WO2006111562A1

WO2006111562A1 - Novel materials used for emitting light

Info

Publication number: WO2006111562A1
Application number: PCT/EP2006/061711
Authority: WO
Inventors: Frank Kubel; Andreas Rief
Original assignee: Etech Ag; Technische Universitaet Wien
Priority date: 2005-04-20
Filing date: 2006-04-20
Publication date: 2006-10-26
Also published as: AU2006237367A1

Abstract

An luminescent composition emitting electromagnetic radiation when subject to stimuli, wherein the ability to emit electromag- netic radiation is induced and/or enhanced by the preparation of the sample in a nanocrystalline state.

Description

Frank Kubel "II"

NOVEL MATERIALS USED FOR EMITTING LIGHT

Technical Field of the Invention

The invention relates to materials emitting electromagnetic radiation, particularly visible light, when provided with a stimulus .

Technical Background

It is known that certain materials, including natural minerals, emit electromagnetic radiation, particularly visible light (electromagnetic radiation in the human-visible part of the spectrum, wavelengths approximately 400nm-700nm) , when provided with an appropriate stimulus. This stimulus can be electromagnetic radiation of a differing nature, normally of a lower wavelength (higher frequency) , where the phenomenon is termed fluorescence or phosphorescence, and where the energizing radiation may be e.g., ultra-violet light: the stimulus may also be of e.g., energetic electrons or ions, the former involving either direct (electrical circuit) or indirect (electron bombardment) electrical contact. Other stimuli are also possible.

For the purposes of lighting, particularly the lighting of interior or partially enclosed spaces, it has for a long time been desirable to find or create materials which, singly or in mixtures, produce white light in the human visible region. Many such materials have been found, but they have tended to be regarded as less than ideal because of consideration of longevity, spectral shift over time, limited range of conditions of use, etc. Consequently the search for improved materials continues.

One particular application for which improved materials are re- quired is that of fluorescent lamp bulbs. These (usually a solid solution of Mn & Sb in calcium fluoroapatite) currently work by means of ionic bombardment and/or ultraviolet light stimulation from a gas containing mercury vapour. Mercury is classified as a hazardous material, and it is desirable (and, indeed, in some legal jurisdictions, mandated) that the manufacture and use of lamp bulbs containing mercury should cease once a suitable (economically sensible, and environmentally less damaging) substitute is found, e.g., a fluorescent lamp bulb which works with nitrogen gas and noble gas without using mercury vapour. One problem with implementing this change is that the known and existing phosphors, largely developed for use with mercury vapour, do not perform well in other systems.

Most systems known and studied which are capable of electromagnetic radiation emission under certain stimuli are oxides, where the number of disclosures is great. For instance, a new blue- white material, Sr₂CeO₄ (and its Eu-doped form) were announced by Symyx in 1998 after having tested 25,000 rare earth mixed oxides for fluorescence using combinatorial chemistry.

The class of materials which does not use oxides but which uses halides has received much less study, but has been previously disclosed. Much of this work has concentrated on substitution of halides and doping in the system BaF₂, a well-known phosphorescent material, to create hitherto unknown structures, superlat- tices and consequent effects.

Summary of the invention

Based on the above mentioned prior art it is an object of the invention to provide a better fluorescent material . A further object is to provide a better material for a luminescent composition. A further object is to provide a method to induce emis- sion of electromagnetic radiation.

The inventors have the insight that the light emission from these structures can, contrary to previous expert opinion, be strongly stimulated by the introduction of defects, especially external surfaces, in the absence of any doping elements, (which are for preference rare-earth cations, for preference europium, residing in a position in the lattice which is strongly polar i.e., non-centro-symmetric, to show strong optical character and confer this on the material as a whole) .

It is thus possible to induce fluorescent ability in materials not otherwise noted as being fluorescent simply by preparing the materials as extremely fine powders (nanocrystals) , where the surface area to volume ratio is very high, and thus the influence of the polarity induced in the body of the crystal by the cessation of the crystal at the surface pervades much of the volume of the crystal .

The observations in such systems are recent, and so the exact nature of the chemical compounds and their structures are still the subject of theory and academic debate, but their exact nature does not prevent or predetermine this disclosure. It should be noted that, unlike many classical material systems, the optically active systems, like their natural counterparts, are difficult to describe in precise crystallographic terms, their optical activity and thus their usefulness arising rather from the irregularities and defects in the structures than from any regular features .

The present disclosure is thus for an entirely novel class of materials which are capable of emitting electromagnetic radiation under appropriate stimuli. Notwithstanding any other poten- tial uses of the materials, e.g., to emit light under electronic or electromagnetic stimulation, one particular disclosure is that certain of these materials demonstrate the desirable characteristics of stable emission under ultra-violet light / ionic stimulation from ions other than those arising from mercury vapour, thus permitting stable white light produced by fluorescence without involving the use of mercury.

The novel class of materials in particular includes those obtained by the creation of materials at the nanocrystalline size, but does not preclude the use of doping elements.

This disclosure thus claims the novel class of materials obtained by the preparation of the materials in a nanocrystalline form. It further claims the emission of electromagnetic radiation from such materials under suitable stimuli, and devices incorporating these materials and effects.

Synthesis of the systems studied is made by ceramic methods from reagent grade starting materials in inert (corundum, platinum, graphite) crucibles. Reduction is made in a nitrogen-hydrogen furnace .

To demonstrate the validity of this approach a wide number of systems have been studied, which include systems based on Ca₂ZnSi₂O₇ plus Zn₂SiO₄ (this showing a pale blue versus UV stimulation at 254nm and 366nm, although Zn₂SiO₄ (Willemite) is known as a natural strong blue phosphor) .

The effect has also been shown in undoped MgCa₂O₄.

A further compound showing luminescence is the SrMgF system.

Claims

1. A luminescent composition emitting electromagnetic radiation when subject to stimuli, wherein the ability to emit electromagnetic radiation is induced and/or enhanced by the preparation of the sample in a nanocrystalline state.

2. The luminescent composition according to claim 1, made by suitable solid state manufacturing techniques such as but not limited to precipitation, ^x shake and bake' and sol gel.

3. The luminescent composition according to one of claims 1 to 2, wherein said stimuli are from a group comprising pressure, oscillating magnetic fields, introduction of damage, introduction of defects, especially external surface defects, and further procedures to induce emission of electromagnetic radiation.

4. A method to induce emission of electromagnetic radiation from one of the materials as in claims 1-3 or from mixtures of two or more materials, where at least one material is a material as in claims 1-3 which are or have been subject to suitable stimuli .

5. The method according to claim 4, wherein said stimuli includes at least one stimulus comprising electromagnetic radiation or wherein said stimuli includes at least one stimulus comprising electromagnetic radiation falling in the ultra-violet part of the spectrum or wherein said stimuli includes at least one stimulus comprising electromagnetic radiation falling at least partly in the human-visible part of the electromagnetic spectrum.

6. The method according to claim 4, wherein said stimuli in- eludes at least one stimulus comprising electrons or wherein said stimuli includes at least one stimulus comprising electrons supplied via direct electrical circuit or via indirect electron bombardment .

7. The method according to claim 4, wherein said stimuli includes at least one stimulus comprising ions.

8. A light emitting device providing emission of electromagnetic radiation from at least one of the materials of the luminescent composition according to claims 1 to 7.

9. The device according to claim 8, wherein the emitted electromagnetic radiation falls at least partly in the human- visible part of the electromagnetic spectrum and/or wherein said stimuli include at least one stimulus comprising electromagnetic radiation, particularly falling in the ultra-violet part of the spectrum.

10. The device according to claim 8, wherein said stimuli includes at least one stimulus comprising electrons, particularly electrons supplied via direct electrical circuit or electrons supplied via indirect electron bombardment or said stimuli includes at least one stimulus comprising ions.

11. The device according to one of claims 8 to 10, wherein such device is a light/lamp bulb or a fluorescent light/lamp bulb or a light-emitting diode or a solid full color display.

12. The device according to one of claims 8 to 11, wherein the light emitting composition comprises a fluorescent paint or ink or colorant or dye ort dyestuff .

13. The device according to one of claims 8 to 12, wherein the such device produces directly 'white light' either and/or by use of a mixture and/or an admixture of materials and/or by using filters and/or absorption and re-emission to achieve 'white light' .