Patent Application
20080192176
This invention provides a sintered electrode for a cold cathode tube, a cold cathode tube comprising this sintered electrode for a cold cathode tube, and a liquid crystal display device.
Sintered electrodes for cold cathode tubes and cold cathode tubes provided with this electrode have hitherto been used, for example, as backlights for liquid crystal display devices. In addition to high luminance and high efficiency, a long service life is required of such cold cathode tubes for liquid crystal applications.
In general, the construction of cold cathode tubes useful as backlights for liquid crystal applications is such that very small amounts of mercury and rare gas are filled into a glass tube comprising a fluorescent substance coated onto the inner surface thereof, and an electrode and a lead-in wire (for example, KOV+dumet wire) are mounted on both ends of this glass tube. In such cold cathode tubes, upon the application of voltage to both end electrodes, mercury sealed in the glass tube is evaporated, resulting in emission of ultraviolet light which is absorbed by the fluorescent substance to emit light.
Nickel materials have hitherto been mainly used as the electrode. This Ni (nickel) electrode, however, is disadvantageous in that a cathode drop voltage necessary for electron emission from the electrode to a discharge space is relatively high and, in addition, the occurrence of the phenomenon of the so-called “sputtering” is likely to deteriorate the service life of the lamp. The sputtering phenomenon refers to a phenomenon that the electrode undergoes ion collision during lighting of the cold cathode tube to cause scattering of an electrode material, and the scattered material and mercury and the like are accumulated on the internal wall surface within the glass tube.
Mercury is introduced into the sputtering layer formed by the sputtering phenomenon, making it impossible to utilize mercury in luminescence. Accordingly, when the cold cathode tube is lighted for a long period of time, the luminance of the lamp is extremely lowered to reach the end stage of the service life. Therefore, if the sputtering phenomenon could be reduced, the mercury consumption could be suppressed and, thus, the service life could be prolonged even in the same mercury sealing amount.
This has led to an attempt to simultaneously realize both cathode voltage drop reduction and sputtering suppression. In a recent effort, an electrode design, in which an electrode in a closed-end cylindrical form is adopted to attain a holocathode effect for realizing both cathode voltage drop reduction and sputtering suppression, has been proposed (Japanese Patent Laid-Open No. 176445/2001). Further, a proposal has been made in which, instead of nickel in the prior art technique, Mo (molybdenum) or Nb (niobium) or the like, which can lower the cathode voltage drop by about 20V, has been used as the electrode material.
Patent document 1: Japanese Patent Laid-Open No. 176445/2001
As compared with the conventional nickel electrode, the above closed-end cylindrical cold cathode electrodes are advantageous in terms of cathode voltage drop and service life.
Since, however, for all the closed-end cylindrical cold cathode electrodes, the closed-end cylindrical form is produced by drawing from plate materials (thickness: generally about 0.07 mm to 0.2 mm), the yield of the material is low and, in addition, for metals having poor drawability, disadvantageously, cracking and the like are likely to occur during working. Further, drawing of plate materials disadvantageously incurs high cost.
In the closed-end cylindrical electrode, the sputtering-derived consumption of the bottom part is likely to be more significant than the consumption of the side wall part. In the drawing, however, the control of the thickness or form of the bottom part and the side wall part is so difficult that the production of an electrode having a bottom part and a side wall part each having the optimal thickness and form is difficult. As a result, in some cases, the thickness is insufficient in some part and is excessive in other part. When the bottom part and the side wall part is excessively thick, disadvantageously, the surface area of the electrode is insufficient or the size of the electrode per se is large.
Thus, in order to provide a high-luminance, high-efficiency and long-service life cold cathode tube, there is a demand for a cold cathode tube electrode that can easily be mass produced at low cost while enjoying a high level of properties required as the electrode.
In general, a lead wire is welded to the bottom part of the closed-end cylindrical electrode. In the case of the conventional electrode produced by drawing of a plate material, disadvantageously, the closed-end part disappears or is deformed at the time of welding of the lead wire, or the level of lowering in weld strength caused by recrystallization is so high that it is difficult to provide a cylindrical electrode to which a lead wire has been welded with satisfactory strength.
The present invention has been made with a view to solving the above problems of the prior art, and an object of the present invention is to provide a cold cathode tube electrode, which has properties favorably comparable with those of the electrode produced by drawing of the plate material, has high weld strength in the welding of a lead wire, and can be produced with good mass productivity at low cost, and to provide a cold cathode tube and a liquid crystal display device.
According to the present invention, there is provided a sintered electrode for a cold cathode tube, comprising a cylindrical side wall part, a bottom part provided at one end of the side wall part, and an opening provided at another end of the side wall part, characterized in that the surface roughness (Sm) of the inner surface of the electrode is not more than 100 μm.
In the sintered electrode for a cold cathode tube according to the present invention, preferably, said side wall part has an average thickness of not less than 0.1 mm and not more than 0.7 mm.
In the sintered electrode for a cold cathode tube according to the present invention, preferably, said bottom part has an average thickness of not less than 0.25 mm and not more than 1.5 mm.
The sintered electrode for a cold cathode tube according to the present invention is preferably formed of a metal selected from tungsten (W), niobium (Nb), thallium (Ta), titanium (Ti), molybdenum (Mo), and rhenium (Re), or its alloy.
The sintered electrode for a cold cathode tube according to the present invention preferably has a relative density of not less than 80%.
In a preferred embodiment of the present invention, the sintered electrode for a cold cathode tube comprises a sinter of a high-melting metal containing a rare earth element (R)-carbon (C)-oxygen (O) compound.
In a preferred embodiment of the present invention, the sintered electrode for a cold cathode tube has a rare earth element (R)-carbon (C)-oxygen (O) compound content of more than 0.05% by mass and not more than 20% by mass in terms of the rare earth element (R).
In a preferred embodiment of the present invention, the sintered electrode for a cold cathode tube has a carbon content of more than 1 ppm and not more than 100 ppm.
In a preferred embodiment of the present invention, the sintered electrode for a cold cathode tube has an oxygen content of more than 0.01% by mass and not more than 6% by mass.
In a preferred embodiment of the present invention, the sintered electrode for a cold cathode tube is such that the rare earth element (R)-carbon (C)-oxygen (O) compound is present as particles having an average particle diameter of not more than 10 μm in the sinter.
In the sintered electrode for a cold cathode tube according to the present invention, preferably, in a section perpendicular to the longitudinal axis direction of the sintered electrode for a cold cathode tube, the inner wall surface of the cylindrical side wall part is in a concave-convex form.
In a preferred embodiment of the present invention, the sintered electrode for a cold cathode tube is such that, in a section perpendicular to the longitudinal axis direction of the sintered electrode for a cold cathode tube, the form of the inner wall surface of the cylindrical side wall part is such that the ratio b/a, wherein a represents the outer diameter distance from an imaginary center O calculated from the outer diameter of the sintered electrode for a cold cathode tube and b represents the inner diameter maximum length, is more than 0.50 and not more than 0.95, and the ratio c/b, wherein c represents the inner diameter minimum length and b is as defined above, is more than 0.50 and not more than 0.95.
According to the present invention, there is provided a sintered electrode for a cold cathode tube, comprising a lead wire welded to the bottom part of any of the above sintered electrode for a cold cathode tube, the weld strength per unit sectional area of the lead wire being not less than 400 N/mm2.
According to the present invention, there is provided a cold cathode tube characterized by comprising: a hollow tubular light transparent bulb into which a discharge medium has been sealed; a fluorescent material layer provided on the inner wall surface of the tubular light transparent bulb; and a pair of the above sintered electrodes for a cold cathode tube provided respectively on both ends of the tubular light transparent bulb.
According to the present invention, there is provided a liquid crystal display device characterized by comprising: the above cold cathode tube; a light guide body disposed closely to said cold cathode tube; a reflector disposed on one surface side of the light guide body; and a liquid crystal display panel disposed on another surface side of the light guide body.
In the sintered electrode for a cold cathode tube according to the present invention, since the surface roughness (Sm) of the inner surface of the electrode is not more than 100 μm, the surface area is large and sputtering during operation can be suppressed. Therefore, the sintered electrode for a cold cathode tube according to the present invention can provide a long-service life cold cathode tube that is low in operating voltage and can significantly suppress mercury consumption.
In the sintered electrode for a cold cathode tube according to the present invention, the amount of the electrode scattered material produced by sputtering is reduced, and illuminance lowering caused by the formation of an amalgam of this scattered material and mercury, and illuminance lowering caused by mercury consumption can be effectively prevented, whereby a high-luminance, high-efficiency and long-service file cold cathode tube can be provided.
Further, for the sintered electrode for a cold cathode tube according to the present invention, the mass productivity is better than that of the conventional electrode produced by drawing from a plate material, and, thus, the sintered electrode for a cold cathode tube according to the present invention can be produced at low cost.
In particular, when the sintered electrode for a cold cathode tube according to the present invention is formed of a sinter of a high-melting metal containing a rare earth element (R)-carbon (C)-oxygen (O) compound, the cathode voltage drop can be lowered to a very low level. Therefore, the sintered electrode for a cold cathode tube according to the present invention can provide a long-service life cold cathode tube that the operating voltage is further low and the consumption of mercury is significantly suppressed. In the sintered electrode for a cold cathode tube formed of the specific rare earth compound-containing sinter, the recrystallization of a sinter structure under welding conditions has been suppressed. Therefore, in the present invention, high-voltage welding conditions, which cannot be substantially adopted in conventional electrodes produced by conventional drawing, can be adopted. A sintered electrode for a cold cathode tube having a higher lead wire weld strength than the conventional sintered electrode can easily be prepared.
When the sintered electrode for a cold cathode tube according to the present invention is such that, in a section perpendicular to the longitudinal axis direction of the sintered electrode for a cold cathode tube, the inner wall surface of the cylindrical side wall part is in a concave-convex form, the cathode voltage drop further lowered. Therefore, this sintered electrode for a cold cathode tube can provide a long-service life cold cathode tube that the operating voltage is lower and the amount of mercury consumption has been significantly suppressed.
So far as the present inventors know, neither focusing on the surface properties of the sintered electrode for a cold cathode tube nor any study on the relationship between the surface properties of the sintered electrode and the properties of the cold cathode tube has been made in the prior art. Therefore, it is surprising that a cold cathode tube having low operating voltage and significantly suppressed consumption of mercury can be provided by focusing on the surface properties of the sintered electrode, for a cold cathode tube, particularly surface properties of the inner surface of the sintered electrode for a cold cathode tube, and regulation of the surface roughness (Sm) in a specific range.
Further, it is unexpected that, in a sintered electrode for a cold cathode tube in which the surface roughness (Sm) has been regulated to a specific range, the use of a sinter of a high-melting metal containing a rare earth element (R)-carbon (C)-oxygen (O) compound can significantly lower the cathode voltage drop and, in addition, in the sintered electrode for a cold cathode tube in which the surface roughness (Sm) has been regulated to a specific range, when the inner wall surface of the cylindrical side wall part is in a concave-convex form, the cathode voltage drop is further lowered and, further, the lead wire weld strength is higher than that in the prior art.
The reduction in operating voltage can render temperature conditions and voltage conditions of the sintered electrode mild, and sputtering of the electrode can be effectively prevented. As a result, the consumption of the electrode per se and the consumption of mercury within the cold cathode tube can be significantly suppressed. At the same time, accumulation of the material scattered by sputtering on the inner wall surface of the cold cathode tube can be prevented. By virtue of the above synergistic effect, in the cold cathode tube according to the present invention, the performance deterioration by the use of the cold cathode tube is small, and the service life until the cold cathode tube is no longer usable is significantly improved. When the operating voltage of the cold cathode tube is reduced, the voltage of a display device with the cold cathode tube incorporated therein can be reduced, contributing to size reduction, weight reduction, and thickness reduction and cost reduction of the device.
The sintered electrode for a cold cathode tube, the cold cathode tube, and the liquid crystal display device according to the present invention is suitable particularly, for example, for not only battery-driven portable electronic device but also display devices which should be of power saving type and should provide stable high-quality display for a long period of time.
1: sintered electrode for cold cathode tube
2: side wall part
3: bottom part
4: opening
5: inner surface of electrode
6: deepest part
7: dumet wire
8: protrusion
20: liquid crystal display device
21: cold cathode tube
22: light guide body
23: reflector
24: liquid crystal display panel
25
a, 25b, 25c: light diffuser
<Sintered Electrode for Cold Cathode Tube (Part 1)>
As described above, the sintered electrode for a cold cathode tube according to the present invention comprises a cylindrical side wall part, a bottom part provided at one end of the side wall part, and an opening provided at another end of the side wall part, characterized in that the surface roughness (Sm) of the inner surface of the electrode is not more than 100 μm.
In the present invention, “surface roughness (Sm)” is specifically one based on “average spacing of profile irregularities (Sm)” specified in JIS B 0601-1994, that is, means that “the portion equal to the reference length I is sampled from the roughness curve in the direction of its mean line, and within this sampled portion, the sum of the lengths of mean lines corresponding to one of the profile peaks and one profile valley adjacent to it is obtained and the arithmetical mean value of many spacings of these irregularities is expressed in millimeter (mm).
The sintered electrode (1) for a cold cathode tube according to the present invention shown in
As shown in
In the present invention, one of main features is that the surface roughness of the inner surface (5) is in a predetermined Sm range. However, it should be noted that, in the present invention, each area in the inner surface (5) is not always required to have an identical Sm value. Further, in the present invention, so far as substantially the whole area (preferably not less than 30%, particularly preferably not less than 50% of the area of the inner surface (5)) of the inner surface (5) falls within the predetermined Sm range, the whole area of the inner surface (5) is not always required in a predetermined Sm range. Accordingly, in some cases, the area of a part of the inner surface (5) is not required to fall within the predetermined Sm range.
On the other hand, regarding the outer surface of the sintered electrode (1) for a cold cathode tube [that is, including, for example, the outer surface of the cylindrical side wall part (2) and the outer surface of the bottom (3) and the surface of the edge end face (4′)], Sm is not specified. Specifically, Sm on the outer surface of the sintered electrode (1) for a cold cathode tube is any desired value and may be the same as or different from the above Sm range specified on the inner surface of the sintered electrode (1) for a cold cathode tube.
The term “thickness” of the bottom as used herein refers to the distance (L2) in the bottom between the above deepest part (6) and the outer surface of the bottom of the sintered electrode for a cold cathode tube. Further, the term “thickness” of the side wall part refers to the distance (L3) in the side wall part between the inner surface and the outer surface of the sintered electrode for a cold cathode tube.
Further, for the side wall part, as shown in
wherein “(i) LMAX” represents “the maximum thickness (LMAX) of “section (i)””, “(i) LMIN” represents “the minimum thickness (LMIN) of the section (i)”; and the same shall apply to “(ii) LMAX”, “(ii) LMIN”, “(iii) LMAX”, “(iii) LMIN”, “(iv) LMAX”, and “(iv) LMIN”.
For the bottom, the term “average thickness” as used herein refers to an average thickness value obtained by measuring the maximum thickness (LMAX) and the minimum thickness (LMIN) for each bottom of four sections obtained from the first section and the second section in the same manner as described above, and calculating the average value based on the measured data according to the above equation.
In general, a wire rod or/and a foil material formed of any one of molybdenum (Mo), W (tungsten), and KOV (kovar alloy) is joined to substantially the center part of the bottom (3) in the sintered electrode (1) for a cold cathode tube. A dumet wire or a nickel (Ni) wire (7) is further joined to the wired rod or foil material. Voltage is applied to the sintered electrode (1) for a cold cathode tube through the dumet wire (7). In some cases, as shown in
As described above, in the sintered electrode for a cold cathode tube according to the present invention, the surface roughness (Sm) of the inner surface is not more than 100 μm. The reason for this is that, in a closed-end electrode, in order to lower the operating voltage, in particular, a larger electrode surface area is more advantageous, and, in particular, since discharge occurs around the inner side of the electrode, increasing the inner side surface area of the electrode is preferred. When the Sm value exceeds 100 μm, the advantageous effect on the operating voltage is poor. Further, the mercury consumption is also likely to be significantly increased, making it difficult to attain the object of the present invention, that is, to provide a long-service life cold cathode tube which has low operating voltage and significantly suppressed mercury consumption. The Sm range is preferably not less than 70 μm and not more than 90 μm, particularly preferably not less than 40 μm and not more than 50 μm.
The surface roughness (Sm) of the inner surface can be provided by setting sinter production conditions (for example, particle diameter of raw material powder) so as to provide a sintered electrode having the above inner surface, or by providing a sinter and subjecting the sinter to suitable processing (for example, polishing such as barreling or blasting, or etching) after the preparation of sinter.
The average thickness of the side face part is preferably not less than 0.1 mm and not more than 0.7 mm. This is so because, in the operation as a cold cathode tube, when the average thickness is less than 0.1 mm, problems sometimes occurs such as unsatisfactory strength or hole formation. When the average thickness exceeds 0.7 mm, the surface area on the inner side of the sintered electrode for a cold cathode tube is reduced and, consequently, the effect of reducing the operating voltage cannot be satisfactorily attained. The average thickness of the side face part is preferably not less than 0.3 mm and not more than 0.6 mm, particularly preferably not less than 0.35 mm and not more than 0.55 mm.
On the other hand, the average thickness of the bottom face part is preferably not less than 0.25 mm and not more than 1.5 mm. The reason for this is as follows. Since the inner side of the bottom face part of the electrode is significantly consumed, the thickness is preferably more than 0.25 mm. When the thickness exceeds 1.5 mm, the surface area of the inner side is reduced. In this case, as with the above case, the effect of reducing the operating voltage cannot be satisfactorily attained. The average thickness of the bottom face part is preferably not less than 0.4 mm and not more than 1.35 mm, particularly preferably not less than 0.6 mm and not more than 1.15 mm.
The sintered electrode for a cold cathode tube according to the present invention may be formed of any purposive high-melting metal. For example, the sintered electrode for a cold cathode tube may be formed of a simple substance of a metal preferably selected from tungsten (W), niobium (Nb), thallium (Ta), titanium (Ti), molybdenum (Mo), and rhenium (Re), or at least one alloy of the above metals. Mo is a preferred metal. Further examples thereof include oxides of rare earth elements such as lanthanum (La), cerium (Ce), and yttrium (Y), rare earth carboxides (particularly preferably “rare earth element (R)-carbon (C)-oxygen (O) compounds” (details thereof will be described later), and Mo to which oxides of light elements such as barium (Ba), magnesium (Mg), and calcium (Ca) have been added. Examples of preferred alloys include W—Mo alloys, Re—W alloys, and Ta—Mo alloys. Further, if necessary, a mixture of an electron emission substance with a high-melting metal may be used. Further, a very small amount (for example, not more than 1% by mass) of nickel (Ni), copper (Cu), iron (Fe), phosphorus (P) and the like may be added as a sintering aid. In general, in the production process of the cold cathode tube, since nitrogen gas is used at an elevated temperature for replacement or other purposes, as compared with the Nb-based or Ta-based metal, the Mo-based or W-based metal, which is less likely to be nitrided, is preferred. In the Mo-based and W-based metals, the Mo-based metal which can be sintered at a low temperature is more preferred than the W-based metal.
The average diameter of crystal grains of the sinter is preferably not more than 100 μm. The aspect ratio (major axis/minor axis) of the crystal grains of the sinter is preferably not more than 5.
The relative density is preferably not less than 80%, particularly preferably not less than 90% and not more than 98%. The relative density is measured by the following method.
Measurement of Relative Density
1. The bottom of the sintered electrode for a cold cathode tube is cut off by wire discharge machining or the like to obtain a sample.
2. Subsequently, the sample of the side wall part obtained in the above step 1 is halved by axisymmetrical cutting by wire discharge machining or the like. The reason why the bottom is cut is that, when the bottom is present, air bubbles enter closed spaces within the sintered electrode for a cold cathode tube and, consequently, accurate measurement is impossible.
3. Measurement is carried out for the sample obtained in the step 2 (N=5) by an Archimedes method specified in JIS Z 2501-2000, and the average is determined as a representative value.
The length of the sintered electrode for a cold cathode tube according to the present invention [that is, length between the surface of the edge end face (4′) and the outer surface of the bottom farmost from the edge end face (4′) (when a protrusion part is present, the surface of the front end of the protrusion part)] is mainly determined depending, for example, upon the size and performance of the cold cathode tube in which the electrode is incorporated. Preferably, however, the electrode length is not less than 3 mm and not more than 8 mm, particularly preferably not less than 4 mm and not more than 7 mm.
Likewise, the diameter of the sintered electrode for a cold cathode tube is determined depending, for example, upon the size and performance of the cold cathode tube in which the electrode is incorporated. Preferably, however, the diameter is not less than 1.0 mmΦ and not more than 3.0 mmΦ, particularly preferably not less than 1.3 mmΦ and not more than 2.7 mmΦ. The sintered electrode according to the present invention is useful in such small electrodes.
The ratio between the length and the diameter of the sintered electrode for a cold cathode tube (length/diameter) is preferably not less than 2 and not more than 3, particularly preferably not less than 2.2 and not more than 2.8.
For the sintered electrode for a cold cathode tube according to the present invention, the shape of the cylindrical space in a section parallel to the longitudinal axis direction is preferably rectangular as shown in
The above construction can provide a long-service life cold cathode tube which has low operating voltage and significantly suppressed mercury consumption.
<<Production Process of Sintered Electrode for Cold Cathode Tube and Cold Cathode Tube (Part 1)>>
The sintered electrode for a cold cathode tube according to the present invention may be produced by mixing raw material powders, granulating the mixture, molding the granules into a desired shape, and then sintering the molded product.
A preferred production process of a sintered electrode for a cold cathode tube according to the present invention will be described by taking molybdenum as a representative example.
The molybdenum powder as the raw material powder has an average particle diameter of not less than 1 μm and not more than 5 μm and a purity of not less than 99.95%. This powder is mixed with pure water, a binder (preferably polyvinyl alcohol (PVA)), and the mixture is granulated. Thereafter, a cup-shaped molded product [for example, 3.0 mm in diameter×7.0 mm in length, average thickness of side face part 0.5 mm, average thickness of bottom face part 1.0 mm, bottom face protrusion R 0.6 mm (this protrusion part is not included in the length 7.0 mm)] is produced by a single action press, a rotary press, or injection molding. When injection molding is used, the protrusion part may if necessary be in a lead form.
Subsequently, degreasing is carried out in a dry hydrogen atmosphere of 800° C. to 1000° C. The degreasing time is preferably 4 hr or less. When the degreasing time exceeds 4 hr, the content of carbon in the rare earth carboxide is disadvantageously lowered. Sintering is then carried out in a hydrogen atmosphere under conditions of 1700 to 1800° C.×4 hr or longer and further is if necessary subjected to hot isostatic pressing (HIP) under conditions of 1100 to 1600° C.×100 to 250 MPa. When the surface roughness of the inner side of the closed-end shape part is not in the predetermined Sm range, or in order to bring the surface roughness to a more preferred Sm range, the surface roughness (Sm) of the inner side of the closed-end shape part may be regulated. An example of a surface roughness regulation method is barrel polishing or blasting. In this case, for example, the abrasive material used and work content may be properly selected or regulated.
Thereafter, washing is carried out, followed by annealing at a temperature of 700° C. or above and 1000° C. or below. Regarding the product to which a lead part has been attached during molding, for example, welding to a dumet rod having a size of 0.6 mm in diameter×25 mm in length is carried out. On the other hand, regarding the lead part-free product, for example, welding of a molybdenum rod having a size of 0.8 mm in diameter×2.6 mm in length and a dumet rod having a size of 0.6 mm in diameter×40 mm in length are carried out to complete assembling of the electrode. In the welding of the electrode on the bottom to the Mo rod, a foil material of Ni, KOV or the like may be inserted for welding. The construction of the lead part (diameter or length) may be any desired one.
<Sintered Electrode for Cold Cathode Tube (Part 2)>
In one preferred embodiment of the present invention, as described above, the sintered electrode for a cold cathode tube is formed of a sinter of a high-melting metal containing a rare earth element (R)-carbon (C)-oxygen (O) compound. The “rare earth element (R)-carbon (C)-oxygen (O) compound” refers to a compound containing a rare earth element (R), carbon (C), and oxygen (O) as constituents.
Rare earth elements (R) include, for example, lanthanum (La), cerium (Ce), samarium (Sm), praseodymium (Pr), and neodymium (Nd). Among them, lanthanum (La), cerium (Ce), and samarium (Sm) are particularly preferred. In the “rare earth element (R)-carbon (C)-oxygen (O) compound” may contain a plurality of rare earth elements in an identical compound. Further, in the sinter of the sintered electrode for a cold cathode tube according to the present invention may contain a plurality of types of “rare earth element (R)-carbon (C)-oxygen (O) compounds” which are different from each other in type of rare earth element, its content, or carbon and/or oxygen content.
The composition of the sinter constituting the sintered electrode for a cold cathode tube can easily be judged by color mapping using EPMA (electron probe micro analyzer). Accordingly, in the sintered electrode for a cold cathode tube according to the present invention, the presence of the above “rare earth element (R)-carbon (C)-oxygen (O) compound” in the sinter is observed as at least one of the sinter constituents other than the high-melting metal, as judged by color mapping using EPMA.
This “rare earth element (R)-carbon (C)-oxygen (O) compound” may be represented by chemical formula RxCyOz or RxOy(COz)a wherein R represents a rare earth element; x, y, z, and a are any number. Possible such compounds include, for example, (i) La-based compounds such as LaCO, La2O(CO3)2, La2O2CO3, La2CO5, La2O(CO3)2, and La2O2CO3, (ii) Ce-based compounds such as CeO2C2 and Ce4O2C2, (iii) Sm-based compounds, for example, SmO0.5C0.4 and Sm2CO5Sm2O2CO3, (iv) compounds having an indefinite structure, (5) mixtures or compounds comprising the above compounds (1) to (4), and (6) other compounds.
In the sintered electrode for a cold cathode tube according to the present invention, the content of the rare earth element (R)-carbon (C)-oxygen (O) compound is preferably more than 0.05% by mass and not more than 20% by mass in terms of the rare earth element (R), particularly preferably more than 0.5% by mass and not more than 10% by mass. When the content is not more than 0.05% by mass, the cathode voltage drop is disadvantageously high, while, when the content is more than 10% by mass, sintering is disadvantageously less likely to proceed. For the above reason, both the above content ranges are unfavorable.
The content of carbon in the sinter constituting the sintered electrode for a cold cathode tube according to the present invention is preferably more than 1 ppm and not more than 100 ppm, particularly preferably more than 5 ppm and not more than 70 ppm. When the carbon content is not more than 1 ppm, the cathode voltage drop is high, while, a carbon content exceeding 100 ppm is disadvantageous in that, when the sinter is used as the electrode, gas (mainly CO2 gas) release has an adverse effect on discharge. For the above reason, the carbon content is preferably in the above-defined range. The carbon content can be determined by measuring infrared absorption properties of a sample in a state free from carbon contamination from environment (for example, preferably within a clean room). The amount of the sample should be not less than 5 g to enhance detection accuracy.
The content of oxygen in the sinter constituting the sintered electrode for a cold cathode tube according to the present invention is preferably more than 0.01% by mass and not more than 6% by mass, particularly preferably more than 0.1% by mass and not more than 3% by mass. When the oxygen content is not more than 0.01% by mass, disadvantageously, the rare earth metal is likely to evaporate during use. On the other hand, an oxygen content of more than 3.0% by mass is disadvantageous in that, when the sinter is used as the electrode, gas (mainly CO2 gas) release has an adverse effect on discharge. For the above reason, the oxygen content is preferably in the above-defined range.
In the sinter constituting the sintered electrode for a cold cathode tube according to the present invention, the rare earth element (R)-carbon (C)-oxygen (O) compound is preferably present, in the sinter, as particles having an average particle diameter of not more than 10 μm, particularly preferably not more than 5 μm. When the average particle diameter is more than 10 μm, the diffusion of the above compound on the electrode surface is unsatisfactory and, further, the distribution quantity of the above compound on the electrode surface is reduced, resulting in increased cathode voltage drop. For this reason, the above-defined particle diameter range is preferred. Here the term “average particle diameter” is determined. by conducting measurement in three or more places of 40 μm×40 μm under an electron microscope and determining the average value of the maximum diameters of the projected particles.
In the sintered electrode for a cold cathode tube according to the present invention formed of the above sinter, the recrystallization of the sintered structure upon the application of a high voltage current has been suppressed. Accordingly, in the present invention using the specific sinter, higher-voltage welding conditions can be adopted in welding a lead wire to the electrode. Therefore, in a conventional electrode produced by conventional drawing, high-voltage welding conditions, which could not have been substantially adopted in the conventional electrode produced by conventional drawing, can be adopted in the present invention, and, thus, a sintered electrode for a cold cathode tube having a higher lead wire weld strength than the conventional cold cathode tube can easily be prepared.
In the present invention, as described above, a sintered electrode for a cold cathode tube, which can provide a long-service life cold cathode tube having low operating voltage and significantly suppressed mercury consumption and, at the same time, can realize a lead wire weld strength of not less than 400 N/mm2 per unit sectional area, can easily be provided.
As shown in
<Sintered Electrode for Cold Cathode Tube (Part 3)>
In one preferred embodiment of the present invention, as described above, in a section perpendicular to the longitudinal axis direction of the sintered electrode for a cold cathode tube, the inner wall surface of the cylindrical side wall part is in a concave-convex form. In this sintered electrode for a cold cathode tube according to the present invention, the inner surface area of the electrode (that is, surface area within the tube in a tubular electrode) is large, and the utilization of a hollow cathode effect derived from the tubular shape of the electrode can be maximized.
Accordingly, the sintered electrode for a cold cathode tube according to the present invention can further lower the operating voltage of the cold cathode tube.
In a sintered electrode 1 for a cold cathode tube according to the present invention, the concave-convex shape on the inner wall surface of the cylindrical side wall part may be any one. Specific examples of preferred concave-convex shapes include, for example, a corrugated shape as shown in
In preferred sintered electrodes for a cold cathode tube in the present invention (including both sintered electrodes shown in
The imaginary center (O) is a value determined with a roundness measuring device by “minimum area method” specified in JIS B 7451. The “outer diameter distance a” refers to an average distance between the imaginary center (O) and a plurality of points (preferably 8 points or more) present on the outer surface of the cylindrical side wall part in a section (the same section) perpendicular to the longitudinal axis direction of the sintered electrode for a cold cathode tube. The “inner diameter maximum length b” refers to a distance between the above imaginary center (O) and the farthermost point present on the inner surface of the side wall part in the same section. The “inner diameter minimum length c” refers to a distance between the imaginary center (O) and the nearmost point present on the inner surface of the side wall part in the same section.
When the ratio between the inner diameter maximum length b and the outer diameter distance a, i.e., b/a, is not more than 0.50, it is difficult to ensure a satisfactory surface area on the inner wall surface of the electrode. Further, in this case, the mold used in the production of the electrode is likely to be broken. On the other hand, when the b/a ratio exceeds 0.95, in the production of the electrode, cracking is likely to occur in the electrode and, consequently, the reject rate is enhanced. When the ratio between the inner diameter maximum length b and the outer diameter distance a, i.e., c/b, is not more than 0.50, cracking is likely to occur in the electrode during the production of the electrode. On the other hand, when the c/b ratio exceeds 0.95, the effect of improving the surface area of the internal wall surface is reduced. For the above reason, the b/a range and the c/b range are preferably in the above-defined respective ranges.
The concave-convex shape of the inner wall surface of the electrode is such that identical or similar concaves and/or convexes are regularly arranged, or concaves and convexes which are quite different from each other in size and shape are irregularly present. Further, in the whole section of a part extending from the opening to bottom in the cylindrical electrode, concaves and convexes having a substantially identical shape are provided on the inner wall part, or alternatively concaves and convexes may be changed in a some portion between the opening and the bottom, or further alternatively concave-convex shape-free parts may be present. In this case, the inner diameter maximum length b and the inner diameter minimum length c, b/a, and c/b, vary depending upon the cylindrical electrode part (that is, sectional position).
When the convenience in the production of the electrode, stability in use as the electrode, durability and the like are taken into consideration, the concave-convex shape of the inner wall surface in the electrode is preferably such that work for taking out the resultant sinter from the mold is easy and, further, the strength is even over the whole area without a local lack of strength. Accordingly, the concave-convex shape of the inner wall surface of the electrode is particularly preferably such that, in a section perpendicular to the longitudinal axis direction of the electrode, the concave and convex are relatively gently continued and, in a section parallel to the longitudinal axis direction of the electrode, the same concave-convex shape is continuously formed. An example of this is shown in
The sintered electrode for a cold cathode tube in which the inner wall surface of the cylindrical side wall part has the above shape may be produced by any desired method. In the present invention, in the production of the sinter, a method using a mold constructed so as to form a cylindrical sinter having the above inner wall surface shape is preferably adopted. In the present invention, after the production of the sinter, for example, barrelling, washing, and annealing are carried out to fabricate the inner side of the cylindrical side wall part into the above shape.
<<Production Process of Sintered Electrode for Cold Cathode Tube, and Cold Cathode Tube (Part 2)>>
The sintered electrode for a cold cathode tube according to the present invention in which the inner wall surface has the above predetermined shape may be produced by mixing raw material powders together, granulating the mixture, molding the granules into a predetermined shape and then sintering the molded product.
A preferred production process of the sintered electrode for a cold cathode tube according to the present invention will be described by mainly taking molybdenum as an example.
The molybdenum powder as the raw material powder has an average particle diameter of not less than 1 μm and not more than 5 μm, a purity of not less than 99.95%, and an oxygen content of not more than 0.5% by mass. When the raw material powder has a high oxygen content, the oxygen content after sintering is also large. For this reason, the above-defined content range is preferred. The rare earth metal (usually oxide) has an average particle diameter of not less than 0.1 μm and not more than 2 μm. Pure water and a binder (the binder being preferably polyvinyl alcohol (PVA)) are mixed in the powder, followed by granulation.
Next, a molded product is produced from the granules by a single press, a rotary press, or injection molding using a mold suitable for the formation of an inner wall surface having a predetermined shape. Thereafter, degreasing treatment is carried out in dry hydrogen at a temperature of 800° C. or above and 1000° C. or below for 4 hr or less. In this case, when degreasing is carried out for more than 4 hr, the carbon content is sometimes excessively lowered. Subsequently, sintering is carried out in hydrogen at a temperature of 1700° C. or above and 1800° C. or below for not less than 4 hr. If necessary, barreling, washing and annealing are carried out to prepare a sinter (for example, 1 to 3 mm in diameter×3 to 6 mm in length) having predetermined concaves and convexes in its inner wall surface.
Subsequently, a molybdenum rod having a diameter of 0.8 mm and a length of 2.6 mm is welded to a dumet rod having a diameter of 0.6 mm and a length of 40 mm to complete the assembly of the electrode. For example, a kovar alloy and nickel may be used as an insert metal for the electrode and the molybdenum rod.
<Cold Cathode Tube>
The cold cathode tube according to the present invention is characterized by comprising: a hollow tubular light transparent bulb into which a discharge medium has been sealed; a fluorescent material layer provided on the inner wall surface of the tubular light transparent bulb; and a pair of the above sintered electrodes for a cold cathode tube provided respectively on both ends of the tubular light transparent bulb.
In the cold cathode tube according to the present invention, for example, a discharge medium, a tubular light transparent bulb, and a fluorescent material layer, which are indispensable constituent elements other than the sintered electrode for a cold cathode tube, those which have hitherto been used in this type of cold cathode tubes, particularly cold cathode tubes for backlight in liquid crystal displays, may be used either as such or after suitable alteration.
Regarding elements which can be applied and are preferred in the cold cathode tube according to the present invention, examples of discharge media include rare gas-mercury systems (examples of rare gases including argon, neon, xenon, krypton, and mixtures thereof), and examples of fluorescent materials include fluorescent materials which emit light upon ultraviolet light stimulation, preferably calcium halophosphate fluorescent materials.
Examples of hollow tubular light transparent bulbs include glass tubes having a length of not less than 60 mm and not more than 700 mm and a diameter of not less than 1.6 mm and not more than 4.8 mm.
<Liquid Crystal Display Device>
The liquid crystal display device according to the present invention is characterized by comprising: the above sintered electrode for a cold cathode tube; a light guide body disposed closely to the sintered electrode for a cold cathode tube; a reflector disposed on one surface side of the light guide body; and a liquid crystal display panel disposed on another surface side of the light guide body.
A liquid crystal display device 20 shown in
In the present invention, the number of cold cathode tubes may be any desired one. For example, as shown in
In the present invention, the cold cathode tube 21 should satisfy predetermined requirements which have been described above in detail. However, various constituent members (for example, the light guide body 22, the light reflector 23, the liquid crystal display panel 24, the light diffuser 25a, 25b, 25c, the support substrate 26, the reflector 27 for a cold cathode tube, the surface protector 28, the antireflector 29, the antistatic body 30, the heat radiating member 31, the frame, the case, and the seal member) other than the cold cathode tube 21 may be those which have hitherto been used in the art.
Electrodes were prepared under varied conditions as shown in Tables 1 to 4 and were incorporated in a cold cathode tube for the evaluation of properties.
The cold cathode tube had an outer diameter of 3.2 mm and an interelectrode distance of 350 mm, and a mixed gas composed of mercury and neon/argon was sealed into the tube. Regarding initial properties, the results of measurement of the operating voltage are shown in Tables 1 to 4.
Regarding the service life of the cold cathode tube, “rare gas discharge mode” in which mercury within the tube is consumed as a result of the formation of an amalgam with the sputtering material is dominative. Therefore, the service life of the cold cathode tube was evaluated by evaluating the amount of mercury consumed.
The results of measurement of the amount of mercury consumed after 15000 hr are also shown in Tables 1 to 4.
When the Sm value exceeds 100 μm, the operating voltage and the amount of mercury evaporated are rapidly increased. When the Sm value is not more than 100 μm, this phenomenon disappears.
In the case of Mo with La2O3 added thereto, the operating voltage is considerably lowered.
Very good properties are provided when the thickness of the side wall part and the thickness of the bottom face part are 0.4 mm and 0.5 mm, respectively.
The results of measurement of the surface roughness (Sm) of the inner surface of the sintered electrode for a cold cathode tube in Example 1 are shown in
Measuring instrument: S4 manufactured by Taylor Hobson
Measuring conditions: cutoff=0.8 mm, evaluation length=1.6 mm, filter=Gaussian filter, stylus tip=R 2 μm, stylus shape=60° cone.
Electrodes were prepared under varied conditions as shown in Tables 5 to 7 and were incorporated in a cold cathode tube for the evaluation of properties.
For all the sintered electrodes for a cold cathode tube prepared in the Examples and Comparative Examples, the shape was as shown in
The cold cathode tubes had an outer diameter of 2.0 mm and an interelectrode distance of 350 mm, and a mixed gas composed of mercury and neon/argon was sealed into the tube. Regarding the service life of the cold cathode tube, “rare gas discharge mode” in which mercury within the tube is consumed as a result of the formation of an amalgam with the sputtering material is dominative. Therefore, the service life can be evaluated by evaluating the amount of mercury consumed.
The results of measurement of the amount of mercury consumed after 10000 hr are also shown in Tables 5 to 7.
The relationship between the average particle diameter (μm) and the initial discharge voltage (V) for an La—C—O compound in an Mo sinter containing the composition of Example 59 (2% La—O—C compound (O2 content 0.4% by mass, C content 30 ppm)) is as shown in
The results of analysis by color mapping by EPMA for this sinter (that is, 2% La—O—C compound (O2 content 0.4% by mass, C content 30 ppm)) is as shown in
In
Sintered electrodes for a cold cathode tube, which comprise an Mo sinter containing the composition of Example 59 (2% La—O—C compound (O2 content 0.4% by mass, C content 50 ppm) and has a corrugated shape as shown in
Each electrode was incorporated in a cold cathode tube in the same manner as in Example 59, and the properties thereof were evaluated in the same manner as in Example 59.
The results were as described in Table 8
For the electrodes of Example 60 and Comparative Example 34, the weld strength was measured. For the weld strength, the electrode was welded to an Mo lead of 0.8 mm in diameter×2.6 mm through a kovar foil of 1.0 mm in diameter×0.1 mm in length, and welding was carried out using a direct current of 500 A×30 ms. For each of the example and the comparative example, 10 assemblies were prepared. These assemblies were subjected to a tensile test at a speed of 10 mm/min (
As is apparent from Table 9, the sintered electrode in the example of the present invention has a high strength of joining to the lead wire.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2004-139559 | May 2004 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP05/08306 | 5/2/2005 | WO | 00 | 3/3/2006 |
