deletions | additions       diff --git a/section_introduction_The_emerging_field__.tex b/section_introduction_The_emerging_field__.tex  index ab69785..4654211 100644  --- a/section_introduction_The_emerging_field__.tex  +++ b/section_introduction_The_emerging_field__.tex 
...
High-temperature solar thermochemical reactors are designed for maximum solar-to-chemical energy conversion efficiency, to minimise the required size of the optical concentrators, and thus to improve the economic viability of the process \cite{Palumbo_2004}. The rapid and efficient transfer of energy from a concentrated solar source to the redox material---i.e. the transfer of energy from photons to chemical bonds---is therefore a critical research aspect for efficient reactor design. To this end, we study here the performance of a class of porous particles of cerium dioxide redox materials under highly-concentrated, direct solar irradiation. We believe these materials to hold promise for efficient energy transfer for a number of reasons we outline here. More specifically, we will answer the questions: "Why particles?", "Why porous?", and "Why ceria?" while outlining the relevant literature in the process.   Unlike bulk materials, particles can have an absorption efficiency (potentially much) greater than unity.   Note that this may seem impossible given the fact that  absorption efficiency must be equal to emissivity by Kirchoff's law and that a blackbody---the perfect emitter---cannot have an emissivity greater than one. However, the idea of a perfect blackbody is restricted to surfaces with low curvature. Both Planck and Kirchoff were clear on this when deriving their famous laws \cite{bohren1998absorption}. Thus, for a material to absorb very well relative to its size---the trait needed to achieve high energies within a particle with relatively little input---we look at materials with features on the scale of the wavelength of light. Perhaps the simplest conceivable geometry is the sphere as we consider here. Ceria has received significant attention as a redox material because of its relatively fast reaction rates and ability to remain in a solid state throughout the redox process which relieves previous problems associated with ferrite-based oxides and volatile metal oxides such as $\mathrm{ZnO}$ and $\mathrm{SnO_2}$  \cite{Chueh_2010} \cite{Chueh_2010a}. The demonstrated ability of ceria to be used as a redox active material and the availability of relevant data make it a very strong candidate to be optimized for high-efficiency thermochemical cycling. The spectral optical properties of ceria are well-known \cite{Patsalas_2002} at room temperature in its stoichiometric form and the thermodynamic properties of non-stoichometric ceria are available allowing detailed modeling in pursuit of this goal. Indeed, researchers have already begun to explore the potential for ceria-particle-based solar thermochemical reactor designs using numerical modeling \cite{Scheffe_2014} \cite{Oles_2015} \cite{Bader_2014}.