Plate glass and substrate glass for electronics

Details Plate glass and substrate glass for electronics Plate glass and substrate glass for electronics

1999-04-28

2001-07-31

Sample, David R. (Department: 1755)

Compositions: ceramic

Ceramic compositions

Glass compositions, compositions containing glass other than...

C501S063000, C501S064000, C501S066000, C501S067000, C501S069000, C501S070000, C501S072000

Reexamination Certificate

active

06268304

ABSTRACT:

The present invention relates to a plate glass having high resistance against the progress of fracture, i.e. a plate glass having high fracture toughness. Particularly, it relates to a plate glass suitable as a substrate for electronics, for which high thermal resistance and a thermal expansion coefficient of a same level as usual soda lime silica glass, are required.
In recent years, industry of large size flat display panels represented by color plasma display panels (hereinafter referred to as color PDP) has shown a remarkable growth, and glasses to be used as substrates thereof have been diversified. Heretofore, usual soda lime silica glass has been widely used for substrates for large sized flat display panels. One of the reasons is such that the thermal expansion coefficients of various glass frit materials to be used as constituting part materials for panels, including inorganic sealing materials, can easily be adjusted to the thermal expansion coefficient of soda lime silica glass.
On the other hand, in order to reduce deformation or thermal shrinkage of glass substrates in the heat treatment process during the production of large sized flat display panels, it is strongly desired to improve the thermal resistance of the glass for substrates. For this purpose, a so-called high strain point glass has been widely used as a substrate, which has a thermal expansion coefficient of the same level as soda lime silica glass and has a higher strain point and which has an alkali content controlled to be low in order to improve the electrical insulating property.
However, such a high strain point glass is brittle as compared with soda lime silica glass and thus has had a problem that it is likely to break in the production process. Further, such a high strain point glass has had another problem that its density is large, whereby it has been difficult to reduce the weight of the large sized flat display panel.
To solve such problems, for example, JP-A-9-301733 proposes a glass for substrate which has a low density and which is hardly flawed. The characteristic of being hardly flawed is effective in a case where a flaw as a fracture origin is likely to be imparted in the process for producing a panel, but it is not necessarily effective in a case where a flaw is imparted during processing treatment such as cutting prior to such a production process. It is usual that many flaws which are likely to be fracture origin, are already present at an edge portion of the glass in the processing treatment such as cutting. In such a case, in order to prevent breakage of the glass, it has been necessary to present a plate glass which essentially has high resistance against the progress of fracture due to a tensile stress, i.e. a plate glass having high fracture toughness.
Usually, there is the following relation between the above mentioned fracture toughness (K
IC
) and the fracture strength (&sgr;):
&sgr;=
K
IC
/(
Y×c
½
)
where c is the size of the flaw which causes fracture, and Y is a constant determined by the shape of the flaw.
As methods for evaluating the fracture toughness of brittle materials, many proposals have heretofore been made. JIS R1607 stipulates two methods i.e. a single edge precracked beam method (SEPB method) and an indentation fracture method (IF method), as methods for testing the fracture toughness of fine ceramics. According to a commentary of the above stipulations, the SEPB method is recommended as a method which should be employed mainly for the reason that the theoretical ground is clear. On the other hand, the IF method is described as a method for inspecting the length of a crack formed when a Vickers indenter is pressed against the surface of the material, which is employed for the reason that it is very simple, and the interrelation with the values measured by the SEPB method has been generally confirmed. In this connection, the discussion in JP-A-9-301733 is made on the basis of the IF method, and although a substrate glass which is hardly flawed, is disclosed, it can hardly be said as aimed at increasing the resistance against the progress of fracture due to a tensile stress.
Accordingly, a discussion of the fracture toughness itself is considered to be preferably made on the basis of a method for measuring the resistance against the growth of a crack due to a tensile stress. As a typical method therefor, the above mentioned SEPB method, a chevron notch method (BBCN method) or a double cantilever method (DCB method) may, for example, be mentioned. Here, the BBCN method is a method for measuring fracture toughness by carrying out a bending test with respect to a test specimen having a chevron-type notch formed at its center portion, and the DCB method is a method for measuring the load required for the progress of a crack by directly pulling both ends of the crack with respect to a test specimen having the crack introduced. The fracture toughness mentioned hereinafter is a value measured by the SEPB method, the BBCN method or the DCB method.
According to J. Am. Ceram. Soc. 52 (1969), No.2, p99-105 (hereinafter referred to as Document 1), the fracture toughness of soda lime silica glass is 0.75 MPa·m
½
.
Silicate glasses having fracture toughness equivalent to or higher than the fracture toughness of soda lime silica glass, have heretofore been known. For example, Document 1 discloses an aluminosilicate glass having a fracture toughness of 0.91 MPa·m
½
, and J. Am. Ceram. Soc. 61 (1978), No.1-2, p27-30 (hereinafter referred to as Document 2) discloses an alkali borosilicate glass having a fracture toughness of 0.94 MPa·m
½
. Further, SID'96 Digest of Technical Papers (1996) p518 discloses that the fracture toughness of an alkali free glass for TFT-LCD substrate is from 0.8 to 0.83 MPa·m
½
. The fracture toughness of these glasses is higher than the fracture toughness of soda lime silica glass, and such glasses may be regarded essentially as glasses having the resistance against the progress of fracture improved. However, each of such glasses having high fracture toughness has a thermal expansion coefficient which is too small as compared with the thermal expansion coefficient of soda lime silica glass, or a strain point which is as low as lower than 550° C., whereby, in many cases, such a glass has not been applicable to a substrate glass for electronics such as a substrate for color PDP.
On the other hand, the relation between the glass composition and the fracture toughness is discussed, for example, in Document 1 or 2. Generally, the relation between the fracture toughness (K
IC
), and the Young's modulus (E) and the fracture surface energy (&ggr;) of the material, is represented by the following formula:
K
IC
=(2
×E
×&ggr;)
½
From this formula, it is evident that in order to improve the fracture toughness, the Young's modulus and the fracture surface energy may be increased. Among them, the Young's modulus is a physical value which can relatively easily be measured, and its relation with the glass composition has been known to some extent. However, the fracture surface energy is a physical value which is rather difficult to measure, and its relation with the glass composition has little been reported. In fact, Document 1 discloses a relation between the Young's modulus and the fracture surface energy, but the relation differs depending upon the glass system, and no clear interrelation between the two has been obtained. Therefore, it is difficult to estimate the fracture toughness from the relation between the glass composition and the Young's modulus. Further, a glass containing a substantial amount of e.g. Si, Al or B which has a high single bond strength with oxygen, is believed to usually have high fracture toughness, but as disclosed in Document 2, such characterization is not applicable to some glasses.
As mentioned above, the mechanism for development of fracture toughness in a glass having high resistance against the progress of fracture, has not

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