  We consider the formation of binary black hole mergers through the evolution
of field massive triple stars. In this scenario, favorable conditions for the
inspiral of a black hole binary are initiated by its gravitational interaction
with a distant companion, rather than by a common-envelope phase invoked in
standard binary evolution models. We use a code that follows self-consistently
the evolution of massive triple stars, combining the secular triple dynamics
(Lidov-Kozai cycles) with stellar evolution. After a black hole triple is
formed, its dynamical evolution is computed using either the orbit-averaged
equations of motion, or a high-precision direct integrator for triples with
weaker hierarchies for which the secular perturbation theory breaks down. Most
black hole mergers in our models are produced in the latter non-secular
dynamical regime. We derive the properties of the merging binaries and compute
a black hole merger rate in the range (0.3- 1.3) Gpc^{-3}yr^{-1}, or up to
~2.5Gpc^{-3}yr^{-1} if the black hole orbital planes have initially random
orientation. Finally, we show that black hole mergers from the triple channel
have significantly higher eccentricities than those formed through the
evolution of massive binaries or in dense star clusters. Measured
eccentricities could therefore be used to uniquely identify binary mergers
formed through the evolution of triple stars. While our results suggest up to
~10 detections per year with Advanced-LIGO, the high eccentricities could
render the merging binaries harder to detect with planned space based
interferometers such as LISA.
