EP1115144A1

EP1115144A1 - Method for manufacturing fluorescent lamp and phosphor suspension

Info

Publication number: EP1115144A1
Application number: EP00927810A
Authority: EP
Inventors: Koji Kitamura; Hirokazu Tachibana
Original assignee: Matsushita Electronics Corp
Current assignee: Panasonic Corp
Priority date: 1999-05-25
Filing date: 2000-05-19
Publication date: 2001-07-11
Also published as: EP1115144A4; JP2000340181A; CN1319249A; JP3430971B2; CN1288705C; WO2000072356A1

Abstract

In a phosphor lamp manufacturing method, a phosphor suspension that is not prone to deterioration is prepared, and applied to an inner surface of a glass bulb. Then, the phosphor suspension is dried and baked to form a phosphor film. In addition to a phosphor, metal oxide used as a bonding agent, pure water used as a dispersant and ammonia used as a pH regulator are mixed into the phosphor suspension, and pH is regulated in a range of no less than pH8 and no more than pH10. Aluminum oxide particles with a specific surface area of no less than 1.5 m²/g and no more than 30 m²/g are used as the metal oxide. This makes the phosphor suspension less prone to deterioration and enables a high film strength of 1.5 gf/cm² or more to be obtained even if the phosphor film is formed using a phosphor suspension that has been left for a long period after mixing.

Description

Technical Field

The present invention relates to a fluorescent lamp manufacturing method, and to an improvement in a phosphor suspension used in a phosphor film forming process thereof.

Background Art

In a fluorescent lamp manufacturing process, a phosphor suspension including a phosphor is applied to an inner surface of a glass tube, dried, and baked to form a phosphor film.
The phosphor suspension is conventionally composed of phosphor particles and minute particles of a metal oxide formed mainly from aluminum oxide, mixed together with water used as a dispersant. The metal oxide is added to the phosphor suspension as a bonding agent to increase the connecting surface areas of neighboring particles. This increases both the connectivity of the phosphor particles and their ability to bond to the inner wall of the glass tube, thereby forming a phosphor film with a high film strength.
As a result, ensuring that the metal oxide has a large surface area has conventionally been considered a precondition for maintaining film strength, and a metal oxide with a large specific surface area of around 100 m²/g is used. An aluminum oxide with a high specific surface area, such as Degussa AG's 'Aluminum Oxide C' (product name; specific surface area of around 100 m²/g) is one example of a metal oxide in general use.
However, one characteristic of the phosphor suspension is that phosphor particles usually sink down to the bottom of the suspension and harden at a pH value of less than pH8. When a fluorescent lamp manufacturing process is repeated at long intervals, it is extremely difficult to redisperse the phosphor once it has hardened in this way. Meanwhile, the phosphor suspension suddenly changes into a gel when the pH value exceeds pH10, and obtaining a uniform phosphor film is impossible if a gelled phosphor suspension is applied.
Therefore, in order to prevent hardening of the phosphor, and gelling of the phosphor suspension, an appropriate amount of alkaline solution is conventionally mixed into the phosphor suspension in order to keep it in a range of no less than pH8 and no more than pH10.
However, when the pH value of the phosphor suspension is kept in the range of no less than pH8 and no more than pH10 using such a prior art technique, the aluminum oxide tends to react with the alkaline solution over time and deteriorate, thereby causing degeneration of the phosphor suspension. A phosphor film formed from such a degenerated phosphor suspension will have a particularly weak film strength, and is likely to become detached from the inner wall of the glass tube as a result of changes in pressure when the glass tube is evacuated or shocks occurring during transportation of the lamp.
In order to solve the problem of deterioration in film strength, a method in which only the necessary amount of phosphor suspension is mixed immediately prior to forming the phosphor film may be considered. However, mixing the phosphor suspension on each occasion is extremely time-consuming. Consequently, this is not a viable solution if the mass production that is essential for cost-cutting is to be performed. Therefore, a phosphor suspension that can be prepared in large quantities beforehand and that does not deteriorate over time is required.
The present invention has been developed to overcome such problems, and has as its object the provision of a phosphor suspension in which the deterioration over time that occurs after mixing is limited, and of a fluorescent lamp manufacturing method that uses such a phosphor suspension to form a phosphor film with a high film strength.

Disclosure of the Invention

A fluorescent lamp manufacturing method of the present invention may be a manufacturing method for forming a phosphor film on an inner surface of a glass tube by applying a phosphor suspension including a phosphor and a metal oxide. Here, a pH value of the phosphor suspension is adjusted so as to be in a range of no less than pH8 and no more than pH10, and the metal oxide includes particles of aluminum oxide each having a specific surface area of no less than 1.5 m²/g and no more than 30 m²/g.
By setting the specific surface area of the particles of aluminum oxide at no less than 1.5 m²/g and no more than 30 m²/g, this being smaller than the size conventionally used, the specific surface area that reacts to the alkaline solution is less. Therefore, deterioration in the phosphor suspension over time are restricted and the film strength of a phosphor film formed using such particles as a raw material is improved.
Here, virtually all metal oxide particles may satisfy a condition 0.5≦b/a≦1.0, a (µm) being a length of a metal oxide particle and b (µm) being a breadth of a metal oxide particle.
Furthermore, virtually all metal oxide particles may satisfy a condition 0.05≦c≦1.00, c being an average diameter of a metal oxide particle.

Brief Description of the Drawings

FIG. 1 shows a conventional structure of a fluorescent lamp having a straight tube, which has been partially cut away;
FIG. 2 is a design drawing showing the condition of particles in a phosphor suspension coating an inner surface of a glass tube in the fluorescent lamp prior to drying;
FIG. 3 is a design drawing showing the condition of particles in the phosphor suspension coating the inner surface of the glass tube after a baking process has been performed;
FIG. 4 is a graph showing the relationship in the present embodiment between a specific surface area of aluminum oxide mixed into the phosphor suspension, and film strength;
FIG. 5 is a table showing, as film strength values, the degree of deterioration over time experienced by (a) the phosphor suspension of the present embodiment, and (b) a conventional phosphor suspension including aluminum oxide with a specific surface area of 100 m²/g;
FIG. 6 is a table showing the relationship between types of additive added to the metal oxide included in the phosphor suspension, and film strength of a phosphor film formed by the phosphor suspension produced in each case; and
FIG. 7 is a process diagram showing the procedure for manufacturing the fluorescent lamp.

Best Mode for Carrying Out the Invention

The following is a description of embodiments of a fluorescent lamp manufacturing method of the present invention with reference to the drawings.

Structure of Fluorescent Lamp

First, an example of a structure for a fluorescent lamp 100 that is formed by the manufacturing method of the present embodiment is described with reference to FIG. 1. In the drawing, the fluorescent lamp 100 is of the straight tube type, and uses a long slim glass tube 1 as an arc tube. Part of the external surface of the glass tube 1 has been cut away to show its internal structure.
A glass stem 4, in which the bases of electrodes 2 are embedded, is sealed at each end of the glass tube 1. The electrodes 2 have a tungsten coil 3 whose surface is coated with an emitter. Oxides such as barium, strontium, and calcium are conventionally used as emitters. Furthermore, a phosphor film 8 is formed on the inner surface of the glass tube 1, and appropriate quantities of mercury and an inert gas such as argon are enclosed within the tube.
A base pin 5 is connected to each of the outer ends of electrodes 2, and a voltage is applied from a stabilizer (not shown) to the electrodes 2 via the base pins 5. Bases 6 hold the base pins 5 in place via plates 9, and also serve to protect each end of the glass tube 1.
When a high voltage is applied to the electrodes 2 by the stabilizer, the tungsten coil 3 heats up, causing electrons to be emitted from the emitter coating its surface. The electrons collide with the mercury vapor enclosed in the tube, generating ultraviolet light. The phosphor film 8 coating the inner surface of the glass tube 1 receives the ultraviolet light, and phosphor in the phosphor film 8 is excited, generating visible light.
If the phosphor film 8 is not firmly attached to the inner surface of the glass tube 1, it will easily become detached if even a slight shock is felt during evacuation of the tube, assembly or transportation, thereby resulting in a defective product. Consequently, when the fluorescent lamp manufacturing process is performed, the phosphor film 8 needs to be formed so as to have a film strength of at least a certain level.
The phosphor film 8 is formed by coating the phosphor suspension explained hereafter over the inner surface of the glass tube 1 to form a layer of an approximately uniform thickness, and then drying and baking the phosphor suspension to form a phosphor film with an extremely high film strength.

Components of the Phosphor Suspension and Formation of the Phosphor Film

The phosphor suspension in the present embodiment is formed from a mixture of phosphor particles, metal oxide used as a bonding agent, polyethylene oxide used as a binder to increase the viscosity of the phosphor suspension, pure water used as a dispersant, and ammonia used as a pH regulator, in the following proportions by weight and in the stated order: 1 : 0.013 : 0.009 : 1.850 : 0.0001. Here, the polyethylene oxide is dissolved in the pure water using an agitator. Then, the phosphor, metal oxide, and ammonia are added, in that order, and mixed together. A phosphor suspension formed from such components has a final pH of 9, and viscosity of 40 mPas (when shearing speed is 20 s^-1).
The phosphor is formed by compounding europium-activated yttrium oxide, cerium terbium-activated lanthanum phosphate, and europium-activated strontium halophosphate in the proportions of 40 : 50: 10 by weight in the stated order. Here, the metal oxide is aluminum oxide having an α-alumina crystal structure with a specific surface area of 15 m²/g and an average particle diameter of 0.1 µm (the ratio of breadth to length is 0.9).
When the phosphor film 8 is formed, the glass tube 1 is first stood on end, and the above-described phosphor suspension applied to the inner surface of the glass tube 1. FIG. 2 is a design drawing showing the condition of particles in the phosphor suspension coating the inner surface at this stage in the manufacturing process. In the drawing, large particles of a phosphor 82 are suspended in a binder 81 of the phosphor suspension, and minute particles of a metal oxide 83 enter the gaps between particles.
The phosphor suspension attached to the inner surface of the glass tube 1 is then dried by applying air heated to a temperature of around 70°C. Then the glass tube 1 is heated in a gas furnace to a temperature of around 550°C to form the phosphor film 8 on the inner surface of the glass tube 1.
FIG. 3 is a design drawing showing the state of particles in the phosphor film 8 after baking. As shown in the drawing, the binder 81 vaporizes during baking, and the metal oxide 83 acting as a bonding agent melts, bonding neighboring particles of the phosphor 82 together, and attaching the phosphor 82 to the inner surface of the glass tube 1. This enables a phosphor film 8 that has a high film strength and does not easily become detached to be formed.
Hereafter, a fluorescent lamp 100 with a phosphor film 8 formed from a phosphor suspension having the above components is also referred to as 'the invention'.

Specific Surface Area and Film Strength of the Metal Oxide

Next, let us consider the relationship between the specific surface area and the film strength of the metal oxide.
A number of experimental lamps having the same basic structure as the fluorescent lamp 100 were manufactured, each having a differing aluminum oxide specific surface area in a range of between 0.2 m²/g and 100 m²/g. The film strengths of each lamp were then investigated, and the results of this experiment plotted to form the curve of the graph in FIG. 4.
The horizontal axis of the graph in Fig. 4 shows the value of the specific surface area used in each case, and the vertical axis shows the corresponding film strengths formed. Here, the method used to evaluate the film strength of the phosphor film involved vertically-sectioning each fluorescent lamp, and blowing high-pressure air onto the phosphor film via a stainless steel tube. The stainless steel tube had an internal diameter of 0.5 mm and was positioned perpendicular to the phosphor film at a center of a vertical direction of the fluorescent lamp, with the end of the stainless steel tube from which the air is blown being 15 mm away from the phosphor film. Then the pressure at which the phosphor film became detached from the inner surface of the fluorescent lamp was recorded as an indicator of the film strength of the phosphor film, in units of kgf/cm².
Moreover, it was found that the phosphor film did not become detached if shocks were received during evacuation or transportation, provided that the film strength was at least 1.5 kgf/cm². Therefore, this film strength of 1.5 kgf/cm² was used as a standard value when determining whether the film strength of the phosphor film in a particular fluorescent lamp was satisfactory, with fluorescent lamps having a film strength of at least 1.5 kgf/cm² being judged as satisfactory.
Note that the phosphor suspension used in this experiment had been mixed 16 days previously.
As is clear from the experimental results shown in FIG. 4, aluminum oxides with a specific surface area of no less than 1.5 m²/g and no more than 30 m²/g produce satisfactory results in which the film strength is at least as much as the standard value of 1.5 kgf/cm². In particular, a maximum film strength of 1.67 kgf/cm² can be obtained when the specific surface area is 15 m² /g, and the graph shown in FIG. 4 slopes gently at specific surface areas in a range of no less than 4.5 m²/g and no more than 15 m²/g, so that high film strengths of between 1.64 kgf/cm² and 1.67 kgf/cm² can be stably obtained. Consequently, if particles of a metal oxide having a specific surface area within this range as a standard value are used, a high film strength can be obtained even if there is a degree of error in the actual specific surface area of various particles. This means that process management of metal oxide particle generation can be performed more easily.
If, however, the specific surface area of the aluminum oxide is less than 1.5 m² /g, or exceeds 30 m²/g, it can be seen that the film strength of the phosphor film will not reach 1.5 kgf/cm².
The reasons for this are thought to be as follows. When specific surface area is less than 1.5 m²/g, this is too small to obtain sufficient contact area. Meanwhile, when a specific surface area exceeding 30 m²/g is used, the aluminum oxide hydrates and elutes within the phosphor suspension, thereby generating aluminum hydroxide. As a result, the thickness of the electrical double layer in the aluminum oxide is reduced, making the aluminum oxide coagulate electrostatically, and the density of the aluminum oxide increases over time.
Next, two sets of experimental lamps were manufactured, lamps in each set using phosphor suspensions that had been mixed 1, 2, 4, 8, 16, 30, and 60 days previously. One set of lamps corresponded to the fluorescent lamp of this invention and used aluminum oxide with a specific surface area of 15 m²/g as a bonding agent, and the other set were fluorescent lamps having an identical structure to the invention apart from using aluminum oxide with a specific surface area of 100 m²/g as a bonding agent (hereafter these lamps are referred to as the 'prior art'). The film strengths of the phosphor film of corresponding lamps in each set were investigated and compared to obtain the results shown by a table 1 in FIG. 5.
Note that the method used to evaluate the film strength of the phosphor film was identical to that described above.
As shown in the table 1, the film strength of the phosphor film in the case of the present invention was 1.73 kgf/cm² after 60 days, showing a drop of only 1% in comparison with the film strength after 1 day (1. 75 kgf/cm²). However, the film strength of the prior art was 1.15 kgf/cm²) after 60 days, a drop of 36% in comparison with the film strength after 1 day (1.80 kgf/cm²).
Therefore, if a metal oxide having a specific surface area in the range shown in this invention is used, a sufficiently high film strength can be obtained without the use of a metal oxide with a large specific surface area used in the prior art, and changes in the phosphor suspension over time limited, so that a phosphor film with a high film strength can be formed, regardless of the number of days that has passed since the phosphor suspension was mixed.

PH of Phosphor Suspension

Furthermore, the pH value of the phosphor suspension should preferably be kept within a range of no less than pH8 and no more than pH9, in order to prevent deterioration over time, and obtain a higher film strength. As explained above, deterioration in the phosphor suspension over time is mainly caused by alkaline erosion of the metal oxide. Consequently, alkalinity should be kept at the lower end of the range of pH8 to pH10 that is required for the phosphor suspension.

Shape of Metal Oxide Particles

Furthermore, when the metal oxide, that is aluminum oxide, has diameters of a µm x b µm (a being particle length and b particle breadth), particle size should preferably fulfil the relation 0.5≦b/a≦1.0. This is because even in cases where the average diameter of metal oxide particles is identical, when b/a<0.5, the specific surface area of the metal oxide is larger, and the transformation of the aluminum oxide into hydrogen proceeds at a marked rate. As a result, the film strength of the phosphor film shows a great amount of deterioration over time. Therefore, defining b/a by the above range limits the changes in film strength of the phosphor film over time.
As shown in FIG. 3, the metal oxide acts as a bonding agent when it is melted during the baking process, and seeps down into the gaps between phosphor particles, attaching itself to the surface of these particles. As a result the metal oxide acts as a kind of bond. From this perspective, metal oxide particles should preferably be of at least a certain volume, but if surface area is increased, there is more likelihood of deterioration occurring, as described above. If b/a is near to 1.0, that is a particle is nearly spherical, a small surface area can be achieved with the same volume. Consequently, the shape of metal oxide particles should preferably be within the range 0.5≦b/a≦1.0.

Average Diameter of Metal Oxide Particles

Furthermore, when an average diameter of aluminum oxide particles is c µm, the relation 0.05≦c≦1.00 should preferably be satisfied. The reason for this is when c<0.05, the particles of aluminum oxide become too minute, and coagulate together, making it difficult to disperse particles in the phosphor suspension, and so use of such minute particles is impractical. Furthermore, when c>1.00, particles of aluminum oxide cannot fit satisfactorily into the narrow gaps between neighboring phosphor particles, markedly reducing connecting surface areas of neighboring phosphor particles. This in turn reduces bonding strength, and consequently the film strength of the phosphor film. Therefore, defining c using the above range facilitates dispersion and ensures that a phosphor film with a film strength that is sufficient for actual use can be obtained.

Crystal Structure of Aluminum Oxide

Use of aluminum oxide with an α-alumina crystal structure is preferable in order to further restrict changes in the film strength of the phosphor film over time. The reason for this is that aluminum oxide having this structure is less likely to react with alkaline solution than a comparable γ-alumina crystal structure, and so generation of changes causing deterioration in the phosphor film is also less likely.

Additives Other than Aluminum Oxide

Furthermore, the inventors have discovered that addition of metal oxides other than aluminum oxide to the phosphor suspension further increases film strength.
The following experimental lamps were manufactured to demonstrate the effect of a fluorescent lamp manufacturing method when other metal oxides were added. Aluminum oxide with a γ-alumina crystal structure (specific surface area 15 m²/g, average particle diameter 0.1 µm, ratio of particle length to breadth 0.9) was used as the main component of the metal oxide in each case. Each of (a) strontium oxide, (b) lanthanum oxide, (c) boron oxide, and (d) both lanthanum oxide and boron oxide were added to aluminum oxide, and the resulting mixtures included in the phosphor suspensions to form phosphor films in lamps A, B, C, and D respectively. In all other respects these lamps are identical to the invention.
Here, the atomic ratio of aluminum oxide to strontium oxide in Lamp A is 1:0.02.
The atomic ratio of aluminum oxide to lanthanum oxide in Lamp B is 1:0.02.
The atomic ratio of aluminum oxide to boron oxide in Lamp C is 1:0.1.
The atomic ratio of aluminum oxide to lanthanum oxide is 1:0.02, and the atomic ration of aluminum oxide to boron oxide is 1:0.1 in Lamp D.
Note that the invention, in other words a fluorescent lamp using only aluminum oxide as the metal oxide, is referred to as lamp E.
The film strengths of the phosphor films in each of lamps A to E were investigated using the same method, and the results shown in table 2 of FIG. 6 obtained.
Note that a phosphor suspension that had been mixed 16 days previously was used in this experiment.
Table 2 indicates that the film strengths of the phosphor films in Lamps A, B, C, and D have improved by 5%, 8%, 9%, and 14% respectively when compared with lamp E. This is believed to be due to the fact that addition of strontium oxide, lanthanum oxide, and boron oxide lowers the melting point of the metal oxide and improves the ability of the phosphor film to bond with the inner surface of the glass tube 1.
Here, the main component of the metal oxide is aluminum oxide, and it is preferable to add one of the above mentioned additives at an atomic ratio of no less than 1:0.001 and no more than 1:1.00. The reason for this is that when the atomic ratio of aluminum oxide to the additive is less than 1:0.001, the amount of additive is too small, so that a sufficient reduction in the melting point cannot be obtained, and the film strength of the phosphor film does not increase. If the atomic ratio is more than 1:1.00, however, the phosphor film becomes stained during the baking process, and luminous flux is reduced.
Therefore, keeping the atomic ratio of aluminum to the additive within the above range prevents reduction of luminous flux as well as increasing the film strength of the phosphor film.

Total Metal Oxide Content

Furthermore, the total metal oxide content of the phosphor suspension, should be no less than 1% and no more than 10% by weight of the phosphor content. The reason for this is that if the total metal oxide content is less than 1% by weight of the phosphor content, the amount of metal oxide is too small, and sufficient increase in connecting surface areas of neighboring particles cannot be obtained, so that the film strength of the phosphor film cannot be sufficiently increased. If, however, the total metal oxide content is more than 10% by weight, the metal oxide stains the phosphor film, reducing luminous flux.
Therefore, keeping the total metal oxide content within the above described range in relation to the phosphor content, prevents reduction of luminous flux, and increases the film strength of the phosphor film. Keeping within this range is still preferably even if the metal oxide consists solely of aluminum oxide.

Outline of Manufacturing Process

The above explanation focuses on components of the phosphor suspension, and in particular on metal oxide used as a bonding agent. In conclusion, a brief explanation of the process used to manufacture the fluorescent lamp 100 is made.
The manufacturing method for the fluorescent lamp 100 mainly includes the following seven processes, as shown in FIG. 7: (a) phosphor suspension preparation process, (b) application process, (c) drying process, (d) baking process, (e) electrode attaching process, (f) sealing process, and (g) pin attaching process.
Firstly, a phosphor suspension with the components described above is prepared ((a) phosphor suspension preparation process). Next, the glass tube 1 is stood up, and the phosphor suspension applied to the inner surface of the glass tube 1 by squirting it in via a nozzle ((b) application process).
Following this, the phosphor suspension attached to the inner surface of the glass tube 1 is dried by being blasted for five minutes with hot air heated to a temperature of 70°C ((c) drying process). Then the entire glass tube 1 is heated at a temperature of 550°C in a gas furnace for three minutes, forming a phosphor film on the inner surface of the glass tube 1 ((d) baking process). This causes the binder 81 to evaporate, and melts the metal oxide 83, bonding particles of phosphor 82 to each other and to the inner surface of the glass tube 1, forming a phosphor film that has a high film strength and will not easily become detached (see FIG. 3).
Following this, the electrodes 2, that are held by stems 4 are attached to each end of the glass tube 1((e) electrode attaching process). Then, the inside of the glass tube 1 is evacuated via an exhaust port 7, predetermined amounts of mercury and argon gas are introduced into the inside of the glass tube 1, and the exhaust port 7 sealed ((7) sealing process). Finally, the base pins 5 are attached to the electrodes 2, and the bases 6 fixed to either end of the glass tube 1 ((g) base attaching process), completing the fluorescent lamp 100 shown in FIG. 100.

Modifications

The invention need not of course be limited to the above embodiments, and the following modifications are also possible.
1. In the above embodiments, the additive included in the metal oxide is described as being one of strontium oxide (an alkaline earth metal oxide), lanthanum oxide (a rare earth element oxide), and boron oxide (a group 13 element oxide). However, similar effects can be achieved by using alkaline earth metal oxides such as barium oxide, calcium oxide, and magnesium oxide, rare earth metal oxides such as cerium oxide, terbium oxide, yttrium oxide, scandium oxide, europium oxide, and ytterbium oxide, and group 13 element metal oxides such as potassium oxide, and indium oxide. These oxides may be added singly or in combination.
2. In the above embodiments, a metal oxide that is a mixture of aluminum oxide and a specific additive is described as being used. However, a similar effect may be achieved by using a compound metal oxide consisting of aluminum oxide in which one or more additives have been diffused.
3. The above embodiments describe the use of polyethylene oxide as a binder for increasing the viscosity of the phosphor suspension. However, a similar effect can be achieved by using one of polyethylene glycol, hydrocellulose, nitrocellulose, carboxymethyl cellulose, polyacrylate, an acrylic acid-maleate copolymer, or an oleic acid-maleate copolymer.
4. The above embodiments describe the use of europium-activated yttrium oxide, cerium terbium-activated lanthanum phosphate, and europium-activated strontium halophosphate as a phosphor. However, a similar effect may be obtained by using at least one of europium-activated barium magnesium aluminate, europium manganese-activated barium magnesium aluminate, terbium-activated cerium aluminate, terbium-activated cerium magnesium aluminate, and antimony-activated calcium halophosphate as a phosphor.
5. In the above embodiments, when the metal oxide particles have a length of a µm, a breadth of b µm, and a average size of c µm, the size of particles of metal oxide is described as being within a numerical range that satisfies the conditions 0.5≦b/a≦1.0, and 0.05≦c≦1.00. Of course all particles of metal oxide should preferably satisfy these conditions. There is a risk, however, that particles may break up during the manufacture of the metal oxide and the mixing of the phosphor suspension, so that a proportion of particles that do not satisfy the conditions may be generated. However, provided that the majority of metal oxide particles satisfy the conditions, bonding strength can still be increased as described above.
6. The present invention need not be restricted to a fluorescent lamp having a straight tube, and may also be applied to a circular fluorescent lamp, or a fluorescent lamp formed from a plurality of fine U-shaped arc tubes, connected by discharge channels.

Industrial Applicability

The fluorescent lamp manufacturing method of the present invention uses a phosphor suspension in which deterioration over time is limited. As a result, a phosphor film with a high film strength can be formed even if a long period of time has passed since the phosphor suspension was mixed, providing favorable conditions for mass production of high quality fluorescent lamps.

Claims

A fluorescent lamp manufacturing method for forming a phosphor film on an inner surface of a glass tube by applying a phosphor suspension including a phosphor and a metal oxide, wherein:

a pH value of the phosphor suspension is adjusted so as to be in a range of no less than pH8 and no more than pH10, and

the metal oxide includes particles of aluminum oxide each having a specific surface area of no less than 1.5 m²/g and no more than 30 m²/g.
The fluorescent lamp manufacturing method of Claim 1, wherein virtually all metal oxide particles satisfy a condition 0.5≦b/a≦1.0, where a (µm) is a length of a metal oxide particle and b (µm) is a breadth of a metal oxide particle.
The fluorescent lamp manufacturing method of one of Claims 1 and 2, wherein virtually all metal oxide particles satisfy a condition 0.05≦c≦1.00, where c is an average diameter of a metal oxide particle.
The fluorescent lamp manufacturing method of any one of Claims 1 to 3, wherein the aluminum oxide has an α-alumina crystal structure.
The fluorescent lamp manufacturing method of any one of Claims 1 to 4, wherein the metal oxide includes, in addition to aluminum oxide, an additive for reducing a melting point of the metal oxide.
The fluorescent lamp manufacturing method of Claim 5, wherein the additive includes at least one of an alkaline earth metal oxide, a rare earth element oxide and a group 13 element oxide.
The fluorescent lamp manufacturing method of Claim 6, wherein the alkaline earth metal oxide includes at least one of barium oxide, strontium oxide, calcium oxide, and magnesium oxide.
The fluorescent lamp manufacturing method of Claim 6, wherein the rare earth element oxide includes at least one of lanthanum oxide, cerium oxide, terbium oxide, yttrium oxide, scandium oxide, europium oxide, and ytterbium oxide.
The fluorescent lamp manufacturing method of Claim 6, wherein the group 13 element oxide includes at least one of boron oxide, gallium oxide, and indium oxide.
The fluorescent lamp manufacturing method of any one of Claims 6 to 9, wherein the additive is added to the aluminum oxide so that an atomic ratio of particles of aluminum oxide to particles of additive is no less than 1:0.001 and no more than 1:1.00.
The fluorescent lamp manufacturing method of any one of Claims 1 to 10, wherein the metal oxide is included in the phosphor suspension in a range of no less than 0.1% and no more than 10% by weight of the phosphor content.
A phosphor suspension that is a raw material for a phosphor film, and includes a phosphor and a metal oxide, wherein a pH value of the phosphor suspension is adjusted so as to be in a range of no less than pH8 and no more than pH10, and the metal oxide includes particles of aluminum oxide each having a specific surface area of no less than 1.5 m²/g and no more than 30 m²/g.