Sintering of opals

Sintering of green opals

Bare as-grown opals are solid but present a low mechanical stability. To improve it, a sintering process can be performed. A detailed study of which allows asserting that enhancing the mechanical stability without disturbing the 3D order of the samples is possible. From all these analyses it can be concluded that samples annealed at temperatures up to 950ºC preserve a filling factor of 74%, which corresponds to a perfectly close packed structure.

FIG. 1. SEM images of a sample treated at T=950ºC [(a), (b) and (c)] and at T=1050ºC [(d), (e) and (f)]. Different types of internal fcc crystalline facets are obtained after sample cleavage: {111} [(a) and (d)], {100} [(b) and (e)], and {111} terraces [(c) and (f)].

These PXs offer the interesting possibility of changing the filling factor (ff) through sintering. This allows to modify the optical properties and the accessible pore volume of the samples, which is an important parameter in order to use artificial opals as hosts for other materials. Green samples present a ff of about 60%, which implies that they are very open structures. SEM can observe hardly any structural differences between as-grown opals and those annealed at temperatures up to 950ºC for three hours. Nevertheless, dramatic changes of the optical features and the mechanical stability take place. These changes are caused by modifications in the physico-chemical characteristics of the spheres surfaces. A mild sintering, promoted by the incipient viscous flow, causes the formation of necks between the spheres for 700ºC<T<950ºC. This process largely improves the mechanical stability without affecting the 3D fcc order, as SEM and AFM reveal. Above 950ºC, however, strong structural and optical modifications take place. Above this firing temperature pore volume reduces as spheres start crushing into each other. Diffraction intensity lowers and finally no photonic bands can be observed in samples sintered at 1100ºC. The morphological outcome of the particles interpenetration can be observed in the SEM images of Fig. 1. Samples sintered at 1050ºC present fractures or holes that allow the location of the particles belonging to the complementary plane. These images show that the strong structural modification that takes place does not influence the long-range order existing in the structure. The interpenetration causes a reduction of the pore size, thus increasing the filling fraction of the structure and being responsible for the observed decrease in intensity of the ptical attenuation band. By the experimental procedure described here, it is possible to control de filling factor of the structure between 74%, corresponding to a compact structure, and 100%, which corresponds to a system without pores (samples treated at T=1100ºC). As a consequence of the sintering process the Bragg diffraction peak undergoes a blue shift that results in a change of colour that can be observed with naked eye. In Fig. 2 three stages in the sintering process are shown. In (a) an as-grown sample as viewed under the optical microscope is shown. In (b) and (c) the same is shown when this sample is sintered at T=950ºC and T=1050ºC respectively. Photographs were taken illuminating the samples with white light at q» 0º (normal incidence). Optical transmission spectra performed at q=0º are shown as well. It can be clearly observed that, depending on the thermal treatment, a different range of wavelengths is Bragg reflected. All these results demonstrate that certain photonic crystal properties of artificial silica opals can be easily and accurately controlled by means of a thermal treatment.

FIGURE 2. Thermal control of the optical properties. On the left column, transmission spectra obtained at q=0º for a sample made of spheres of 260 nm diameter (a) as grown, (b) treated at T=950ºC and (c) treated at T=1050ºC. On the right column, the corresponding reflected images, taken under white light at normal incidence.