Estimation of optical heterogeneity of samples in the process of developing nanoporous matrices from two-phase glass

In this paper the research of optical heterogeneity of nanoporous silicate matrices (NPM) by method of digital holographic interferometry (DHI) is presented.


Introduction
Currently, the increase of complexity of devices and expanding of the area of application of optical range radiation sets new requirements to optical materials. One of the main ways of development is the creation of new materials with specified properties. Matrix principle of construction of composite materials is one of the prospective. Nanoporous silicate matrices have a special place among porous structures [1]. At present time, application of nanoporous silica matrices mainly related to the transparency of these glasses in the visible spectral range and to the possibility of obtaining samples of optical quality [2]. The research of nanoporous matrices occupies a certain place in the life of science community. To obtain a material which will possess the specified properties it is important to develop technology that will include comprehensive researches for estimation of quality of the product. The aim of this work was to research the influence of each stage of chemical processing on the final optical heterogeneity of the sample.

Materials and method
In the experiment we used nanoporous silicate matrices that were made of sodiumborosilicate twophase glass formed by two interpenetrating phases: chemically unstable borate and chemically stable silica. We used matrices in the form of polished disks with a diameter of 15 mm and with a thickness of 1 mm and in the form of plane-parallel plates, which were made using proven technology from twophase glass DV-1 after stage of heat processing. The main characteristics of the samples during their development are the average pore diameter and the free pore volume, characterizing the volume of the sample that isn't occupied by silica frame. The average pore diameter of the samples is 17 nm. The free pore volume of such samples is in the range of 48-58% and depends on the time of the procedure of alkaline etching. During developing nanoporous silica matrices technological regulation was observed and it gave an ability to reproduce the specified characteristics of NPM from batch to batch. Control of phase homogeneity of the samples was carried out at each stage of processing by carrying out researches of samples on the stand of DHI -scheme of the stand (figure 1) -in the initial state,  3 -collimator; 4 -object of research; 5 -rotary mirror system; 6 -lens 1; 7aperture diaphragm; 8 -lens 2; 9 -CMOS-matrix; 10 -DPSS laser (532 nm); 11device OPHIR.
An additional problem of the research was to determine the influence of gravity on the result of the procedure of chemical processing. During setting to a chemical processing the samples were oriented so that the action of gravity relative to the sample was always pointing in the same direction. In order to determine the influence of gravity on the result of the etching before alkaline etching sample 1 was rotated on 180° relative to the horizontal axis and sample 2 was left in the same position.
As the main parameter for estimation of optical heterogeneity was used the effective refractive index n eff and its changes throughout the volume of the sample. The effective refractive index is determined as following: where V 1 -the relative free pore volume; n 1 -the refractive index of the immersion of free pore volume (air immersion -n 1 =1, water immersion -n 1 =1.33); V 2 -the relative volume of sample that is occupied by the frame SiO 2 ; n 2 -the refractive index of the sample frame (n 2 =1.45).
Calculation of the free pore volume was carried out by the weight method which was proposed in the following work [3]. The effective refractive index of the sample calculated by the formula 1 represents by itself a refractive index averaged throughout the volume of the sample. In table 1 the parameters of the obtained matrices during filling of pores with air and water after each stage of chemical etching are shown.

Results and discussion
Interferograms characterizing the phase structure of the initial samples were obtained. The samples were set in a cuvette which was included in the stand of DHI. Table 2. Phase structure of billets for disks samples which was measured in the air (left) and in the water (right).

Sample
In the air In the water

Conclusion
Optical heterogeneity of the samples due to the influence of chemical etching is associated with the removal of the soluble phase that is enriched with the oxides of boron and sodium during acid etching and with the removal of "secondary" silica from the area of destruction of borate phase during alkaline etching. The change of the optical heterogeneity is associated with the orientation of the samples in the gravity field and its value is 8х10 -5 in the center and 18х10 -4 on the edge of the sample 1. During turning of the sample before alkaline etching there is a shift to the center, while in the sample that wasn't turned over there is only a gain of heterogeneity obtained during acid etching.