Luminescent Hafnia Composition

This invention relates to phosphors emmitting visible luminescence under ultra violet excitation, and more particularly, to a cubic, yttria-stabilized HfO.sub.2 phosphor, doped with cerium.

The term phosphor is generally employed in the art to describe luminescent or fluorescent solids which are composed of two ingredients; a "host" matrix, and an "activator" impurity. Such phosphors find many diverse uses including laser applications, as fluorescent lamps, and color cathode ray tubes.

The properties of the cubic form of HfO.sub.2 phosphors are unknown since the material normally exists in its monoclinic crystalline state. The only reference in the prior art to a "probable cubic form" of HfO.sub.2 is that of L. Passerini: Gazz, Chem. Ital. 60, 762-766 (1930), who hypothesized that cubic HfO.sub.2 should have a cell edge a.sub.o =5.155 A. This value, however, was extrapolated from measurements of cubic cell-edges of the crystal HfO.sub.2 --CeO.sub.2, where samples ranging from 95 to 5% CeO.sub.2 were measured. Pure HfO.sub.2 has not been actually found to have a cubic (fluorite) structure; when fired up to 1300.degree. C. it is, as we have verified, monoclinic.

In accordance with this invention a new phosphor compound has been discovered, consisting essentially of 99.0 to 99.9 weight percent of yttria and cubic hafnia in a HfO.sub.2 :Y.sub.2 O.sub.3 mole ratio of about 3:1 and including from 0.1 to 1.0 weight percent of cerium dopant.

It is an object of this invention to produce a phosphor that is photoluminescent and cathode luminescent in the blue-green region of the optical spectrum.

It is a further object of this invention to provide a host matrix for trivalent cerium (Ce.sup.3+) which will fluoresce in the blue-green region of the optical spectrum.

Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings wherein:

FIG. 1 discloses the luminescent emmision curve for the phosphor having a composition of 3HfO.sub.2 :1Y.sub.2 O.sub.3 :0.4%Ce.sup.3+, 0.4%Li.sup.+.

FIG. 2 discloses the emmission curve for the phosphor having a composition of 3HfO.sub.2 1:Y.sub.2 O.sub.3 :0.4%Ce.sup.3+, 0.8%Li.sup.+.

As is essential in phosphor chemistry, purity of the ingredients used is of fundamental importance. Until recently, it has been difficult to obtain HfO.sub.2 of the necessary purity (99.999%) required. Currently, pure HfO.sub.2 can be obtained from Ames Laboratory, Ames, Iowa. Alternatively, pure HfO.sub.2 can be prepared by dissolving pure hafnium metal in hydrofluoric acid, then precipitating Hf(OH).sub.4 with ammonium hydroxide, filtering, rinsing first with dilute ammonia, then with H.sub.2 O, and finally baking the resultant white powder in a vacuum furnace. The Y.sub.2 O.sub.3 used should be about 99.999% pure.

Pure cubic, yttria stabilized HfO.sub.2 does not emit in the blue region of the spectrum, in the absence of dopants, under U.V. excitation. However, mixtures of Y.sub.2 O.sub.3 and HfO.sub.2 have been predicted by E. N. Isupova et al., Neorg. Mater. Vol. 5, No. 11, pp 1948-1952, Nov., 1969 as forming a cubic (fluorite) structure. It is critical that the mole ratio Y.sub.2 O.sub.3 :HfO.sub.2 is not less than 1 to 3 in order to maintain this homogeneous cubic phase after firing up to 1300.degree. C.; if more HfO.sub.2 is introduced as a host into the system, other phases, particularly monoclinic, will enter into the crystalline structure.

The doping of the HfO.sub.2 --Y.sub.2 O.sub.3 matrix by Ce.sup.3+ ions is preferably carried out by preparing dilute aqueous solutions, e.g. 0.001 molar, of Ce(NO.sub.3).sub.3, or, other suitable Ce.sup.3+ salts, either as solutions or dry; however, the double nitrates of Ce.sup.3+ and Mg.sup.2+ have also proven satisfactory. After a measured amount of aqueous Ce.sup.3+ ions have been added to the mixture, the resultant paste is preferably dried in air before high-temperature firing. An initial high temperature firing before Ce.sup.3+ doping has been found not to affect the final properties of the phosphor. The addition of small amounts of mineralizers, such as up to 1 weight percent of NaNO.sub.3, LiNO.sub.3, Mg(NO.sub.3).sub.2 or LiCl, for example, have proven useful in creating a more uniform material, but are not essential for luminescense in the blue portion of the spectrum. Preferably, the amount of mineralizers added, usually NaNO.sub.3 or LiCl, is molecularly equivalent to Ce.sup.3+ added. The mineralizer is added by using a measured amount of an aqueous solution, before the drying in air and subsequent high temperature firing.

Once the desired composition has been mixed together, a high temperature firing to about 1300.degree. C. for a suitably long time period (2-100 hours) should commence. Preferably, the phosphor composition is fired after being placed in an extremely pure container of pyrolytic boron nitride (preferred), or, equivalently in quartz (vitreosil), alundum, graphite, and the like. Either an inert atmosphere, such as argon, or a slightly reducing one, such as 99%N.sub.2, 1%H.sub.2, is preferred for the 1300.degree. C. firing, but other equivalent environments can be employed. As was stated previously, the firing at about 1300.degree. C. can occur before the addition of Ce.sup.3+ and the mineralizer.

After completion of this firing and subsequent cooling, a second firing at temperatures of the order of 1000.degree. C. for a period of 1 to 10 hours in an oxidizing atmosphere, such as air, has been found to optimize luminescence. Air firing has been found particularly beneficial when a graphite boat is used for the first firing.

To 2 g of composition 3HfO.sub.2 :1Y.sub.2 O.sub.3 in a pyrex beaker was added a sufficient dilute solution of Ce(NO.sub.3).sub.3 and LiCl to get after drying a powder of composition 3HfO.sub.2 :1Y.sub.2 O.sub.3 :0.4%Ce.sup.3+ :0.4%Li.sup.+. The mixture was allowed to dry on a hot plate. It was then transferred to a boat of pyrolytic boron nitride (PBN) and was fired up to 1300.degree. C. in an argon atmosphere for 40 hours. During the firing the PBN boat was covered with a flat lid of boron nitride. After cooling to room temperature, the boat was transferred to a furnace kept at 1000.degree. C. and was kept there an additional 16 hrs in air. The 3HfO.sub.2 :Y.sub.2 O.sub.3 :0.4%Ce.sup.3+ :0.4%Li.sup.+ obtained is a crystalline cubic powder of light blue color. When excited by 3650 A irradiation it emits blue light, in the form of the broad spectrum shown in FIG. 1. The same spectrum is shown when the phosphor is excited by cathode rays.

A sample of composition 3HfO.sub.2 +Y.sub.2 O.sub.3 (dry mixed powder) was fired 60 hrs in argon as in Example 1. No room temperature luminescence was observed after firing under 3650 A. The powder was fired a second time in air at 1,000.degree. C. for 2 hours. Again no room temperature luminescence was observed. To the powder was now added enough Ce(NO.sub.3).sub.3 and LiCl solutions to get, after drying, the composition 3HfO.sub.2 :Y.sub.2 O.sub.3 :0.4%Ce.sup.3+ :0.8%Li.sup.+. After a 6 hr firing of this mixture at 1300.degree. C. in argon a blue luminescent powder was obtained having the emission spectrum shown in FIG. 2.

We have found phosphors which emit blue photoluminescence and cathodeluminescence in the blue-green region of the spectrum (4700-4900 A) that consist essentially of HfO.sub.2 :Y.sub.2 O.sub.3 with Ce.sup.3+ as a dopant in concentrations ranging from about 0.1-1.0 weight %, and most preferably, about 0.4%Ce.sup.3+. The Ce ions have been verified as Ce.sup.3+ by electron spin resonance techniques; Ce.sup.4+ does not work in the visible spectrum.

While there have been shown and described what are at present considered the preferred embodiments of the invention, it will be apparent to those skilled in the art that various changes and modifications may be made herein without departing from the scope of the invention as defined by the appended claims.

Obviously many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.