Oxynitride glass, method of preparing the same and glass fiber

Transparent oxynitride glass has transmittance of at least 80%/mm in a wavelength region of 400 to 800 nm, with Si particles or Si alloy particles of not more than 1.0.times.10.sup.9 /cm.sup.3 in number. Glass fiber prepared by drawing this glass can attain strength which is by far superior to that of conventional oxide glass fiber, to be preferably applied to reinforcing fiber.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to oxynitride glass which is excellent in 
transparency and strength, and a method of preparing the same. Glass fiber 
which is prepared from the inventive glass is employed as reinforcing 
fiber for various composite materials such as FRP (fiber-glass reinforced 
plastics). 
2. Description of the Background Art 
In oxynitride glass, oxygen forming oxide glass is partially replaced by 
nitrogen having three bond valences. Thus, a larger number of chemical 
bonds are formed in the oxynitride glass as compared with the oxide glass, 
to strengthen the network of the glass. Thus, the oxynitride glass has 
excellent physical properties such as a high elastic modulus, high 
hardness and high strength, and is suitably applied to reinforcing fiber 
or the like. 
In general, such oxynitride glass is prepared by a melting method of mixing 
a nitride raw material such as Si.sub.3 N.sub.4 or AlN with another glass 
raw material of a metal oxide such as SiO.sub.2 and melting the mixture in 
inert or reducing gas for a long time. However, conventional oxynitride 
glass which is prepared by melting under such environment easily causing 
reduction reaction is considerably colored and hazed with low transparency 
in a visible region of 400 to 800 nm, while its strength is inferior as 
compared with the expected physical value. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide oxynitride glass and 
oxynitride glass fiber which are remarkably superior in strength to the 
prior art. 
The inventors have made deep study on improvement in physical properties of 
oxynitride glass and recognized that it is possible to obtain oxynitride 
glass and oxynitride glass fiber of an absolutely novel structure which 
are transparent and excellent in physical strength by employing specific 
preparation conditions, to complete the present invention. 
The present invention provides oxynitride glass containing Si particles or 
Si alloy particles of at least 0.1 .mu.m by not more than 
1.0.times.10.sup.9 /cm.sup.3 in number. The inventive glass has 
transmittance of at least 80%/mm at a wavelength of 400 to 800 nm, in 
particular. The present invention also provides a method of preparing such 
glass, and glass fiber. 
According to the inventive method of preparing oxynitride glass, the 
following raw materials are first mixed with each other: 
(i) SiO.sub.2 ; 
(ii) Si.sub.3 N.sub.4 or another metal nitride; 
(iii) at least one metal oxide; and 
(iv) CaO or CaO+MgO. 
In place of Si.sub.3 N.sub.4, the nitrogen source of the above item (ii) 
can be prepared from a metal nitride such as AlN, Mg.sub.3 N or Li.sub.3 
N, or an silicon oxy-nitride such as Si.sub.2 N.sub.2 O or Si.sub.5 
N.sub.6 O. Further, a metal oxy-nitride (Al.sub.2n+1 O.sub.3n N or the 
like), SIALON (Si.sub.6-x Al.sub.x O.sub.x N.sub.8-x or the like), or a 
compound of an silicon oxy-nitride and a metal (La.sub.4 Si.sub.2 O.sub.7 
N.sub.2 or the like) may be employed. 
The metal oxide of the above item (iii) can be prepared from Al.sub.2 
O.sub.3, BaO, Sb.sub.2 O.sub.3, SrO, Na.sub.2 O, K.sub.2 O, La.sub.2 
O.sub.3, CeO.sub.2, Y.sub.2 O.sub.3, ZrO.sub.2, TiO.sub.2, B.sub.2 
O.sub.3, Cr.sub.2 O.sub.3, PbO, V.sub.2 O.sub.5 or SnO.sub.2, or 
carbonate, hydroxide or oxalate for forming such a metal oxide by thermal 
decomposition may be blended. Further, a compound for forming CaO or MgO 
by thermal decomposition such as carbonate, hydroxide or oxalate may be 
employed in place of CaO or MgO. 
The above raw materials are sufficiently mixed with each other, and the 
mixture is heated and melted to obtain oxynitride glass. The mixture is 
preferably melted at a temperature of 1400.degree. to 1900.degree. C. for 
3 to 100 hours at a heating speed of 10.degree. to 800.degree. C./min. 
under an atmosphere of inert or reducing gas such as nitrogen or argon, to 
be glassified. 
After such melting, the inventive oxynitride glass is refined at a 
prescribed temperature under an atmosphere of inert or reducing gas. The 
refinement temperature is 1400.degree. to 1690.degree. C., preferably 
1500.degree. to 1690.degree. C. The refinement time is 1.0 to 100 hours, 
preferably 2 to 60 hours, and most preferably 4 to 6 hours. 
Then, the oxynitride glass as obtained is quenched or slowly cooled. The 
oxynitride glass as cooled is drawn by a drawing machine which is heated 
to 1100.degree. to 1600.degree. C. under an inert or reducing atmosphere 
at a drawing speed of 20 to 3000 m/min., to obtain continuous fiber. 
Alternatively, the oxynitride glass may be cooled from the melting 
temperature to 1100.degree. to 1600.degree. C. in a melting furnace of the 
drawing machine, to be drawn under an inert or reducing atmosphere. In the 
latter method, the raw materials for the oxynitride glass are continuously 
supplied by a feeder into the melting furnace of the drawing machine, to 
be directly drawn. 
The glass fiber is preferably 3 to 50 .mu.m in diameter. If the diameter is 
below this range, it is difficult to form fiber. If the diameter exceeds 
the above range, on the other hand, the glass fiber is extremely reduced 
in strength. The glass fiber may be continuous fiber, or short fiber of 
1.5 to 100 mm. Further, the glass fiber may be in any form of cloth, 
roving, yarn, staple, a chopped strand, wool, paper or a mat. 
Conventional glass, particularly oxynitride glass contains a number of Si 
particles and Si alloy particles not having any bonds with oxygen atoms. 
It is estimated that the conventional oxynitride glass is so colored and 
hazed that it is impossible to obtain glass fiber exhibiting expected 
excellent physical strength, due to such internal defects. On the other 
hand, the inventive oxynitride glass is not colored despite involving 
nitrogen, and has extremely high transmittance of at least 80%/mm in a 
visible region (400 to 800 nm in wavelength). In this glass, further, Si 
particles or Si alloy particles are reduced to not more than 
1.0.times.10.sup.9 /cm.sup.3 in number, and hence glass fiber which is 
obtained by drawing this glass is extremely superior in strength to the 
conventional glass fiber, and preferably applied to reinforcing fiber. 
Preferable oxynitride glass obtained by the inventive method includes 
Si-Ca-M-O-N based or Si-Ca-Mg-M-O-N based glass, where M represents at 
least one metal selected from Al, Sr, La, Ba, Y, Ti, Zr, Ce, Na, K, Sb, B, 
Cr, Pb, V and Sn. The glass as obtained has a nitrogen content of 0.1 to 
15 percent by weight, a elastic modulus of 10,000 to 22,000 kg/mm.sup.2, 
and tensile strength of 100 to 700 kg/mm.sup.2. 
The foregoing and other objects, features, aspects and advantages of the 
present invention will become more apparent from the following detailed 
description of the present invention when taken in conjunction with the 
accompanying drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention is now described in more concrete terms, with 
reference to Examples. 
[EXAMPLE 1] 
Powder materials of 50.31 percent by weight of SiO.sub.2, 9.81 percent by 
weight of CaO, 11.54 percent by weight of MgO, 20.83 percent by weight of 
Al.sub.2 O.sub.3 and 7.51 percent by weight of Si.sub.3 N.sub.4 were 
employed as glass raw materials. 3 kg of a mixture of these raw materials 
was melted and refined in a carbon crucible of 24 cm by 15 cm by 10 cm 
which was lined with BN(Boron Nitride), under an atmosphere containing Ar 
and N.sub.2 in a ratio of 1:9 at 1660.degree. C. for 5 hours and at 
1550.degree. C. for 3 hours respectively. The glass as obtained was 
cooled, drawn out from the crucible, partially cut out and subjected to 
double optical polishing, and thereafter subjected to measurement of 
transmittance. FIG. 1 shows the result. 
The above glass was pulverized, introduced into a molybdenum bushing, which 
was lined with BN, having 72 nozzle tips, entirely heated to 1600.degree. 
C. under a nitrogen atmosphere, and discharged from the nozzle tips. It 
was possible to form glass fiber of 8.0 .mu.m in mean fiber diameter by 
drawing the glass at 2000 m/min. The mean strength of this glass fiber was 
420 kg/mm.sup.2. Transmittance of this glass fiber was measured in the 
following method: 100 bundles of strands each of which includes 72 fibers 
were impregnated with epoxy resin containing carbon, and drawn into the 
form of a cylinder. The glass fiber reinforced epoxy resin was partially 
cut out along its section into a thickness of not more than 1 mm, and 
subjected to double optical polishing. An effective glass area was 
measured by an image processor, and the glass fiber was subjected to 
measurement of transmittance, similarly to the bulk glass. The glass fiber 
exhibited the same transmittance as the bulk glass. 
[EXAMPLE 2] 
Powder materials of 46.04 percent by weight of SiO.sub.2, 9.32 percent by 
weight of CaO, 6.22 percent by weight of MgO, 25.90 percent by weight of 
Al.sub.2 O.sub.3 and 12.52 percent by weight of Si.sub.3 N.sub.4 were 
employed as glass raw materials. A mixture of these raw materials was 
melted and refined under an atmosphere containing He and N.sub.2 in a 
ratio of 1:9 at 1680.degree. C. for 5 hours and at 1580.degree. C. for 3 
hours respectively. This glass was cooled and drawn out, to be subjected 
to measurement of transmittance. FIG. 1 shows the result. 
This glass was drawn similarly to Example 1, whereby the glass fiber as 
obtained exhibited mean strength of 450 kg/mm.sup.2. The transmittance of 
this glass fiber was similar to that of the bulk glass. 
[EXAMPLE 3] 
Powder materials of 56.74 percent by weight of SiO.sub.2, 10.42 percent by 
weight of MgO, 25.33 percent by weight of Al.sub.2 O.sub.3 and 7.51 
percent by weight of Si.sub.3 N.sub.4 were employed as glass raw 
materials. A mixture of these raw materials was melted and refined under a 
nitrogen atmosphere at 1700.degree. C. for 5 hours and at 1600.degree. C. 
for 3 hours respectively. This glass was cooled and thereafter partially 
drawn out, to be subjected to measurement of transmittance. FIG. 1 shows 
the result. 
This glass was drawn similarly to Example 1, whereby the glass fiber as 
obtained exhibited mean strength of 580 kg/mm.sup.2. The transmittance of 
this glass fiber was similar to that of the bulk glass. 
[COMATIVE EXAMPLE 1] 
Oxynitride glass was prepared with compositions and conditions which were 
similar to those of Example 3, except that refinement was carried out at 
1700.degree. C. for 8 hours. The glass as obtained was partially drawn out 
and subjected to measurement of transmittance. FIG. 1 shows the result. 
The glass was drawn similarly to Example 1, to obtain extremely weak glass 
fiber having mean strength of 220 kg/mm.sup.2. The transmittance of this 
glass fiber was similar to that of the bulk glass. 
[COMATIVE EXAMPLE 2] 
Powder materials of 51.24 percent by weight of SiO.sub.2, 10.57 percent by 
weight of MgO, 25.68 percent by weight of Al.sub.2 O.sub.3 and 12.52 
percent by weight of Si.sub.3 N.sub.4 were employed as glass raw 
materials. A mixture of these raw materials was melted and refined under a 
nitrogen atmosphere at 1750.degree. C. for 8 hours and 1700.degree. C. for 
2 hours respectively. The glass as obtained was subjected to measurement 
of transmittance. FIG. 1 shows the result. The glass was drawn similarly 
to Example 1, to obtain glass fiber having low mean strength of 240 
kg/mm.sup.2. The transmittance of this glass fiber was similar to that of 
the bulk glass. 
[EXAMPLE 4] 
Raw materials of 50.31 percent by weight of SiO.sub.2, 9.81 percent by 
weight of CaO, 11.54 percent by weight of MgO, 20.83 percent by weight of 
Al.sub.2 O.sub.3 and 7.51 percent by weight of Si.sub.3 N.sub.4 were 
employed as glass raw materials. 1 kg of a mixture of these raw materials 
was melted and refined at 1650.degree. C. through a carbon crucible of 14 
cm by 10 cm by 10 cm which was lined with BN, under an atmosphere 
containing Ar and N.sub.2 in a ratio of 1:9. The melting and refining 
times were varied to count the number of Si particles contained in the 
glass by the following method. Table 1 shows the results. 
Si Particle Counting Method: 
The interior of an optically polished glass sample was observed with an 
optical microscope, the Si particles were counted in several portions, and 
the average number was obtained in terms of a unit observation volume per 
1 cm.sup.3. 
The glass as obtained was introduced into a molybdenum bushing, which was 
coated with chromium, having 36 nozzle tips, entirely heated to 
1550.degree. C., discharged from the nozzle tips, and drawn at 2000 m/min. 
The mean fiber diameter was 8.0 .mu.m. Table 1 shows strength levels of 
the glass fiber as obtained. 
TABLE 1 
______________________________________ 
Number of Si Particles 
Melting Time 
of at least 0.1 .mu.m 
Mean Strength 
(hours) (.times. 10.sup.8 /cm.sup.3) 
(kg/mm.sup.2) 
______________________________________ 
1 15.1 220 
4 9.5 350 
8 3.0 420 
16 2.2 430 
24 1.1 450 
______________________________________ 
As clearly understood from Table 1, it is conceivably effective to reduce 
Si particles of at least 0.1 .mu.m to not more than 10.times.10.sup.8 
/cm.sup.3 in number. 
[EXAMPLE 5] 
Raw materials of 56.74 percent by weight of SiO.sub.2, 10.42 percent by 
weight of MgO, 25.33 percent by weight of Al.sub.2 O.sub.3 and 7.51 
percent by weight of Si.sub.3 N.sub.4 were employed as glass raw 
materials. 3 kg of a mixture of these raw materials was melted and refined 
in a molybdenum- tungsten crucible of 24 cm by 15 cm by 10 cm under a 
nitrogen atmosphere at 1720.degree. C. for 6 hours and at 1600.degree. C. 
for 2 hours respectively. The glass as obtained was cooled and Si 
particles contained therein were counted, to obtain a result of 
1.8.times.10.sup.8 /cm.sup.3. Glass fiber which was obtained by drawing 
this glass similarly to the above exhibited mean strength of 580 
kg/mm.sup.2. 
Although the present invention has been described and illustrated in 
detail, it is clearly understood that the same is by way of illustration 
and example only and is not to be taken by way of limitation, the spirit 
and scope of the present invention being limited only by the terms of the 
appended claims.