Colour display tube and display device comprising such a colour display tube

Abstract: A colour display tube comprising a colour selection structure which is provided with a layer of getter material. The surface on which the layer of getter material is provided is rough. In this manner heat radiation is increased and doming of the colour selection structure is reduced.

Description: The invention relates to a colour display tube comprising an electron gun, a getter, a display screen and a colour selection structure which is arranged in front of said display screen and which has a surface facing away from the display screen. A colour display tube of the type described in the opening paragraph comprises an electron gun and a colour selection structure. In operation, electrons emitted by the electron gun and impinging on the colour selection structure heat said colour selection structure. This heating of the colour selection structure causes deformations of the colour selection structure, the so-called "doming", which adversely affects picture quality. The side of the colour selection structure facing away from the display screen may have been treated such that it has favourable properties as regards doming. The colour display tube further comprises a getter. The getter material is vaporised from the getter in a gettering process and is deposited on surfaces of the colour display tube. The layer of getter material thus formed improves the vacuum in the colour display tube. It has been found that said layer of getter material influences doming. An increase of doming caused by the layer of getter material can be precluded by taking steps which prevent that getter material is deposited on the colour selection structure, for example vaporising the getter material in a direction away from the colour selection structure. However, this imposes restrictions on the location and/or shape of the getter, and a part of the surface inside the colour display tube is not covered by getter material, which adversely affects the vacuum inside the tube. It is an object of the invention to provide a colour display tube of the type mentioned in the opening paragraph, in which the above drawbacks are overcome. To this end, the colour display tube according to the invention is characterized in that the said surface is rough and a layer of getter material is applied to said surface. A rough surface is to be understood to mean herein a surface having a roughness, i.e. a difference between "hillocks" and "pits" on the surface in the order of 0.2 to 20 µm. It has been found that a layer of getter material which is applied to a smooth surface has a low coefficient of infrared emission, so that the colour selection structure can radiate only little heat, which results in a relatively high level of doming. When the layer of getter material is applied to a rough surface, the emission coefficient is higher. The said surface may be roughened by means of etching or scouring. An embodiment of the colour display tube according to the invention, in which said surface is formed by a glass layer is characterized in that said glass layer comprises particles of another material. The surface of the glass layer is roughened in a simple manner. It is to be noted that a colour display tube having a glass layer on which a layer of getter material is provided is known per se from European Patent Application 133411. In said Application, the colour selection structure is provided with a glass layer of a lead-borate glass on the side facing away from the display screen. The layer of lead-borate glass reduces doming. A layer of getter material is applied to the glass layer. The layer of getter material prevents that the glass layer is charged electrically. In said Application it is not stated whether the layer of getter material has any influence on doming. However, without special measures a glass layer is smooth. It has been found that a layer of getter material on a smooth glass layer has a low coefficient of infrared emission. The particles in the glass layer may consist of materials having a higher melting point than that of the glass layer, for example Bi₂O₃, Al₂O₃ or WC, or of particles having a lower melting point than that of the layer, for example metal particles such as tin particles or bismuth particles. Preferably, materials are used such that the glass layer bonds to the colour selection structure at a temperature of approximately 450°C. Said temperature is approximately equal to the firing temperature of the colour selection electrode. Foreign particles having a melting point below that of the glass layer meet this requirement, as do Al₂O₃ particles. An embodiment of the colour display tube according to the invention is characterized in that the glass layer is composed of a type of glass which forms a rough glass layer when it is provided. An example hereof is a type of glass comprising approximately, i.e. within a margin of a few percent, 52% of PbO, 16% of B₂O₃, 14% of S i O₂, 7% of ZnO, 4% of MnO, 4% of Fe₂O₃ and 3% of Al₂O₃, which glass bonds to the colour selection structure at a temperature of 490°C, but which remains granular and forms a rough glass layer. In an embodiment of the colour display tube according to the invention, the layer of getter material is applied to a granular layer, for example to a layer comprising Al₂O₃ grains or Bi₂O₃ grains. Preferably, the layer of getter material comprises an element having an atomic number above 50. In this case, the coefficient of electron reflection is relatively high. The invention also relates to a display device comprising a colour display tube according to the invention. The invention will be explained in greater detail by means of a few exemplary embodiments of the colour display tube according to the invention and with reference to a drawing, in which Fig. 1 is a partly perspective elevational view of a display device comprising a colour display tube according to the invention; Fig. 2 is a sectional view of a detail of a colour display tube, which Figure illustrates the effect of locally heating the colour selection structure; Fig. 3 is a sectional view of a known colour selection structure; Fig. 4 is a sectional view of a colour selection structure which can suitably be used in a colour display tube according to the invention; Figs. 5 and 6 are sectional views of further examples of colour selection structures which can suitably be used in a colour display tube according to the invention, Fig. 7 shows the coefficient of infrared thermal emission as a function of the thickness of the getter layer for various colour selection electrodes, Figs. 8 and 9 show two ways of distributing a granular intermediate layer over a colour selection electrode. The Figures are diagrammatic representations and are not drawn to scale, corresponding components in the various embodiments generally bearing the same reference numerals. Fig. 1 is a sectional view of a display device comprising a colour display tube according to the invention. In a glass envelope 1 which is composed of a display window 2, a cone 3 and a neck 4, an inline electron gun 5 is arranged in said neck 4, which electron gun generates three electron beams 6, 7 and 8 whose axes are located in the plane of the drawing. In the undeflected condition, the axis of the central electron beam 7 coincides with the axis 9 of the tube. The display window is provided on the inside with a screen 10 having a large number of triads of phosphor elements. Said elements may consist of, for example, lines or dots. In the present case, the elements are composed of linear triads. Each triad comprises a line having a phosphor emitting in green, a line having a phosphor emitting in blue, and a line having a phosphor emitting in red. The phosphor lines extend perpendicularly to the plane of the drawing. A colour selection structure 11 in which a great number of elongated apertures 12 for passing electron beams 6, 7 and 8 are formed is arranged in front of the display screen. The three coplanar electron beams are deflected by a system of deflection coils 13. The colour display tube further comprises a getter 14. In operation, getter material is vaporised from the getter. Fig. 2 is a sectional view of a detail of a colour display tube. As an example of doming, this Figure shows the effect of a local heating of the colour selection structure 11, which effect is called "local doming". In the "cold" state the electron beam 7 is incident on display screen 10 on the inside of the display window 2 at location 15. A local heating of the colour selection electrode 11, which may take place, for example, when the image displayed exhibits large differences in intensity, i.e. dark and light surfaces, leads to a local bulging of the colour selection structure 11, as represented by bulge 11a in Fig. 2. The aperture in the colour selection structure 11 through which the electron beam 7 passes is displaced such that the electron beam 7 is incident on the screen 10 at location 16. Consequently, a local heating of the colour selection structure leads to a displacement of the target spot of the electron beam on the screen, which effect will be termed "local doming" hereinafter. Besides "local doming" for example "overall doming" also occurs in a colour display tube. Even if substantially the entire colour selection structure 11 is irradiated with an equal electron current density, temperature differences between the central part of the colour selection structure and the edges of the colour selection structure will still occur; in general the edges are colder than the central part. This brings about bulging of the colour selection structure as a whole, which causes a displacement of the target spot. Fig. 3 is a sectional view of a colour selection electrode. At the side 17 facing the electron gun 5, the colour selection structure 11 is provided with a glass layer 18 to which a layer of getter material 19 is applied. In this example, the layer of getter material is a layer of barium. It has been found that the layer of getter material 19 has an influence on the "local doming" of such a colour display tube. Table 1 lists the "local doming" (in µm) for a 26 inch 30AX tube at various thicknesses of the layers of lead-borate glass, at two points on the display screen, one point on the longitudinal axis of the colour display tube at a distance from the centre of the display screen equal to half the distance between the centre and the edge of the display screen measured along the longitudinal axis (1/2 OW), and the other point on the longitudinal axis at a distance from the centre of the display screen equal to 2/3 rd of the distance between the centre and the edge of the display screen measured along the longitudinal axis (2/3 OW). The shadow mask is composed of iron. influence of barium getter layer on local doming lead-borate glass layer thickness in µm local doming in µm with barium getter layer without barium getter layer 1/2 OW 2/3 OW 1/2 OW 2/3 OW 0.9 119 156 73 106 1.1 115 147 71 105 It is obvious that local doming is less before the application of the barium getter layer than after the application of said layer. The heat supplied by the electrons is dissipated either by radiation, in which case in particular infrared radiation having a wavelength between 3 µm and 80 µm is important, or by heat conduction by the colour selection structure. In these tests, the barium getter layer has a very low coefficient of infrared emission (< 0.1), so that only little heat can be radiated. Fig. 4 shows a colour selection structure which can be suitably used in a colour display tube according to the invention. The surface 20 is rough. A layer of getter material 21 is applied to said surface 20. Rough is to be understood to mean herein, that the surface is rough relative to the wavelength of the radiated heat. Heat is radiated by means of infrared radiation having a wavelength in the range from 3 to 80 µm. The surface 14 has a roughness of the order of 0.2 to 20 µm. The layer of getter material preferably has a thickness below 2 µm. A thicker layer of getter material leads to a levelling of said layer of getter material. Consequently, the coefficient of thermal emission is reduced. If the colour selection structure comprises a glass layer, said glass layer preferably contains foreign particles. These particles bring about a roughening of the surface of the glass layer. A colour selection structure comprising a glass layer 22 having foreign particles 23 on which a layer of getter material 24 is provided is shown in Fig. 5. Table 2 lists the measured coefficients of infrared (= thermal) emission after a barium getter layer is applied, for a number of selection electrodes which are composed of invar (an iron-nickel compound having a very low coefficient of thermal expansion) (invar is a trademark) and which comprise a (lead-borate) glass layer which is mixed with foreign particles. It is obvious that the coefficients of thermal emission are higher than in the case of a smooth barium getter layer. Preferably, a proper bond between the borate-containing layer and the rest of the colour selection structure is obtained at a temperature which is approximately equal to or lower than the temperature at which the display screen and the cone are secured to each other. A suitable bond is obtained if the foreign particles are moistened by the glass. This is attained at a temperature of approximately 450°C (dependent on the type of glass used for the display tube). In this case, a separate high-temperature treatment of the colour selection structure can be omitted. It was found that in the case of layers containing Bi₂O₃ particles and WC particles a suitable bond was obtained at a temperature of approximately 600°C (in air). In this respect, a layer containing Al₂O₃ particles is to be preferred because it provides a proper bond at lower temperatures. The layers having a material with a melting temperature below that of borate glass were all properly bonded to the colour selection structure at approximately 450°C. It is alternatively possible to provide the colour selection structure with a glass layer of a type of glass which bonds to the colour selection structure in the form of grains at the bonding temperature. An example of such a type of glass is glass comprising approximately 52% of PbO, 16% of B₂O₃, 14% of SiO₂, 7% of ZnO, 4% of MnO, 4% of Fe₂O₃ and 3% of Al₂O₃, which glass bonds to a colour selection structure in the form of grains at a temperature of 490°C. Also in such an examplary embodiment of the invention the main aspect of the invention is met, i.e., that the surface on which the layer of getter material is to be provided is so rough that after providing said layer of getter material a relatively high coefficient of thermal emission (> 0.5 and preferably > 0.7) is obtained. In an exemplary embodiment, the surface on which the layer of getter material is provided is a granular layer. Fig. 6 shows a selection electrode comprising a rough layer 25 having particles which are deposited on the colour selection structure. The barium getter layer 26 is sprayed thereon. The barium layer may be present on the granular layer and/or diffused into the granular layer. As is shown in the drawing, said barium getter layer is not plane. Table 3 compares local doming results of various 51 FS (Flat Square) colour display tubes. In table 3 the quantities of Bi₂O₃ and of Al₂O₃ are indicated in gr/colour selection structure. In the case of Bi₂O₃,1 gr/colour selection structure for a 51 FS screen corresponds to approximately an average layer thickness of 1.1 µm. In the case of Al₂O₃, 1 gr/colour selection structure corresponds to approximately an average layer thickness of 2.6 µm. Consequently, the average layer thicknesses are of the order of 0.2 to 1 µm. The point 2/3 OD, the local doming of which is indicated in Table 3, is located on the diagonal at a distance from the centre of the display screen which is equal to 2/3 of the distance between the centre of the display screen and the corner of said display screen. In the case of the invar colour selection structure without a rough layer (see table 3B) the application of a barium getter layer leads to an improvement in local doming. Invar has a low coefficient of thermal emission (approximately 0.25) and a low coefficient of electron reflection (approximately 0.22). A smooth barium getter layer has an approximately equally high emission coefficient and a higher coefficient of electron reflection, so that local doming is reduced. Fig. 7 shows the coefficient of infrared thermal emission ε as a function of the layer thickness δ of the getter material. Line 71 shows ε for an invar colour selection structure having a thin (approximately 0.1 µm) oxide layer without a granular layer, line 72 shows ε for an iron colour selection structure without a granular layer. There is a remarkable strong negative influence of the layer of getter material on the coefficient of infrared thermal emission. Line 73 shows ε for the invar colour selection structure of line 71, but now provided with 0.6 gr of Bi₂O₃ grains (which corresponds approximately to 0.33 mg of Bi₂O₃/cm²). Line 74 shows ε for the iron colour selection structure of line 72, but now provided with 0.6 gr of Bi₂O₃. Lines 75 and 76 show ε for the invar colour selection structure and the iron colour selection structure provided with 1.0 gr of Bi₂O₃, respectively. Finally, line 77 shows ε for an invar colour selection structure having a thick (approximately 3 µm) oxide layer and provided with 0.73 gr of Bi₂O₃. The positive influence of the granular intermediate layer, in this example Bi₂O₃, can be clearly observed. For the lines 71, 72, 74, 76 and 77, ε decreases as a function of the layer thickness. It is remarkable and surprising that for a colour selection structure whose uncovered surface has a low emission coefficient, and which is covered with a granular layer, in this example an invar colour selection structure having a Bi₂O₃-containing layer, ε as a function of the layer thickness exhibits an extremum at a layer thickness of approximately 100 nm, as is shown by the lines 73 and 75. The invention is important to in particular colour selection electrode structures of this type. It has also been found that the grain size distribution is important. A colour selection structure comprising a getter layer with a granular intermediate layer having an average grain size of approximately 0.25 µm exhibits approximately 7% less local doming than when a granular intermediate layer having an average grain size of 0.75 µm is used. A preferred embodiment is characterized in that the particles have an average grain size which is smaller than 0.5 µm. The average grain size is the value of the grain size for which it holds that 50%, of the particles is smaller and 50%, of the particles is larger. Further, the average particle size is preferably larger than 0.05 µm. If the particles are too small it is very likely that a reflecting getter layer having a low ε is formed on the intermediate layer. It has further been found that the distribution of the granular intermediate layer over the colour selection electrode has an influence on doming. Such layers can be applied in a simple and rapid manner by means of a spraying process in which a solution which contains the granular particles is provided on the colour selection electrode. Figs. 8 and 9 show two ways of distributing a granular layer over a colour selection electrode. Approximately 1 gr of Bi₂O₃ is sprayed on both colour selection electrodes. The values shown in the lines indicate the quantity of Bi₂O₃ in 10⁻⁴ gr/cm². In Fig. 8, the variation in the quantity of Bi₂O₃ per unit area along the longitudinal axis is approximately 50% and between the points 2/3 O and 2/3 W approximately 25%. In Fig. 9, this variation along the longitudinal axis is less than 25%, in this example approximately 20%, and between the points 2/3 O and 2/3 W less than 12.5%, in this example approximately 10%. It has been found that a distribution as shown in Fig. 9 leads to a reduction in local doming of approximately 7% relative to a distribution as shown in Fig. 8. The quantity of Bi₂O₃ which was sprayed wide of the colour selection electrode differed only little. Consequently, a preferred embodiment of the display tube is characterized in that the granular layer is provided in a manner, for example by means of spraying, such that the variation in quantity per unit area along the longitudinal axis is less than 25%, and, preferably, less than 12.5% between the points 2/3 O and 2/3 W. In Table 4 local doming in a 26 inch 30AX tube having an iron colour selection structure is compared for a colour selection structure with a smooth lead-borate layer, an uncovered colour selection structure, a colour selection structure comprising a granular layer of Bi₂O₃, the Bi₂O₃ particles being distributed over the surface of the colour selection structure as uniformly as possible, and a colour selection electrode in which the Bi₂O₃ particles are coagulated in agglomerates on the surface of the colour selection structure. 1.0 gr of lead-borate glass and 0.8 gr of Bi₂O₃ was provided per colour selection structure. As can be derived from the table below, doming is reduced according as the surface on which the barium getter layer is provided is rougher. The particles may also consist of other materials (for example a metal carbide or metal nitride). Al₂O₃ is a suitable material because it is cheap and it can be obtained in many particle sizes. Preferably, compounds of a metal having a low atomic number are used because, apart from the fact that elements having a high atomic number are generally more rare and hence more expensive than elements having a low atomic number, the use of heavy metals may adversely affect the environment. It will be obvious that within the scope of the invention many variations are possible to those skilled in the art. The shape of the colour display tube is not to be regarded as limitative, it may for example be flat, nor is the type of electron gun to be regarded as limitative, it may be, for example, a so-called delta electron gun or the tube may comprise more than one electron gun. Electron gun is to be understood to mean herein a system for generating one or more electron beams. In the examples described above a barium getter layer is shown. This is not to be regarded as limitative. The getter layer may be composed of a different material, for example cesium or titanium.

Claims: A colour display tube comprising an electron gun, a getter, a display screen and a colour selection structure which is arranged in front of said display screen and which has a surface facing away from the display screen, characterized in that the said surface is rough and a layer of getter material is applied to said surface. A colour display tube as claimed in Claim 1, in which said surface is formed by a glass layer, characterized in that the glass layer comprises particles of a different material. A colour display tube as claimed in Claim 2, characterized in that the said particles consist of a material whose melting point is below that of the glass layer. A colour display tube as claimed in Claim 2, characterized in that the particles consist of Al₂O₃. A colour display tube as claimed in Claim 1, in which said surface is formed by a glass layer, characterized in that the glass layer is composed of a type of glass which forms a rough layer when it is applied. A colour display tube as claimed in Claim 1, characterized in that the layer of getter material is provided on a layer containing granular particles. A colour display tube as claimed in Claim 6, characterized in that the granular layer comprises Al₂O₃ particles. A colour display tube as claimed in Claim 2, 3, 4, 6 or 7, characterized in that the average grain size of the particles is smaller than 0.5 µm. A colour display tube as claimed in Claim 8, characterized in that the average grain size of the particles is larger than 0.05 µm. A colour display tube as claimed in Claim 6, 7 or 8, characterized in that the quantity of material per unit area of the layer containing granular particles varies less than 25% along the longitudinal axis. A colour display tube as claimed in any one of the preceding Claims, characterized in that the colour selection structure consists at least partly of an alloy having a low coefficient of thermal expansion. A colour display tube as claimed in Claim 11, characterized in that the alloy is an iron-nickel alloy. A display device comprising a colour display tube as claimed in any one of the preceding Claims.

Inventors: COMPEN JOHANNES MARIA AZALINA, HENS THEODOOR CHRISTIAAN ANNA, VAN UDEN MARIA CHRISTIAAN, VRANCKEN THOMAS DANIEL MARIE, COMPEN, JOHANNES MARIA AZALINA ANTONIUS, HENS, THEODOOR CHRISTIAAN ANNA, VAN UDEN, MARIA CHRISTIAAN, VRANCKEN, THOMAS DANIEL MARIE
Applicants: PHILIPS NV, N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1990
Main classification: H01J29
IPC codes: H01J29