A medium- or high-voltage circuit breaker comprising: - at least two arcing contacts and that are movable axially relative to each other, between an open position of the circuit breaker in which the arcing contacts and are separated from each other and a closed position of the circuit breaker in which the arcing contacts and are in contact with each other, - an electric arc-blast nozzle (5, 17) for a circuit breaker comprising: - a middle portion forming a throat defining internally an axial passage for breaking an electric arc, and - two end portions (9, 11) extending on either side of the middle portion and being designed to receive respective arcing contacts and that are movable axially relative to each other, between an open position of the circuit breaker in which the arcing contacts and are separated from each other and a closed position of the circuit breaker in which the arcing contacts and are in contact with each other and in which one of the arcing contacts partially closes the axial passage of the middle portion, an arc-control gas flowing through the axial passage of the middle portion in order to interrupt an electric arc that is likely to form during movement of the arcing contacts and from the closed position to the open position of the circuit breaker, the middle portion together with the two end portions (9, 11) being made of a same dielectric material obtained from a composition comprising a matrix, at least one inorganic filler and micro-capsules, and - an arc-control gas flowing through the axial passage of the middle portion of the nozzle (5, 17) in order to interrupt an electric arc that is likely to form during movement of the arcing contacts and from the closed position to the open position of the circuit breaker, characterised in that the matrix is a fluorocarbon polymer matrix, the microcapsules are microcapsules of liquid heptafluoro-iso-butyronitrile and in that the arc-control gas comprises heptafluoro-iso-butyronitrile in a mixture with a dilution gas. - at least two arcing contacts and that are movable axially relative to each other, between an open position of the circuit breaker in which the arcing contacts and are separated from each other and a closed position of the circuit breaker in which the arcing contacts and are in contact with each other, - an electric arc-blast nozzle (5, 17) for a circuit breaker comprising: - a middle portion forming a throat defining internally an axial passage for breaking an electric arc, and - two end portions (9, 11) extending on either side of the middle portion and being designed to receive respective arcing contacts and that are movable axially relative to each other, between an open position of the circuit breaker in which the arcing contacts and are separated from each other and a closed position of the circuit breaker in which the arcing contacts and are in contact with each other and in which one of the arcing contacts partially closes the axial passage of the middle portion, an arc-control gas flowing through the axial passage of the middle portion in order to interrupt an electric arc that is likely to form during movement of the arcing contacts and from the closed position to the open position of the circuit breaker, the middle portion together with the two end portions (9, 11) being made of a same dielectric material obtained from a composition comprising a matrix, at least one inorganic filler and micro-capsules, and - a middle portion forming a throat defining internally an axial passage for breaking an electric arc, and - two end portions (9, 11) extending on either side of the middle portion and being designed to receive respective arcing contacts and that are movable axially relative to each other, between an open position of the circuit breaker in which the arcing contacts and are separated from each other and a closed position of the circuit breaker in which the arcing contacts and are in contact with each other and in which one of the arcing contacts partially closes the axial passage of the middle portion, an arc-control gas flowing through the axial passage of the middle portion in order to interrupt an electric arc that is likely to form during movement of the arcing contacts and from the closed position to the open position of the circuit breaker, the middle portion together with the two end portions (9, 11) being made of a same dielectric material obtained from a composition comprising a matrix, at least one inorganic filler and micro-capsules, and - an arc-control gas flowing through the axial passage of the middle portion of the nozzle (5, 17) in order to interrupt an electric arc that is likely to form during movement of the arcing contacts and from the closed position to the open position of the circuit breaker, characterised in that the matrix is a fluorocarbon polymer matrix, the microcapsules are microcapsules of liquid heptafluoro-iso-butyronitrile and in that the arc-control gas comprises heptafluoro-iso-butyronitrile in a mixture with a dilution gas.