Phosphor-converted white light-emitting diodes (pc-WLEDs) are emerging as an indispensable solid-state light source for the next generation lighting industry and display systems due to their unique properties including but not limited to energy savings, environment-friendliness, small volume, and long persistence. Until now, major challenges in pc-WLEDs have been to achieve high luminous efficacy, high chromatic stability, brilliant color-rending properties, and price competitiveness against fluorescent lamps, which rely critically on the phosphor properties. A comprehensive understanding of the nature and limitations of phosphors and the factors dominating the general trends in pc-WLEDs is of fundamental importance for advancing technological applications. This report aims to provide the most recent advances in the synthesis and application of phosphors for pc-WLEDs with emphasis specifically on: (a) principles to tune the excitation and emission spectra of phosphors: prediction according to crystal field theory, and structural chemistry characteristics (e.g. covalence of chemical bonds, electronegativity, and polarization effects of element); (b) pc-WLEDs with phosphors excited by blue-LED chips: phosphor characteristics, structure, and activated ions (i.e. Ce 3+ and Eu 2+ ), including YAG:Ce, other garnets, non-garnets, sulfides, and (oxy)nitrides; (c) pc-WLEDs with phosphors excited by near ultraviolet LED chips: single-phased white-emitting phosphors (e.g. Eu 2+ –Mn 2+ activated phosphors), red-green-blue phosphors, energy transfer, and mechanisms involved; and (d) new clues for designing novel high-performance phosphors for pc-WLEDs based on available LED chips. Emphasis shall also be placed on the relationships among crystal structure, luminescence properties, and device performances. In addition, applications, challenges and future advances of pc-WLEDs will be discussed.

Phosphors in phosphor-converted white light-emitting diodes: Recent advances in materials, techniques and properties

Hybrid processes in manufacturing

A new precision finishing process for complex internal geometries using smart magnetorheological polishing fluid is developed. Magnetorheological abrasive flow finishing (MRAFF) process provides better control over rheological properties of abrasive laden magnetorheological finishing medium. Magnetorheological (MR) polishing fluid comprises of carbonyl iron powder and silicon carbide abrasives dispersed in the viscoplastic base of grease and mineral oil; it exhibits change in rheological behaviour in presence of external magnetic field. This smart behaviour of MR-polishing fluid is utilized to precisely control the finishing forces, hence final surface finish. A hydraulically powered experimental setup is designed to study the process characteristics and performance. The setup consists of two MR-polishing fluid cylinders, two hydraulic actuators, electromagnet, fixture and supporting frame. Experiments were conducted on stainless steel workpieces at different magnetic field strength to observe its effect on final surface finish. No measurable change in surface roughness is observed after finishing at zero magnetic field. However, for the same number of cycles the roughness reduces gradually with the increase of magnetic field. This validates the role of rheological behaviour of magnetorheological polishing fluid in performing finishing action.

Design and development of the magnetorheological abrasive flow finishing (MRAFF) process

Technological Advances in Fine Abrasive Processes

Study on Magnetic Abrasive Finishing

Study of the surface modification resulting from an internal magnetic abrasive finishing process

This study presents the application of a new technique, magnetic field assisted finishing, for finishing of the inner surfaces of alumina ceramic components. The experiments performed on alumina ceramic tubes examine the effects of volume of lubricant, ferrous particle size, and abrasive grain size on the finishing characteristics. The finished surface is highly dependent on the volume of lubricant, which affects the abrasive contact against the surface; on the ferrous particle size, which changes the finishing force acting on the abrasive; and on the abrasive grain size, which controls the depth of cut. By altering these conditions, this process achieves surface finishes as fine as 0.02 μm in surface roughness ( R a ) and imparts minimal additional residual stress to the surface. This study also reveals the mechanism to smooth the inner surface of alumina ceramic tube and to improve the form accuracy, i.e. the roundness of inside the alumina ceramic tube.

Internal finishing process for alumina ceramic components by a magnetic field assisted finishing process

The automatic precision polishing technique of three-dimensional complicated micro-curved surfaces of components in extremely low surface roughness and high efficiency is greatly demanded by advanced industrial fields. The existing polishing methods have great difficulty in satisfying these demands. Therefore, three modes (horizontal vibration, vertical vibration and compound vibration) of vibration-assisted magnetic abrasive polishing processes have been developed. Previous research focused on each polishing characteristic. The aims of this paper are to characterize effects of vibration of workpiece on magnetic field, polishing pressure, in-process abrasive behavior and polishing performances in three vibration modes and to describe their machining mechanism. Furthermore, a realization of efficient polishing of a 3D micro-curved surface was confirmed to be possible by the process.

A comparative study: polishing characteristics and its mechanisms of three vibration modes in vibration-assisted magnetic abrasive polishing

The ultimate goal of this project is to develop an efficient finishing process enabling unskilled operators to finish automatically the complicated micro-curved surface and edge surface of the magnesium alloy. The results achieved in the first phase as described in this paper focus on the basic characteristics of the plane and edge surface finishing and deburring of this alloy by the use of vertical vibration-assisted magnetic abrasive finishing process. It demonstrates that realization of efficient finishing of magnesium alloy is possible by the process. The removal volume per unit time of magnesium alloy is larger than that of other materials such as brass and stainless, that is, high-efficiency finishing could be achieved. Micro-burr of magnesium alloy could be removed easily in a short time by the use of the magnetic abrasive finishing process. Furthermore, deburring efficiency considerably increases with vertical vibration assistance.

Takeo Shinmura

Papers

Study on Magnetic Abrasive Finishing

Study of the surface modification resulting from an internal magnetic abrasive finishing process

Internal finishing process for alumina ceramic components by a magnetic field assisted finishing process

A comparative study: polishing characteristics and its mechanisms of three vibration modes in vibration-assisted magnetic abrasive polishing

Vertical vibration-assisted magnetic abrasive finishing and deburring for magnesium alloy