JP3476938B2 - Thermal head - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement of a thermal head incorporated as a printer mechanism of a word processor, a facsimile or the like. 2. Description of the Related Art Conventionally, as shown in FIG. 3, a thermal head incorporated as a printer mechanism such as a word processor includes an electrically insulating substrate 1 made of alumina ceramic or the like.
1, a heating resistor 12 made of tantalum nitride or the like, a pair of conductive layers 13 made of aluminum or the like, and a protective layer 14 made of glass are sequentially applied,
A predetermined electric power is applied between the pair of conductive layers 13 based on a print signal to selectively cause the heating resistor 12 to generate Joule heat and to conduct the generated heat to a heat-sensitive paper or the like pressed by a platen roller or the like. Then, a predetermined print image is formed on thermal paper or the like to function as a thermal head. The protective layer 14 protects the heating resistor 12 and the pair of conductive layers 13 from abrasion due to sliding contact with thermal paper or the like and oxidative corrosion due to contact with moisture contained in the atmosphere. Protective layer 1 mainly composed of soft low softening point glass (Vickers hardness: about 180 to 450 Hv)
Hard inorganic particles 14a made of alumina or the like (Vickers hardness: about 1800 H
v). Such a protective layer 14 is usually formed as follows. First, a hard inorganic material such as alumina, which is finely crushed to a particle size of about 1 μm, is added to a predetermined glass paste prepared using a glass having a low softening point, and is mixed. The protective layer 14 is formed by printing and applying on the alumina insulating substrate 11 made of alumina ceramic on which the heating resistor 12 is attached by a screen printing method, and then firing at a predetermined temperature. The reason why the low softening point glass is used to form the protective layer 14 is that if the firing temperature of the glass is too high, the heating resistor 12 and the like may be deformed by heat. However, in this conventional thermal head, the specific gravity of the inorganic particles 14a (alumina) contained in the protective layer 14 is about 3.98.
Since the specific gravity of the low softening point glass is close to the specific gravity (4.0 to 4.5) and the alumina particles have a relatively large particle size of about 1 μm, the protective layer 14 is formed by a thick film method. When formed, a part of the inorganic particles 14a is exposed,
A large number of large projections are formed on the surface of the protective layer 14 according to the shape of the inorganic particles 14a. For this reason, when printing is performed by sliding the thermal paper or the like into contact, there is a disadvantage that the adhesion between the thermal head and the thermal paper or the like is poor, and a clear printed image cannot be formed on the thermal paper or the like. I have. If the above-mentioned large projections are formed on the surface of the protective layer 14, the stress due to sliding contact of thermal paper or the like during printing is concentrated on the large projections. Cracks are formed starting from the large projections, causing a disadvantage that the corrosion resistance of the protective layer 14 is reduced. SUMMARY OF THE INVENTION The present invention has been devised in view of the above-mentioned drawbacks, and has as its object to provide a thermal head having a high corrosion resistance capable of always forming a clear printed image on thermal paper or the like. Is to provide. A thermal head according to the present invention comprises a heating resistor and a pair of conductive layers connected to the heating resistor on an electrically insulating substrate. , A thermal head coated with a protective layer containing glass and inorganic particles made of tungsten carbide having a specific gravity greater than that of the glass and having a high hardness, wherein the inorganic material is provided in the vicinity of the heating resistor and a conductive layer. The particles are intensively distributed, and a silicon nitride layer is interposed between the inorganic particles and the heating resistor and the pair of conductive layers. Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. FIG. 1 is a sectional view showing an embodiment of a thermal head according to the present invention, wherein 1 is an electrically insulating substrate, 1a is a glaze layer, 2 is a heating resistor, 3 is a pair of conductive layers, and 4 is protection. The layers 4a are glass and 4b are inorganic particles. The electrically insulating substrate 1 is made of an electrically insulating material such as alumina ceramics, and functions to support the heating resistor 2 and the like on its upper surface. The electrically insulating substrate 1 is formed into a slurry by adding and mixing an appropriate organic solvent and a solvent to a ceramic raw material powder such as alumina, silica, magnesia, etc., and forming the slurry into a slurry by a conventionally known doctor blade method or calender roll method. A ceramic green sheet is formed by employing the method described above, and thereafter, the ceramic green sheet is punched into a predetermined shape and fired at a high temperature. A glaze layer 1a made of glass or the like is formed on the upper surface of the electrically insulating substrate 1 so as to have a thickness of 15 μm to 60 μm, and the glaze layer 1a appropriately generates heat generated by the heating resistor 2. The thermal head accumulates at a suitable temperature and acts to keep the thermal response characteristics of the thermal head good. The glaze layer 1a is formed by, for example, applying a glass paste obtained by adding and mixing an appropriate organic solvent and an organic resin to a powder of a high softening point glass on the upper surface of the electrically insulating substrate 1 by screen printing or the like. Thereafter, this is baked at a high temperature of about 1000 ° C. to 1200 ° C. to be formed on the upper surface of the electrically insulating substrate 1. A plurality of heating resistors 2 made of tantalum nitride or the like are attached and arranged on the upper surface of the glaze layer 1a, and a pair of conductive layers 3 are provided at both ends of each heating resistor 2.
Is connected. The heating resistor 2 is made of, for example, tantalum nitride or the like, and has a predetermined electric resistivity. Therefore, when electric power is applied through a pair of conductive layers 3, Joule heat is generated. This causes heat to be generated at a predetermined temperature required to form a printed image on thermal paper or the like, for example, a temperature of 200 ° C. to 350 ° C. A pair of conductive layers 3 connected to both ends of the heating resistor 2 are made of a metal material such as aluminum. The pair of conductive layers 3 is used to cause the heating resistor 2 to generate Joule heat. It acts to apply necessary predetermined power. The plurality of heat generating resistors 2 and the pair of conductive layers 3 are formed on the upper surface of the glaze layer 1a by a predetermined pattern and a predetermined thickness (the heat generating resistor 2 is formed by employing a conventionally known sputtering method and photolithography technique). 0.0
A thickness of 1 μm to 0.5 μm, and a pair of conductive layers 3
μm to 2.0 μm). On the upper surfaces of the heating resistor 2 and the pair of conductive layers 3, a protective layer 4 having a thickness of about 4 μm is further provided. The heating resistor 2 and the like are shielded from the atmosphere by the protective layer 4. ing. The protective layer 4 is made of a low softening point glass 4a and inorganic particles 4b having a higher specific gravity and a higher hardness than the low softening point glass 4a. About 1 μm is contained at a ratio of about 60% by volume with respect to the entire protective layer 4. For example, when the specific gravity of the low softening point glass 4a is 4.0 and the hardness is 340 Hv (Vickers hardness), the inorganic particles 4b have a large specific gravity of 6.0 or more and a specific gravity of 1,000 Hv or more. Inorganic material with high hardness,
Specifically, tungsten carbide (specific gravity: 15.
8, Vickers hardness: 2,000 Hv) is used. The protective layer 4 comprises a low softening point glass 4a,
When the protective layer 4 is formed by the thick film method, the inorganic particles 4b are externally formed when the glass paste is fired because the inorganic particles 4b are formed of the inorganic particles 4b having a higher specific gravity than the low softening point glass 4a and having a high hardness. It sinks downward by its own weight in the low softening point glass 4 a softened by the applied heat and is concentratedly distributed near the heating resistor 2 and the conductive layer 3. Therefore, a part of the inorganic particles 4 b is exposed on the surface of the protective layer 4 or the inorganic particles 4 b
A large number of large projections corresponding to the shape of b are not formed, and thus the adhesion between the thermal head and the thermal paper can be improved. Therefore, a clear printed image can be formed on thermal paper or the like. Further, since the above-mentioned large projections are not formed on the surface of the protective layer 4, the stress caused by the sliding contact of the thermal paper or the like during printing is limited to the entire area of the surface of the protective layer 4 where the thermal paper or the like is in sliding contact. And the stress is not concentrated on one place. As a result, the formation of cracks in the protective layer 4 is effectively prevented, and the corrosion resistance of the protective layer 4 can be favorably maintained for a long time. It should be noted that the thermal conductivity of the inorganic particles 4b is substantially equal to that of the low melting point glass 4a which is the main component of the protective layer 4.
Alternatively, if it is set to be larger than that, the heat that is going to be conducted from the heating resistor 2 to the thermal paper or the like will be less than the inorganic particles 4b.
, The thermal efficiency of the thermal head can be improved. Therefore, the thermal conductivity of the inorganic particles 4 b is reduced by the low melting point glass 4 a which is the main component of the protective layer 4.
It is preferable that the distance is approximately equal to or larger than the above. When the inorganic particles 4b are contained in the protective layer 4 at a ratio of 30% by volume to 70% by volume, even if the inorganic particles have a large particle size exceeding 1 mm, the protective layer 4 The surface of the recording medium can be made flat and the adhesion of thermal paper or the like can be kept extremely high, and the protective layer 4 can be protected from damage even when printing is performed on a hard recording medium such as a card. It can be effectively prevented.
Therefore, 30% to 70% by volume of the inorganic particles 4b
Is preferably contained in the protective layer 4 at a ratio of The protective layer 4 is formed by the following method. First, an appropriate organic solvent and a solvent are added to powder of low softening point glass (softening point: 450 ° C. to 880 ° C.) containing 5 to 60% by weight of lead oxide to prepare a glass paste. A predetermined amount of inorganic particles 4b (tungsten carbide) is added to an electrically insulating substrate 1 on which a heating resistor 2 and a pair of conductive layers 3 are applied by a thick film method such as a screen printing method. Inorganic particles 4b
Is applied with a thickness sufficiently larger than the particle size of, for example, about 4 μm. Next, this is baked at a temperature of about 500 ° C. for 0.5 hour to form a protective layer 4. still,
At this time, even if the inorganic particles 4b in the glass paste are present on the surface of the paste when the glass paste is applied, when the low softening point glass is softened by the heat during firing, it sinks by its own weight and generates heat resistance. Body 2 and a pair of conductive layers 3
, Ie, concentratedly in the lower region of the protective layer 4. Thus, in the above-described thermal head, a predetermined electric power is applied between the pair of conductive layers 3 based on a print signal to selectively cause the heating resistor 2 to generate Joule heat, and the generated heat is transferred to a platen roller or the like. The thermal head is made to conduct to thermal paper or the like pressed by the above, and forms a predetermined print image on the thermal paper or the like to function as a thermal head. It should be noted that the present invention is not limited to the above embodiment, and various changes and improvements can be made without departing from the gist of the present invention. For example, in the thermal head of the above embodiment, FIG. As shown in the figure, a hard material having a Vickers hardness of 1000 kg / mm ² or more on the protective layer 4,
For example, a wear-resistant layer 6 made of silicon nitride, silicon carbide, sialon, or the like may be applied to a thickness of, for example, 3 μm. In this way, the same effect as in the above embodiment can be obtained, and even if the thermal head becomes extremely high due to the repetitive heat generated by the heating resistor 2 at the time of printing, the wear-resistant layer 6 has the heating resistance. Body 2, conductive layer 3, and protective layer 4
In order to effectively prevent the deformation of the thermal head, the reliability of the thermal head is further improved. Therefore, it is preferable that the wear-resistant layer 6 made of a hard material having a Vickers hardness of 1000 kg / mm ² or more is applied on the protective layer 4. Further, in the thermal head of the above embodiment, an oxidation preventing layer 7 made of silicon nitride or the like is provided between the heating resistor 2 and the pair of conductive layers 3 and the protective layer 4 by 0.2 to 2 mm.
If the thickness is set to a thickness of μm, the same effect as in the above embodiment can be obtained, and in addition, the oxygen contained in the protective layer 4 is prevented from diffusing into the heating resistor 2 by the antioxidant layer 7. In addition, oxidative corrosion of the heating resistor 2 can be effectively prevented. Therefore, it is preferable to interpose an antioxidant layer 7 made of silicon nitride or the like with a thickness of 0.2 to 2 μm between the heating resistor 2 and the pair of conductive layers 3 and the protective layer 4. According to the thermal head of the present invention, the protective layer is formed of glass and inorganic particles having a higher specific gravity than the glass and having a high hardness. When formed by the thick film method, the inorganic particles sink down due to their own weight in the low softening point glass softened by heat applied from the outside during firing of the glass paste, and are intensively distributed near the heating resistor and conductive layer I do. For this reason,
Part of the inorganic particles is not exposed on the surface of the protective layer, or a large number of large projections corresponding to the shape of the inorganic particles are not formed on the surface of the protective layer. And the adhesiveness with the adhesive can be improved. Therefore, a clear printed image can be formed on thermal paper or the like. Further, according to the thermal head of the present invention, since a large number of large projections are not formed on the surface of the protective layer, the stress caused by sliding contact of the thermal paper or the like during printing is limited by the thermal paper or the like on the surface of the protective layer. The stress will be distributed over the entire sliding contact portion, and the stress will not be concentrated at one location. As a result, the formation of cracks in the protective layer is effectively prevented, and the corrosion resistance of the protective layer can be favorably maintained for a long time.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view showing an embodiment of a thermal head according to the present invention. FIG. 2 is a sectional view showing another embodiment of the thermal head of the present invention. FIG. 3 is a sectional view of a conventional thermal head. [Description of Signs] 1 ... electric insulating substrate 2 ... heating resistor 3 ... a pair of conductive layers 4 ... protective layer 4a ... glass 4b ... inorganic particles 6 ... resistant Wear layer 7: antioxidant layer

Claims (1)

(57) [Claims 1] A heating resistor and an electric heating element on an electrically insulating substrate
While providing a pair of conductive layers connected to the resistor, the heating resistor is made of glass, tungsten carbide having a higher specific gravity than the glass, and having a high hardness.
Samaruhe' coated with a protective layer made containing the inorganic particles
The heating resistor and the conductive layer in the vicinity of the conductive layer.
In addition to intensive distribution of machine particles,
A silicon nitride layer between the particles and the heating resistor and the pair of conductive layers
A thermal head characterized by interposing .