This invention relates to phosphor materials and is particularly concerned with

phosphor materials which emit blue light when energised.

In US Patent No. 3 767 589 a bismuth activated yttrium niobate blue phosphor is

described. An example of such a phosphor is referred to with the formula

Yø 9 ₇ B10 ₀ 3NDO4 and its brightness /wavelength characteristics are quoted. The example is described as being inferior to a phosphor of different composition, namely bismuth

activated gadolinium niobate.

It is an object of the invention to provide a bismuth activated yttrium niobate blue

phosphor of improved properties.

According to the invention a bismuth activated yttrium niobate phosphor material is characterised in that the proportion of niobium to yttrium is greater than 1.2 to 1 , and is

preferably greater than 1.7 to 1.

In one preferred embodiment the ratio of niobium to yttrium is 2.5 to 1. The accompanying drawings show characteristics of phosphor materials embodying

the invention compared with other phosphor materials. In the drawings:

Fig. 1 and Fig. 2 show voltage/lurninance relationships and current/luminance relationships for a phosphor material embodying the invention in comparison with the P47 phosphor (yttrium silicon oxide : cerium), and

Fig. 3 and Fig. 4 show voltage luminance relationships and current luminance relationships for a phosphor material embodying the invention in comparison with the known yttrium niobate : bismuth phosphor.

One method of preparing a phosphor embodying the invention comprises taking the

relevant oxides in appropriate proportions. The oxides Y2®? _>> Nb2θ5 and B12O3 are

mixed together as a slurry with 2-propanol. The mixture is then fired in open alumina crucibles at 1000°C for 30 minutes. After cooling, the weakly luminescent product is ground and then refired for 30 to 60 minutes at 1250°C to 1400°C. The exact temperature and time for optimum brightness depends on the proportions of the constituents used. The powder is then ground and sieved through a fine nylon mesh after which it is ready for use.

The effect of changing the ratio of niobium to yttrium from between 1.0 and 3.0, as well as varying the amount of bismuth oxide from between 0.4% to 1.0%, is shown in

Table 1. The optimum concentration of bismuth oxide was found to be around 0.4%, although the concentration of bismuth is not critical. Table 1

Luminance(L) of Yttrium Niobate : Bi over a range of Nb/Y Ratios (with P47 for comparison)

L(ftL) at 1500V and 320uAcnT ²

0.4 % Bi _{2 3} 1.0% Bi ₂ O ₃

Nb Y

O min. 30 min. O min. 30 min.

1 43 42 43 40 1.7 88 92 103

2 120 98 65 53

2.5 168 159 75 67

3 131 112 92 88

P47 62 71 (Y ₂ SiO ₅ :Ce)

For a blue material, the luminance of phosphors embodying the invention is high and compares very well with the commercial phosphor P47. A comparison of results on

yttrium niobate (Nb/Y=2.5) with P47 is given in Fig. I (for luminance against voltage) and Fig. 2 (for luminance against current). Fig. 1 shows values of luminance for various values of voltage for an electron beam current of 5μA forming a spot of diameter 1 41mm. Fig. 2 shows values of luminance for various values of corrected electron beam current (in μA) at 1500V and forming a spot of diameter 1.41 mm. In each figure the luminance is in ft-L. It will be readily seen that the luminance of a phosphor embodying the invention is more than double that of P47 under a wide range of voltages and currents. The actual value of luminance, as shown in Table 1 , depends on the Nb/Y ratio in the starting mixture. In most cases the phosphors were aged (after measuring) for 30 minutes at 1500 V at a current density of 320μA cm ^-2 .

Both the 0 min and 30 minutes values are shown to give some indication of the stability. The 30 minute figure corresponds to the passage of 0.58 coulombs cm" ² .

Fig. 3 shows the voltage/luminance characteristics for two yttrium niobate : bismuth phosphors. The first phosphor is one with a Nb:Y ratio of 2.5: 1 as described herein while the other phosphor is the yttrium niobate:bismuth phosphor of formula Y ₀ 9 ₇ Bi _{0 03} NDO4 previously described. Voltage values for both phosphors for a 5μA electron beam current and spot diameter of 1.41mm against luminance are shown. Fig. 4 shows current luminance characteristics for the same two phosphors. The horizontal axis gives the corrected beam current (in μA) at 1500V for a spot diameter of 1.41mm. It can be seen that the previously known phosphor is markedly inferior to the sample embodying the invention.

A ground sample of bismuth activated yttrium niobate embodying the invention (Nb/Υ = 2.5, 0.4% B12O3) was viewed under the electron microscope and consisted of irregular particles in a range of sizes from about 1 to lOμm.

The spectral peak of the phosphors changes slightly as the Nb/Y ratio is increased. There is also a smaller shift as the bismuth concentration is increased. Both features are

illustrated in Table 2.

Table 2

Peak Wavelengths (λ) for Range of Nb/Y

Peakλ(nm)

Nb/Y

0.4%Bi ₂ O ₃ 1.0%Bi ₂ O ₃

1 425 430

1.7 415 415

2.0 425 -

2.5 430 440

3.0 445 450

There is an apparent minimum peak wavelength of 415nm at a Nb/Y value of 1.7. The dominant wavelength also drifts upwards from 472nm to 478nm as the Nb/Y ratio increases from 1.0 to 3.0. Phosphor materials described herein have use both in low voltage excitation devices such as field-emission displays as well as in high voltage applications such as cathode ray tubes. The phosphors also have application as electroluminescent and X-ray

devices.