Sealing glass

Abstract: A sealing glass which comprises 1 to 75 % by weight of TeO₂ , 1 to 30 % by weight of B₂O₃ , 1 to 75 % by weight of PbO, 1 to 25 % by weight of CdO, 0 to 15 % by weight of ZnO, 0 to 25 % by weight of at least one oxide selected from the group of Bi₂O₃ and Tl₂O , 0 to 5 % by weight of A₂O wherein A is an alkali metal, 0 to 35 % by weight of ARO wherein AR is an alkaline earth metal, 0 to 35 % by weight of La₂O₃ , 0 to 5 % by weight of at least one oxide selected from the group consisting of Al₂O₃ , SiO₂ , TiO₂ and ZrO₂ , 0 to 10 % by weight of at least one oxide selected from the group consisting of Ta₂O₅ , Nb₂O₅ , WO₃ and MoO₃ , 0 to 30 % by weight of PbF₂ , and 0 to 2 % by weight of CoO, which glass has a coefficient of thermal expansion close to a ferrite and good water resistance.

Description: The present invention relates to a sealing glass for a magnetic head. With the progress of electronics, need for various kinds of glass has been increased. Conventional sealing glasses will be explained by making reference to a sealing glass for magnetic heads. Fig. 1 shows a perspective view of a typical magnetic head for a VTR which comprises a core made of ferrite. Ferrite core parts 1 are faced to each other with a constant gap distance therebetween, and a formed gap is filled with a gap glass 2 and bonded with a sealing glass 3. Around the ferrite core parts, coil 4 is wound. Although, the sealing glass should be sufficiently softened and flowed to bond the cores when heated, a softening or working point is preferably as low as possible to prevent interaction between the core material and the glass. Further, the sealing glass has a coefficient of thermal expansion close to that of the ferrite. In general, the ferrite has a coefficient of thermal expansion of 80 x 10⁻⁷ to 115 x 10⁻⁷/°C. Recently, a ferrite having high saturation magnetic flux density has been developed to be used with a high resolution tape. Such new ferrite has a coefficient of thermal expansion of about 130 x 10⁻⁷/°C (see, for example, Horikawa et al, the Preprint for the Society of Applied Physics, 4a-ZE-8, October, 1985). As magnetic materials as substitutes for the ferrite in the same fields, amorphous alloys, superstructural nitride alloys (see, for example, the Technical Study Reports of the Electronics, Information and Communication Society, MR-86-4, 87-14) and Sendust have been used. Among them, Sendust has a coefficient of thermal expansion of about 140 x 10⁻⁷/°C. Such new magnetic materials 5 are formed near the gap of the magnetic head by spattering as shown in Fig. 2, in which "6" represents a gap depth, and "16" represents an opening for winding. The sealing glass should have a coefficient of thermal expansion suitable for such structure of the magnetic head of Fig. 2. The magnetic head is polished during processing. Since water is used in the polishing step, the sealing glass is required to have good water resistance. In addition, if the glass is darkly colored or devitrified (or crystallized) during processing, an apex 7 is not seen when the gap depth 6 is adjusted at a predetermined depth. In view of the above facts, the sealing glass for the magnetic head should meet at least following requirements: (a) Its working temperature is low. (b) Its coefficient of thermal expansion is close to that of the magnetic material, for example, from 80 x 10⁻⁷ to 140 x 10⁻⁷/°C. (c) It has good water resistance. (d) It is not darkly colored and not devitrified (crystallized) during processing. As a result of preliminary experiments on the water resistance of the glass, it was found that the glass preferably has water resistance of "First Class" when expressed according to the classes determined by the Japan Optical Glass Industries Association (Nippon Kogaku Garasu Kogyokai). From the above view points, various sealing glasses have been proposed. (1) Japanese Patent Kokai Publication Nos. 255643/1985, 36135/1986 and 111935/1986 disclose a glass having a coefficient of thermal expansion of 70 x 10⁻⁷ to 130 x 10⁻⁷/°C and a working temperature of from 400 to 600°C. (2) Japanese Patent Kokai Publication No. 36040/1987 discloses a glass having a coefficient of thermal expansion of 112 x 10⁻⁷ to 155 x 10⁻⁷/°C and a working temperature of from 400 to 560°C. (3) Japanese Patent Kokai Publication No. 170240/1988 discloses a glass having a coefficient of thermal expansion of 119 x 10⁻⁷ to 126 x 10⁻⁷/°C and a working temperature of 550°C. (4) Japanese Patent Kokai Publication No. 206330/1988 discloses a glass having a coefficient of thermal expansion of 85 x 10⁻⁷ to 105 x 10⁻⁷/°C and a working temperature of from 500 to 550°C. (5) One of commercially available glass, namely T 015 (manufactured by Iwaki Glass Co., Ltd.) has a coefficient of thermal expansion of 112 x 10⁻⁷ and a working temperature of 450°C, and another, namely T 176 (manufactured by Iwaki Glass Co., Ltd.) has a coefficient of thermal expansion of 120 x 10⁻⁷/°C and a working temperature of 430°C. However, each of the above conventional glasses has its own defects. For example, since the glass (1) contains 30 to 70 % of V₂O₅, it is black. Therefore, this glass does not satisfy the above requirement (d). Since the glass (2) contains a large amount of ZnO, a combination of SiO₂ + SnO₂ + TiO₂ + ZrO₂, and a combination of WO₃ + MoO₃ which crystallize the glass easily, raw materials are not vitrified during melting, or the formed glass is easily devitrified during processing. Then, this glass does not satisfy the requirement (d). Since the glass (3) contains a large amount, for example, 5 % or more of Na₂O, it has insufficient water resistance and does not satisfy the requirement (c). Since the glass (4) contains a large amount of SiO₂ or Al₂O₃ which crystallizes the glass easily, it is easily devitrified during processing and does not satisfy the requirement (c). Since the commercially availabel glasses (5) both contain a large amount of PbO and B₂O₃, they are easily devitrified during processin and do not satisfy the above requirement (c). One object of the present invention is to provide a novel sealing glass which is suitable for sealing electronic parts, in particular magnetic heads. An aspect of this object is to provide a sealing glass which has a comparatively lower working temperature, a coefficient of thermal expansion close to that of a magnetic material and good water resistance and which is not darkly colored or devitrified during processing. According to the present invention, there is provided a sealing glass which comprises: 1 to 75 % by weight of TeO₂, 1 to 30 % by weight of B₂O₃, 1 to 75 % by weight of PbO, 1 to 25 % by weight of CdO, 0 to 15 % by weight of ZnO, 0 to 25 % by weight of Bi₂O₃, 0 to 5 % by weight of A₂O wherein A is an alkali metal, 0 to 35 % by weight of ARO wherein AR is an alkaline earth metal, 0 to 35 % by weight of La₂O₃, 0 to 5 % by weight of at least one oxide selected from the group consisting of Al₂O₃, SiO₂, TiO₂ and ZrO₂, 0 to 10 % by weight of at least one oxide selected from the group consisting of Ta₂O₅, Nb₂O₅, WO₃, and MoO₃, 0 to 30 % by weight of PbF₂, and 0 to 2 % by weight of CoO, and which has a coefficient of thermal expansion from 80 x 10⁻⁷ to 140 x 10⁻⁷/°C in the temperature range between 30 and 300 °C, a working temperature from 430 to 620 °C and a water resistance of "first class" and is not colored and not devitrified during processing. Fig. 1 is a perspective view of a magnetic head, Fig. 2 is an enlarged partial view of a magnetic head, Fig. 3 is a ternary diagram of a sealing glass compositions comprising 10 % by weight of CdO, 5 % by weight of ZnO and varying amounts of TeO₂, B₂O₃ and PbO, and Figs. 4A to 4E are photographs showing conditions of an interface between superstructural nitride alloy and the glass of Sample No. 13 containing CoO. Fig. 1 is a ternary diagram of sealing glass compositions comprising 10 % by weight of CdO, 5 % by weight of ZnO and varying amounts of TeO₂, B₂O₃ and PbO which are major components. The sealing glass has a basic composition in the area A in Fig. 3 and optionally contains other components. The glass having a composition in the area B has a coefficient of thermal expansion of smaller than 80 x 10⁻⁷ and does not satisfy the above requirement (b). The sealing glass of the present invention will be explained in connection with compositions shown in Tables 1 and 2. Table 1 shows the compositions of the glass and also vitrification states, coefficients (α) of thermal expansion, yielding points, working temperatures, water resistance and total evaluation. The vitrification states are evaluated according to the following criteria: O: Vitrified during melting. X: Vitrified during melting but devitrified during processing. XX: Not vitrified during melting. The coefficient of thermal expansion is measured between 30 and 300°C. The yielding point is a temperature at which elongation stops in an alpha curve. The working temperature is a temperature at which the glass is fully expanded when the glass is drawn to a fiber having a diameter of 1 mm and a length of 25 mm and placed on a piece of ferrite having a width of 2.6 mm and a length of 25 mm. Water resistance is expressed in terms of "Class" determined by the Japan Optical Glass Industries Association. Sample No. 1 2 3 4 5 6 Component (wt. %) TeO₂ 18 18 18 18 18 18 B₂O₃ 14 14 14 14 14 14 PbO 45 40 35 25 10 ZnO 13 13 13 13 13 13 CdO 10 10 10 10 10 10 PbF 5 10 20 35 45 Vitrification O O O O X X Coefficient of thermal expansion (x 10⁻⁷/°C) 102 103 105 108 114 118 Yielding point (°C) 408 401 388 381 357 353 Working temp.(°C) 530 520 510 500 470 460 Water resistance (Class) 1 1 1 1 1 1 Total evaluation O O O O XX XX In the Sample Nos. 1 to 9, contents of TeO₂ and PbO were changed. When the content of TeO₂ is zero or exceeded 75 % by weight, it was difficult to vitrify the material. This means that the content of PbO is restricted to a range between 1 and 75 % by weight. In the Sample Nos. 10 to 14, a content of B₂O₃ was changed. When it was zero or exceeded 30 % by weight, it was difficult to vitrify the material. In the Sample Nos. 15 to 18, a content of ZnO was changed. ZnO can decrease the coefficient of thermal expansion while it does not change the working temperature of the glass materially. When the content of ZnO exceeds 15 % by weight, vitrification becomes difficult. In the Sample Nos. 19 to 23, a content of CdO, which is one of the essential components, was changed. CdO improves the water resistance of the glass. When no CdO is present, the water resistance is deteriorated, while when the content of CdO exceeds 25 % by weight, vitrification becomes difficult. In the Sample Nos. 24 and 25, Bi₂O₃ or Tl₂O was added to the glass composition. They can decrease the coefficient of thermal expansion while it does not change the working temperature of the glass materially. They may be contained in an amount of up to 25 % by weight. When their content exceeds 25 % by weight, the water resistance may tend to decrease. In the Sample No. 26, Li₂O which a typical example of the alkali metal oxide (A₂O) was added to the glass composition. The alkali metal oxide increases the coefficient of thermal expansion. When the content of the alkali metal oxide exceeds 5 % by weight, the vitrification becomes difficult. In the Sample Nos. 27 to 30, BaO which is a typical example of the alkaline earth metal oxide (ARO) was added to the glass composition. BaO can increase the coefficient of thermal expansion, while it does not change the water resistance substantially. When the content of BaO exceeds 35 % by weight, the vitrification becomes difficult. In the Sample No. 31, Al₂O₃ was added to the glass composition. Al₂O₃ decreases the coefficient of thermal expansion. When its content exceeds 5 % by weight, the vitrification becomes difficult. In the Sample Nos. 32 to 36, La₂O₃ was added to the glass composition. La₂O₃ increases the water resistance. When its content exceeds 35 % by weight, the vitrification becomes difficult. In the Sample Nos. 37 to 39, SiO₂, TiO₂ or ZrO₂ was added to the glass composition. They decrease the coefficient of thermal expansion. When their content exceeds 5 % by weight, the vitrification becomes difficult. In the Sample Nos. 40 to 47, Nb₂O₅, Ta₂O₅, WO₃ or MoO₃ was added to the glass composition. They decrease the coefficient of thermal expansion. When its content exceeds 10 % by weight, the vitrification becomes difficult. The Sample Nos. 48 to 54 were other various examples of the glass compositions. All the glasses were not devitrified and had the thermal coefficient of from 82 x 10⁻⁷ to 137 to 10⁻⁷/°C, the working temperature of from 430 to 620°C and water resistance of "First" or "Second" Class. Therefore, they satisfy the requirements for the sealing glass for the magnetic head. Table 2 shows the change of the properties when PbO in the glass composition of the Sample No. 49 was replaced with PbF₂. When the content of PbF₂ exceeded 35 % by weight, the glass devitrified during processing. Therefore, the content of PbF₂ should be limited in the range between 0 and 30 % by weight. Figs. 4A to 4E show conditions of an interface between superstructural nitride alloy and the glass of Sample No. 13 to which CoO was added in varying amounts. Figs. 4A to 4E correspond to the glass of the Sample No. 13 to which 0, 0.2, 0.5, 1.0 and 2.0 % by weight of CoO was added, respectively. As understood from Figs. 4A to 4E, the addition of CoO suppressed the interaction at the interface between the superstructural nitride and the glass. However, when 2.0 % by weight of CoO was added, the glass was partly cracked. Then, the content of CoO should not exceed 2.0 % by weight. Comparative Example Table 3 shows the properties of the conventional glass (2) (Sample Nos. C1 and C2), the conventional glass (3) (Sample Nos. C3 and C4) and the conventional glass (4) (Sample No. C5 and C6). Each of them was difficult to vitrify, had poor water resistance or was devitrified during processing. Therefore, they are not practically usable. Since the conventional glasses (1) and (5) have the drawbacks as discussed in the above, they are not tested here. Sample No. C1 C2 C3 C4 C5 C6 Component (wt. %) TeO₂ 21.1 31.2 19 15 B₂O₃ 9.2 11.1 16 14 19 15 PbO 36.9 40.4 48 50 53 60 ZnO 8.1 10.6 9 7 SiO₂ 12 15 5 2 TiO₂ 2.3 Al₂O₃ 3 4 1.5 6 WO₃ 15.3 MoO₃ 9.5 SnO₂ 4.4 Na₂O 12 10 MgO 1.5 Vitrification XX XX O O X X Coefficient of thermal expansion (x 10⁻⁷/°C) 119 121 96 102 Yielding point (°C) 385 390 380 360 Working temp.(°C) 550 550 540 510 Water resistance (Class) 3 3 1 1 Total evaluation XX XX XX XX XX XX The glass of the present invention is used to seal the magnetic head. Also the glass of the present invention can be used to seal metals having the close coefficient of thermal expansion (e.g. platinum, titanium, chromium-iron alloys, 426 alloy, etc.) or other glasses (soda ash glass, lead-potassium glass, borosilicate glass, etc.) and can be used to adhere a CRT tube or as an IC package, a hermetic seal or a sheathed heater.

Claims: A sealing glass for a magnetic head which comprises: 1 to 75 % by weight of TeO₂, 1 to 30 % by weight of B₂O₃, 1 to 75 % by weight of PbO, 1 to 25 % by weight of CdO, 0 to 15 % by weight of ZnO 0 to 25 % by weight of Bi₂O₃, 0 to 5 % by weight of A₂O wherein A is an alkali metal, 0 to 35 % by weight of ARO wherein AR is an alkaline earth metal, 0 to 35 % by weight of La₂O₃, 0 to 5 % by weight of at least one oxide selected from the group consisting of Al₂O₃, SiO₂, TiO₂ and ZrO₂, 0 to 10 % by weight of at least one oxide selected from the group consisting of Ta₂O₅, Nb₂O₅, WO₃, and MoO₃, 0 to 30 % by weight of PbF₂, and 0 to 2 % by weight of CoO, and which has a coefficient of thermal expansion from 80 x 10⁻⁷ to 140 x 10⁻⁷/°C in the temperature range between 30 and 300 °C, a working temperature from 430 to 620 °C and a water resistance of "first class" and is not colored and not devitrified during processing. The sealing glass according to claim 1, which further comprises a ceramic having small thermal expansion.

Inventors: IKEDA MASAKI, MIZUNO YASUO, YOSHIDA AKIHIKO, IKEDA, MASAKI, MIZUNO, YASUO, YOSHIDA, AKIHIKO
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1990
Main classification: C03C8
IPC codes: C03C3, C03C8