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A silica-group composite oxide fiber formed of a composite oxide phase of an oxide phase (first phase) mainly made of a silica component and a metal oxide phase (second phase) excluding silica, in which the existent ratio of at least one metal element of a metal oxide constituting the second phase u

A silica-group composite oxide fiber formed of a composite oxide phase of an oxide phase (first phase) mainly made of a silica component and a metal oxide phase (second phase) excluding silica, in which the existent ratio of at least one metal element of a metal oxide constituting the second phase upward slopingly increases toward the surface layer of the fiber and a process for the production thereof.

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1. A silica-group composite oxide fiber formed of a composite oxide phase of an oxide phase (first phase) mainly made of a silica component and a metal oxide phase (second phase) excluding silica, in which the existent ratio of at least one metal element of a metal oxide constituting the second phas

1. A silica-group composite oxide fiber formed of a composite oxide phase of an oxide phase (first phase) mainly made of a silica component and a metal oxide phase (second phase) excluding silica, in which the existent ratio of at least one metal element of a metal oxide constituting the second phase upward slopingly increases toward the surface layer of the fiber. 2. A silica-group composite oxide fiber according to claim 1, wherein the existent ratio of the first phase based on the fiber as a whole is 98 to 40% by weight and the existent ratio of the second phase based on the fiber as a whole is 2 to 60% by weight. 3. A silica-group composite oxide fiber according to claim 1, wherein the oxide phase mainly made of a silica component is amorphous or crystalline. 4. A silica-group composite oxide fiber according to claim 1, wherein the oxide phase mainly made of a silica component contains a metal element or a metal compound which can form a solid solution or a eutectic compound with the silica. 5. A silica-group composite oxide fiber according to claim 4, wherein the metal element is at least one metal selected from the group consisting of titanium, aluminum, zirconium, yttrium, lithium, sodium, barium, calcium, boron, zinc, nickel, manganese, magnesium and iron. 6. A silica-group composite oxide fiber according to claim 1, wherein the metal oxide phase excluding silica is made of titania, its eutectic compound or an lead/zirconium/titanium type oxide. 7. A silica-group composite oxide fiber according to claim 1, wherein the slope in the existent ratio of at least one metal element of a metal oxide constituting the metal oxide phase excluding silica exists from the fiber surface to a depth in the range of 5 nm to 500 nm. 8. A silica-group composite oxide fiber according to claim 1, wherein the slope in the existent ratio of at least one metal element of a metal oxide constituting the metal oxide phase excluding silica exists to a depth of 1/3 of the diameter of the fiber. 9. A silica-group composite oxide fiber according to claim 1, wherein the metal oxide of the second phase is titania which has a crystal particle diameter of 15 nm or less and has photocatalyst and/or thermal catalyst functions. 10. A process for the production of the silica-group composite oxide fiber recited in claim 1, which process comprises melt-spinning a modified polycarbosilane having a structure obtained by modification of a polycarbosilane having a main chain structure represented by the formula, in which R is a hydrogen atom, a lower alkyl group or a phenyl group and n is an integer of 1 to 30, and having a number average molecular weight of 200 to 10,000 with an organic metal compound, or a mixture of the modified polycarbosilane and an organic metal compound to obtain a spun fiber, infusibilizing the spun fiber, and then calcining the infusible fiber in air or in oxygen. 11. A process according to claim 10, wherein the organic metal compound is a compound having a basic structure of the formula M(OR')n or the formula MR"m, in which M is a metal element, R' is an alkyl group having 1 to 20 carbon atoms or a phenyl group, R" is acetyl acetonate, and each of m and n is an integer of more than 1. 12. A process according to claim 10, wherein the modified polycarbosilane is a product obtained by reacting the polycarbosilane with the organic metal compound at a temperature of 280° C. or lower in an inert gas. 13. A process according to claim 10, wherein the modified polycarbosilane is a product in which the organic metal compound is bonded to the polycarbosilane as a monofunctional polymer. 14. A process according to claim 10, wherein the mixture is formed of the modified polycarbosilane and a low-molecular weight organic metal compound which is a monomer, a dimer or a trimer. 15. A process according to claim 10, wherein the modified polycarbosilane contains a modified polycarbosilane having a low molecular weight.