity, and correlated color temperature CCT are the three main performance indicators of failure of high-power white light-emitting diodes (LEDs) (HPWLEDs). The existing data-driven (DD) method to predict the life of LEDs often considers that three performance indicators are independent, or only one or two performance indicators are used for prediction, which may not be accurate enough. To solve this problem, a method based on Archimedean Copula function is proposed to describe the correlation of multiple performance indicators. The test data reported from LUXEON is used as an example to establish a 3-D life prediction model by combining the thresholds of three performances to predict the life of LEDs, and the application of Copula function is explained. The results show that there is a little influence on life prediction of LEDs among the different Copula function structures, while the difference between considering single performance and multi-performance is obvious. The mean time to failure (MTTF) of which is proven to be more conservative than the L70 life predicted by the conventional method.

Lattice-constant and band-gap tuning in wurtzite and zincblende BInGaN alloys

InGaN light-emitting diodes (LEDs) are more efficient and cost effective than incandescent and fluorescent lighting, but lattice mismatch limits the thickness of InGaN layers that can be grown on GaN without performance-degrading dislocations. In this work, we apply hybrid density functional theory calculations to investigate the thermodynamic stability, lattice parameters, and bandgaps of wurtzite and zincblende quaternary BInGaN alloys. We find that the wurtzite phase is more stable and can be lattice matched to GaN for BInGaN compositions containing up to ∼30% boron. The lattice match with GaN decreases strain and enables thicker active layers that mitigate Auger recombination and increase the efficiency of the LEDs. The bandgap of the alloy remains tunable throughout the visible spectrum. Our results indicate that BInGaN alloys are promising alternatives to InGaN for high-efficiency, high-power LEDs.

Experimental Study on LED Heat Dissipation Based on Enhanced Corona Wind by Graphene Decoration

To enhance the heat dissipation of high-power light-emitting diodes (LEDs), corona wind based on corona discharge was promoted. In this paper, we aimed at reinforcing the intensity of corona wind by modifying the needle electrode with graphene solution. To explore the differences in the graphene-coated electrode and the original bare electrode on the electrical characteristics, the intensity of corona wind, and LED heat dissipation, a corona wind cooling system and a thermal test system were built for the experimental study. The results showed that the graphene-coated electrode system could produce corona wind at a lower voltage. In addition, the discharge current of the graphene-coated electrode system is higher than that of the bare electrode system under the same discharge voltage. The graphene-coated electrode system can induce a faster corona wind and the highest speed is about 3.4 m/s, showing that it is more efficient than the bare electrode system in corona wind production. Moreover, the temperature drop of LEDs for the graphene-coated system is greater, which means that the corona wind cooling system can reduce the junction temperature of the LED chips effectively. Ultimately, as the discharge power increases, the average convective heat transfer coefficient increases. Indeed, the result of this paper can be used for the wide variety of the graphene-coated corona wind generator application as a promising technology for the LED heat management.

YAGG:Ce transparent ceramics with high luminous efficiency for solid-state lighting application

A series of Y2.985Al5–xGaxO12:0.015Ce (YAGG:Ce, x = 0, 1, 2, 3, 4, 5) transparent ceramics were prepared via a solid-state reaction method. Two-step sintering technique was proved to be an effective approach to prepare functional ceramics with high Ga concentration, and Y3Ga5O12 (YGG) transparent ceramic was successfully prepared for the first time. According to the variation of Al/Ga ratio, regulation of band structure and luminescence properties of YAGG:Ce transparent ceramics were effectively investigated. When Ga substitutes Al sites, the tetrahedral site is more favorable compared to the octahedral site for Ga to occupy according to the first-principle calculation. A continuous blue shift of the emission from 565 to 515 nm was achieved as Ga was gradually introduced into Y3Al5O12:Ce matrix. High quality green light was obtained by coupling the YAGG:Ce ceramics with commercial blue InGaN chips. Transparent luminescence ceramics accomplished in this work can be quite prospective for high power LED application.

Thermal conductivity performance of nanoporous plate for high-power LED lighting and power electronics

Comparative results of measurements of the thermal resistance of a new nanoporous material based on aluminium oxide are presented. Low-cost high-heat-conducting printed circuit boards for high-power LEDs, LED lighting devices and power electronics on the basis of the presented material have been developed and manufactured. Comparison of a set of performances of nanoporous aliminium oxide and aluminium nitride ceramics demonstrate advantages of the material developed. Its thermal characteristics is inferior to the AlN ceramics by 30%, but the cost of the proposed material is lower by almost four times.

High Moisture Accelerated Mechanical Behavior Degradation of Phosphor/Silicone Composites Used in White Light-Emitting Diodes.

In a high-power white light emitting diode (LED) package, the phosphor/silicone composite is typically used for photometric and colorimetric conversions, ultimately producing the white light. However, the phosphor/silicone composite is always exposed under harsh environments with high temperature, high blue light irradiation and high moisture when the LED operates. Therefore, its reliability issue has become one of the critical bottlenecks to improve the lifetime of a high-power white LED package. As the curing process and mechanical behavior of phosphor/silicone composite essentially determine its reliability, this paper firstly uses an in situ viscosity monitoring approach combined with Differential Scanning Calorimetry (DSC) and Fourier Transform Infrared Spectroscopy (FTIR) analysis to explain the curing mechanism of a phosphor/silicone composite by taking the effects of temperature and phosphor mass fraction into consideration. Then, the mechanical properties of phosphor/silicone composites aged under a long-term high moisture condition are evaluated by using the tensile test. Meanwhile, the finite element (FE) simulations, the Mori–Tanaka theoretical estimations and the microstructure analysis are applied to investigate the high moisture induced degradation mechanisms. The results show that: (1) the in situ measured isothermal viscosity curves of both pristine silicone and phosphor/silicone composites follow the Arrhenius empirical model, and high temperature and high phosphor mass fraction can increase the curing rate; (2) the hydrosilylation reaction between silicones determines the curing mechanism of phosphor/silicone composite; (3) the tensile test, FE simulation and Mori–Tanaka theoretical prediction results confirm that the Young’s modulus of phosphor/silicone composite increases by gradually adding phosphors; and (4) the Young’s modulus of phosphor/silicone composite increases after the high moisture ageing test, which can be attributed to the oxidation and cross-linking reaction of silicone and the hydrolysis of phosphor powders.

Growth of star gooseberry-like nickel–phosphorus films on aluminium as thermal interface material for light-emitting diode application

Electroless deposition of nickel–phosphorus (Ni–P) on aluminium alloy and the effects of deposition time (1 to 5 min) on the structural, morphological and thermal properties of the coatings are investigated. As the deposition time increases, it leads to growth of star gooseberry-like films. The deposition of Ni–P films has been confirmed by X-ray diffraction spectroscopy and field emission scanning electron microscopy. The prepared films are stacked with high power LED package and the thermal management performance are tested by applying various driving current and temperatures. The Ni–P films shows a better thermal management compared to bare Al substrates. As the deposition time increases up to 4 min, it show gradual increase in thermal conduction, but decreases upon further coating. Ni–P 4 min film, efficiently reduces the rise in junction temperature (Tj) at a driving current of 700 mA from 81.8 to 68.9 °C at 50 °C compared to other ambient condition. Also, the film has very low interface thermal resistance (Rth-int) of 1.09 W/K at 25 °C. The best thermal performance of Ni–P 4 min film is attributed by the high crystallinity, optimized thickness, smooth surface and star gooseberry-like morphology which are effectively acting as conductive pathways for directional thermal transport.

Pinching a triboelectric nanogenerator using soft pottery for powering electronics

In this paper, soft pottery was utilized for the first time as the triboelectric material to prepare a triboelectric nanogenerator (SP-TENG) and the triboelectric polarity of soft pottery was demonstrated through a series of experiments, which can get electrons during the triboelectrification process. Some commercial materials, such as nylon, conductive copper tape, and paper card, were made as the triboelectric pair, conductive electrode, and supporting structure, respectively. According to the results, the approximate values of open circuit voltage (Voc) can extend to 179 V, and the short-circuit current (Isc) can reach 5 μA. To demonstrate the generated electrical output of SP-TENG, an electronic calculator and 54 green high-power light-emitting diodes (LEDs) can be easily powered. This research demonstrated the feasibility to utilize recyclable material to prepare TENG for harvesting energy, which could facilitate the wide and sustainable workable application of TENGs.

Color converter based on transparent Ce:YAG ceramic by different package structure for high-power white LED

Ce: GdYAG ceramics and CaAlSiN3: Eu2+ phosphor were fabricated by solid-state reaction. CaAlSiN3: Eu2+ phosphor films with different content were manufactured on Ce: YAG ceramic by the spin-coating method. The Ce: YAG ceramics doping with different Gd3+ content is labeled as Gd-doping structure. The red phosphor film placing Ce: YAG ceramic close to the chips is labeled as R–Y structure, similarly, the red phosphor film adjacent to the chips is named as Y-R structure. The main emission peak has redshift with Gd content increase. The PL spectra have a shift to the red region with the increasing of the red phosphor content in the R–Y structure and Y-R structure. As-prepared samples were further fixed on the substrate to manufacture the ceramic covered film (CCF)-based LEDs and ceramic-based LEDs. The color temperature can be regulated through the three structure. The luminous efficacy in the R–Y structure, Y-R structure, and Gd-doping structure is 89.16 lm/w, 86.8 lm/w, and 62.9 lm/w at about 3500K, respectively. The CRI in the R–Y structure, Y-R structure, and Gd-doping structure is 64.6,68.8, and 62.2 at about 3500K, respectively. The temperature in the R–Y structure, Y-R structure, and Gd-doping structure 83.3 °C, 81 °C, and 124.7 °C at about 3500K, respectively. The heat dissipation capability of R–Y structure and Y-R structure are superior to that of the Gd-doping structure. The result shows that the R–Y structure has the best luminous efficacy at a similar CCT which improved 41.7% comparing to the Gd-doping structure. The Y-R structure has the best CRI (improve 10.6% comparing to the Gd-doping structure) and heat dissipation capability (decreased 35.1% comparing to the Gd-doping structure) at a similar CCT. The way of Ce: YAG ceramic coated CaAlSiN3: Eu2+ phosphor will be a promising candidate to achieve warm white for high power LEDs.

Numerical Simulation, Machining and Testing of a Phase Change Heat Sink for High Power LEDs.

Thermal management is crucial to guarantee the normal operation of light-emitting diodes (LEDs) Phase change heat sink is superior to traditional metal solid heat sink due to very small thermal resistance. In this study, a new type of phase change heat sink for high power LEDs is first designed. Then, the fabrication process of boiling structures at the evaporation surface of the phase change heat sink is discussed and analyzed. To make a comparison and deep discussion, the machining process is simulated through the FEM (finite element analysis) software, DEFORM-3D. Last but not least, heat transfer performance of the fabricated phase change heat sink is tested. Results have shown that the designed new type of phase change heat sink has superior heat transfer performance and is suitable for heat dissipation of high-power LEDs.

Novel highly efficient and thermally stable Ca2GdTaO6:Eu3+ red-emitting phosphors with high color purity for UV/blue-excited WLEDs

Nowadays, it is very stringent to develop high-performance red-emitting phosphors to accelerate the new generation of high-power white light-emitting diodes (WLEDs). In this regard, we synthesized novel highly efficient Eu3+-activated double perovskite Ca2GdTaO6 phosphors via a conventional solid-state reaction method. The phosphors possessed pure phase according to the analysis of their X-ray diffraction patterns. Under 396 nm near-ultraviolet light or 466 nm blue light excitation, they exhibited a dominant red emission centering at 615 nm, and the values for intensity ratio of red to orange emission of Ca2Gd1-xTaO6:xEu3+ samples were very high, which indicated that Eu3+ ions occupied low symmetric sites in this host. Besides, the CIE chromaticity coordinates of the optimal sample Ca2Gd0.6TaO6:0.4Eu3+ were calculated to be (0.6676, 0.3321) and the color purity could reach up to a high value of 96%. And the internal quantum efficiencies were determined to be 82% and 83% when excited at 396 and 466 nm, respectively. What's more, the thermal quenching effect on the emission spectra of Ca2Gd0.6TaO6:0.4Eu3+ phosphors excited at 396 and 466 nm were also characterized and the corresponding emission intensities at 423 K remained 78% and 82% of that at room temperature, respectively. Consequently, the results indicated that the as-obtained red-emitting Ca2GdTaO6:Eu3+ phosphors were promising red components for warm WLEDs.

Predistortion and pre-equalization for nonlinearities and low-pass effect mitigation in OFDM-VLC systems.

The orthogonal frequency division multiplexing (OFDM) transmission is a promising candidate for visible light communication (VLC) applications. However, the nonlinearity in the optical power emitted by the high-power light-emitting diode (HPLED) as a function of current and temperature implies drastic OFDM-VLC performance degradation. The first part of this work experimentally confirms and models this degradation due to temperature in a white HPLED. The higher attenuation at high frequencies, which is inherent to the HPLED and which is accentuated by the effect of the intrinsic capacitance of the photodiode, is another factor of degradation due to the reduction in the signal-to-noise ratio (SNR) at the receiver for such frequencies. For the mitigation of these combined effects, we propose a predistortion and a digital pre-equalization scheme using a luminous feedback signal at the transmitter side. The VLC system is modeled so that the operating points are mathematically deduced and evaluated by simulations and by an experimental setup. By allowing the linearization of the transmitted light signal and the maintenance of a uniform SNR among all OFDM subcarriers, the performance improvement is confirmed in comparison with other schemes, such as a VLC system without predistortion, predistortion with fixed parameters, as well as a simple post-equalization.