rus
Issue #6/2021
A.O.Ismagilov, N.V.Andreeva, O.V.Andreeva
Nanoporous silicate matrices: optical homogeneity problems
Nanoporous silicate matrices: optical homogeneity problems
DOI: 10.22184/1993-8578.2021.14.6.364.37
The paper discusses issues related to the methodology for assessing the optical quality of nanoporous silicate matrices. The results of the study of samples at various stages of their production using the method of digital holographic interferometry are presented. The gravity effect on the formation of a porous structure in the process of chemical etching was analysed.
The paper discusses issues related to the methodology for assessing the optical quality of nanoporous silicate matrices. The results of the study of samples at various stages of their production using the method of digital holographic interferometry are presented. The gravity effect on the formation of a porous structure in the process of chemical etching was analysed.
Теги: digital holographic interferometry nanoporous silicate matrices optical quality нанопористые силикатные матрицы оптическое качество цифровая голографическая интерферометрия
NANOPOROUS SILICATE MATRICES: OPTICAL HOMOGENEITY PROBLEMS
The paper discusses issues related to the methodology for assessing the optical quality of nanoporous silicate matrices. The results of the study of samples at various stages of their manufacture using the method of digital holographic interferometry are presented. The gravity effect on the formation of a porous structure in the process of chemical etching was analysed.
INTRODUCTION
Nowadays, the increasing sophistication of devices and the expansion of the scope of optical range radiation impose new requirements on optical materials and the development of technologies for creating new optical materials with given properties. The matrix principle of composite material construction is among the promising ones.
Nanoporous silicate matrices have a special place among porous structures [1]. Nanoporous matrices based on silicate glass are a very special and unique tool for investigation of physical and chemical processes in a limited volume, commensurate with the scale of processes and size of objects under study: limited space and effective contact with pore walls cause significant features of filler material as compared with the case of its presence in a free volume. It is this possibility that determines the increasing interest in nanoporous silicate matrices and is currently the subject of various studies [1–3].
The current use of nanoporous silicate matrices (NPSMs) is mainly due to their transparency in the visible region of the spectrum and the possibility of obtaining optical quality samples [1].
The research of nanoporous matrices occupies a niche in the life of the scientific community. In order to obtain a material that will have the desired properties, it is important to develop not only the technology for obtaining samples with stable and reproducible characteristics, but also quality control methods for manufactured samples. As a rule, the existing methods have been developed to characterize the quality of optical surfaces. At the same time, the optical quality of NPSMs is determined by the internal porous structure of the sample, and the methods developed for evaluating the optical parameters of homogeneous (indiscreet) non-porous materials are not always directly applicable to its characterization.
It has been shown in [1, 3] that the optical quality of NPSM samples is determined by the stages of chemical treatment of the blanks: acid etching and alkaline etching.
The aim of the work is to investigate the optical quality of the samples after the necessary chemical etching stages developed for the use of NPSM in optical experiments [3].
RESEARCH METHODS
In this work nanoporous silicate matrices made of sodium borosilicate two-phase glass formed by two interpenetrating phases: chemically unstable borate and chemically stable silica were used. To obtain them, blanks shaped as 15 mm dia. polished disks 15 mm and 1 mm thick were used (plane-parallel plates can also be used), manufactured according to a proven technology from two-phase DV-1 glass, which underwent heat treatment with phase separation and the formation of liquation channels.
The main characteristics of the produced samples are the average pore diameter and the sample volume not occupied by the silica frame (free pore volume). The average pore diameter of the most popular samples (NPSM-17) is 17 nm. The free pore volume is in the range (48–58)% and depends on the time of the alkaline etching procedure. In the production of nanoporous silicate matrices the technological procedure was followed which allows of reproducing the specified characteristics of NPSM from batch to batch.
When studying the optical properties of the internal structure of transparent samples the most common and sensitive are interferometric methods that estimate the state of the sample by its phase changes. The method of digital holographic interferometry (DHI) was used in [4]. The developed method allowed of obtaining interferograms (IG) of the sample state at different stages of NPSM manufacturing, to carry out comparison and analysis of experimental results.
The optical homogeneity of the samples was monitored by examining their phase characteristics at an experimental bench (Fig.1).
The bench is designed to study phase transformations of transparent objects by digital holographic interferometry (DHI) method and is equipped with a terminal for processing experimental data [4]. The object module is placed outside the main interferometric scheme, which makes it possible to study different states of the same object when it is exposed outside the interferometer according to the scheme: blank – control; acid treatment (NPSM-7) – control; alkaline treatment (NPSM-17) – control.
The object module is a cuvette with water where the sample to be examined is placed. The sample is set in the centre of the object beam perpendicular to its central beam. Interference control presents an interferogram (IF) experiment and involves the acquisition of two digital holograms H1 and H2 by shooting two frames (exposure 1/1500 s), one frame, H1 is a hologram with sample, the second, H2 is a hologram without sample. In the time interval between frames a stable state of imaging conditions and absence of phase changes in the object beam were ensured.
The interferogram, describing the state of the object, was created with the bench software by subtracting the hologram H2 from the hologram H1, with the formation of a phase difference expanded over the area of the sample with tangential contrast: IG = H1 – H2.
Each point of the IG with x,y coordinates in the object plane describes the phase change of the object beam φ(z) and is calculated by the formula (1) [5]:
Δφ = 2π × Δ(nl)/λ, (1)
where λ is the radiation wavelength in the sample; Δ(nl) is a change in optical thickness of the sample along the z-axis, n is the average refractive index of the sample, l is the geometric (physical) thickness of the sample.
The technological mode of obtaining NPSM as the basis of volumetric recording media was developed with the purpose of creating matrices with characteristics uniformly distributed over the sample thickness [6]. Therefore, the plane-parallel samples (1÷4 mm thick) were placed during chemical treatment in the position closest to vertical in order to ensure uniformity of characteristics along the thickness (in the horizontal direction). Now it became necessary to describe the quality of porous matrices in terms of improving the optical homogeneity over the area of samples produced by NPSM.
One of the main aims of the study was to determine the effect of gravity on the result of the chemical etching steps of the blanks. When setting up the chemical treatment, the samples were oriented in such a way that the effect of gravity, with respect to the sample, was always directed in one direction. In order to determine the gravity effect on the etching result, sample 1 was rotated 180° around the horizontal axis before alkaline etching and sample 2 was left in the same position.
The optical quality of NPSM samples is determined by both the quality of the blank and the structure of the porous frame. The optical quality of a blank is determined by the optical homogeneity of the material (source glass is DV) and the quality of the optical-mechanical processing. As a rule, the blank has no optical inhomogeneities of the refractive index. The quality of optical surface machining corresponds to the requirements for machining optical parts. However, the small sample size (up to 50 mm) and small thickness (1–3 mm) of the used samples limit the machining possibilities, which leads to the presence of an uncontrolled angle between the optical surfaces, i.e. to the wedging of the sample.
Appearance of refractive index inhomogeneities is caused by chemical treatment that forms a porous structure. Porous matrices are heterogeneous media that consist of two components – a silicate framework and a free pore volume, which can be filled with air (air-dry state sample) or with an immersion liquid with certain characteristics (refractive index and absorption coefficient).
The refractive index in this case is the effective refractive index (peff) of the multicomponent medium, which is determined by partial contributions of each component, and is calculated for a porous matrix with two components according to formula (2):
nэф = Vпорnпор + Vкnк, (2)
where Vpor is the relative free pore volume; nпор is the refractive index of the free pore volume filler (pores filled with air – nпор = 1, pores filled with water – nпор = 1.33); Vк is the relative volume of the sample occupied by the silicate frame; nк is the refractive index of the porous sample frame (nк = 1.45).
The change in the effective refractive index, nэф, which is denoted by Δn in the paper, is used as the main parameter for assessing optical heterogeneity.
The free pore volume was measured by the weight method – the difference in weight between the sample in the air-dry state and when it was filled with water, followed by a calculation.
The effective refractive index of a sample, calculated using formula (2), is the refractive index averaged over the entire sample volume, i.e. over both the sample thickness and the sample area.
Table 1 comprises the parameters of the test samples in air-dry state and when filled with water after each of the chemical etching steps.
The pore size distribution relationships after the chemical etching steps were determined by experimental measurements of the adsorption-desorption isotherms of ethyl alcohol vapour (NPSM-7) and mercury vapour (NPSM-17) [6]. The pore diameters at the maximum value of this function gave names to the samples produced by these technologies – NPSM-7 and NPSM-17.
RESULTS
Interferograms describing the phase structure of the original blank samples are shown in Table 2.
As can be seen from the above data, the interference fringes in the blanks are arranged almost parallel to each other over the sample area. The distance between the fringes determines the wedge shape of the sample blank. It should be noted that this distance over the area of the sample blank practically does not change, which indicates, as one would expect, the uniformity of the distribution of the refractive index of a solid medium sample over its volume.
Interferograms of test samples 1 and 2 after each etching step are shown in Table 3.
Note again, that the interference fringes in the blanks are parallel to each other and the distance between them practically does not change over the sample area. At the same time, after chemical treatment the structure of interferograms changes – a curvature of fringes and a decrease in the distance between them in the direction of gravity are observed. Such changes are characteristic of both NPSM-7 and NPSM-17. The change in sample position during alkaline treatment (sample No.1) compared to its position during acid treatment results in a rather bizarre interference pattern – the appearance of a "trough" in the centre of the sample. This indicates a significant influence of gravity on the dissolution of chemically unstable phase and amorphous (finely dispersed) silica and the removal of reaction products from the liquation channels.
It is important, that the optical inhomogeneity of porous samples is due to the non-uniform distribution of the free pore volume over the sample volume. The sample thickness l does not change in this case.
When considering the interferograms obtained in this scheme (Fig.1), the distance between the two interference fringes corresponds to a phase difference of 2π. This value determines a change in the average effective refractive index of the sample on its surface between two adjacent fringes (Table 4). After the chemical etching stages the distance between the interference fringes changes in the direction of gravity. Moreover, a decrease in the distance between the bands indicates an increase in Δn between the bands and results in an increase in the change in refractive index per unit length in the direction of gravity.
For example (see Table 3), in the centre of sample 2 a phase change of 2π occurs at Δz = 5 mm, and at the upper edge of the sample at Δz = 0.2 mm. It is the value Δn/d (d – distance between two interference fringes) that characterizes optical inhomogeneity of the sample which effective index of refraction, when determined by volume averaging.
Figure 2 shows the change of Δn/d value in section A-A of the investigated samples. It is important to note that for NPSM-17 samples obtained with the specified geometry of the chemical treatment of blanks, only small (a few millimetres) sections of the sample may have a given value of optical homogeneity (e.g. Δn<10–4).
CONCLUSIONS
This study demonstrates a significant influence of gravity on the formation of the porous structure of NPSM silicate matrices and on the distribution of a free pore volume over the sample area. It is shown, that the nature of this effect does not depend on the type of etching: in the direction of gravity the distance between the interference fringes decreases, which leads to an increase in the effective refractive index in the lower part of the sample located vertically in the working volume.
The optical heterogeneity of the samples is due to the effect of chemical etching and is associated with the removal of the readily soluble boron and sodium oxide-rich phase during acid etching and the removal of "secondary" silica from the borate phase breakdown area during alkaline etching.
The analysis shows that the quantitative data on the change in refractive index per unit length is quite high and that only parts of the working area of the sample can be considered optically homogeneous.
The practically important result is in use of the digital holographic interferometry for studying the optical quality of nanoporous silicate matrices. The developed method of investigation of optical characteristics of NPSM can be applied to control samples when developing the technological regimes of production of NPSM with specified values of optical homogeneity. ▪
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
The paper discusses issues related to the methodology for assessing the optical quality of nanoporous silicate matrices. The results of the study of samples at various stages of their manufacture using the method of digital holographic interferometry are presented. The gravity effect on the formation of a porous structure in the process of chemical etching was analysed.
INTRODUCTION
Nowadays, the increasing sophistication of devices and the expansion of the scope of optical range radiation impose new requirements on optical materials and the development of technologies for creating new optical materials with given properties. The matrix principle of composite material construction is among the promising ones.
Nanoporous silicate matrices have a special place among porous structures [1]. Nanoporous matrices based on silicate glass are a very special and unique tool for investigation of physical and chemical processes in a limited volume, commensurate with the scale of processes and size of objects under study: limited space and effective contact with pore walls cause significant features of filler material as compared with the case of its presence in a free volume. It is this possibility that determines the increasing interest in nanoporous silicate matrices and is currently the subject of various studies [1–3].
The current use of nanoporous silicate matrices (NPSMs) is mainly due to their transparency in the visible region of the spectrum and the possibility of obtaining optical quality samples [1].
The research of nanoporous matrices occupies a niche in the life of the scientific community. In order to obtain a material that will have the desired properties, it is important to develop not only the technology for obtaining samples with stable and reproducible characteristics, but also quality control methods for manufactured samples. As a rule, the existing methods have been developed to characterize the quality of optical surfaces. At the same time, the optical quality of NPSMs is determined by the internal porous structure of the sample, and the methods developed for evaluating the optical parameters of homogeneous (indiscreet) non-porous materials are not always directly applicable to its characterization.
It has been shown in [1, 3] that the optical quality of NPSM samples is determined by the stages of chemical treatment of the blanks: acid etching and alkaline etching.
The aim of the work is to investigate the optical quality of the samples after the necessary chemical etching stages developed for the use of NPSM in optical experiments [3].
RESEARCH METHODS
In this work nanoporous silicate matrices made of sodium borosilicate two-phase glass formed by two interpenetrating phases: chemically unstable borate and chemically stable silica were used. To obtain them, blanks shaped as 15 mm dia. polished disks 15 mm and 1 mm thick were used (plane-parallel plates can also be used), manufactured according to a proven technology from two-phase DV-1 glass, which underwent heat treatment with phase separation and the formation of liquation channels.
The main characteristics of the produced samples are the average pore diameter and the sample volume not occupied by the silica frame (free pore volume). The average pore diameter of the most popular samples (NPSM-17) is 17 nm. The free pore volume is in the range (48–58)% and depends on the time of the alkaline etching procedure. In the production of nanoporous silicate matrices the technological procedure was followed which allows of reproducing the specified characteristics of NPSM from batch to batch.
When studying the optical properties of the internal structure of transparent samples the most common and sensitive are interferometric methods that estimate the state of the sample by its phase changes. The method of digital holographic interferometry (DHI) was used in [4]. The developed method allowed of obtaining interferograms (IG) of the sample state at different stages of NPSM manufacturing, to carry out comparison and analysis of experimental results.
The optical homogeneity of the samples was monitored by examining their phase characteristics at an experimental bench (Fig.1).
The bench is designed to study phase transformations of transparent objects by digital holographic interferometry (DHI) method and is equipped with a terminal for processing experimental data [4]. The object module is placed outside the main interferometric scheme, which makes it possible to study different states of the same object when it is exposed outside the interferometer according to the scheme: blank – control; acid treatment (NPSM-7) – control; alkaline treatment (NPSM-17) – control.
The object module is a cuvette with water where the sample to be examined is placed. The sample is set in the centre of the object beam perpendicular to its central beam. Interference control presents an interferogram (IF) experiment and involves the acquisition of two digital holograms H1 and H2 by shooting two frames (exposure 1/1500 s), one frame, H1 is a hologram with sample, the second, H2 is a hologram without sample. In the time interval between frames a stable state of imaging conditions and absence of phase changes in the object beam were ensured.
The interferogram, describing the state of the object, was created with the bench software by subtracting the hologram H2 from the hologram H1, with the formation of a phase difference expanded over the area of the sample with tangential contrast: IG = H1 – H2.
Each point of the IG with x,y coordinates in the object plane describes the phase change of the object beam φ(z) and is calculated by the formula (1) [5]:
Δφ = 2π × Δ(nl)/λ, (1)
where λ is the radiation wavelength in the sample; Δ(nl) is a change in optical thickness of the sample along the z-axis, n is the average refractive index of the sample, l is the geometric (physical) thickness of the sample.
The technological mode of obtaining NPSM as the basis of volumetric recording media was developed with the purpose of creating matrices with characteristics uniformly distributed over the sample thickness [6]. Therefore, the plane-parallel samples (1÷4 mm thick) were placed during chemical treatment in the position closest to vertical in order to ensure uniformity of characteristics along the thickness (in the horizontal direction). Now it became necessary to describe the quality of porous matrices in terms of improving the optical homogeneity over the area of samples produced by NPSM.
One of the main aims of the study was to determine the effect of gravity on the result of the chemical etching steps of the blanks. When setting up the chemical treatment, the samples were oriented in such a way that the effect of gravity, with respect to the sample, was always directed in one direction. In order to determine the gravity effect on the etching result, sample 1 was rotated 180° around the horizontal axis before alkaline etching and sample 2 was left in the same position.
The optical quality of NPSM samples is determined by both the quality of the blank and the structure of the porous frame. The optical quality of a blank is determined by the optical homogeneity of the material (source glass is DV) and the quality of the optical-mechanical processing. As a rule, the blank has no optical inhomogeneities of the refractive index. The quality of optical surface machining corresponds to the requirements for machining optical parts. However, the small sample size (up to 50 mm) and small thickness (1–3 mm) of the used samples limit the machining possibilities, which leads to the presence of an uncontrolled angle between the optical surfaces, i.e. to the wedging of the sample.
Appearance of refractive index inhomogeneities is caused by chemical treatment that forms a porous structure. Porous matrices are heterogeneous media that consist of two components – a silicate framework and a free pore volume, which can be filled with air (air-dry state sample) or with an immersion liquid with certain characteristics (refractive index and absorption coefficient).
The refractive index in this case is the effective refractive index (peff) of the multicomponent medium, which is determined by partial contributions of each component, and is calculated for a porous matrix with two components according to formula (2):
nэф = Vпорnпор + Vкnк, (2)
where Vpor is the relative free pore volume; nпор is the refractive index of the free pore volume filler (pores filled with air – nпор = 1, pores filled with water – nпор = 1.33); Vк is the relative volume of the sample occupied by the silicate frame; nк is the refractive index of the porous sample frame (nк = 1.45).
The change in the effective refractive index, nэф, which is denoted by Δn in the paper, is used as the main parameter for assessing optical heterogeneity.
The free pore volume was measured by the weight method – the difference in weight between the sample in the air-dry state and when it was filled with water, followed by a calculation.
The effective refractive index of a sample, calculated using formula (2), is the refractive index averaged over the entire sample volume, i.e. over both the sample thickness and the sample area.
Table 1 comprises the parameters of the test samples in air-dry state and when filled with water after each of the chemical etching steps.
The pore size distribution relationships after the chemical etching steps were determined by experimental measurements of the adsorption-desorption isotherms of ethyl alcohol vapour (NPSM-7) and mercury vapour (NPSM-17) [6]. The pore diameters at the maximum value of this function gave names to the samples produced by these technologies – NPSM-7 and NPSM-17.
RESULTS
Interferograms describing the phase structure of the original blank samples are shown in Table 2.
As can be seen from the above data, the interference fringes in the blanks are arranged almost parallel to each other over the sample area. The distance between the fringes determines the wedge shape of the sample blank. It should be noted that this distance over the area of the sample blank practically does not change, which indicates, as one would expect, the uniformity of the distribution of the refractive index of a solid medium sample over its volume.
Interferograms of test samples 1 and 2 after each etching step are shown in Table 3.
Note again, that the interference fringes in the blanks are parallel to each other and the distance between them practically does not change over the sample area. At the same time, after chemical treatment the structure of interferograms changes – a curvature of fringes and a decrease in the distance between them in the direction of gravity are observed. Such changes are characteristic of both NPSM-7 and NPSM-17. The change in sample position during alkaline treatment (sample No.1) compared to its position during acid treatment results in a rather bizarre interference pattern – the appearance of a "trough" in the centre of the sample. This indicates a significant influence of gravity on the dissolution of chemically unstable phase and amorphous (finely dispersed) silica and the removal of reaction products from the liquation channels.
It is important, that the optical inhomogeneity of porous samples is due to the non-uniform distribution of the free pore volume over the sample volume. The sample thickness l does not change in this case.
When considering the interferograms obtained in this scheme (Fig.1), the distance between the two interference fringes corresponds to a phase difference of 2π. This value determines a change in the average effective refractive index of the sample on its surface between two adjacent fringes (Table 4). After the chemical etching stages the distance between the interference fringes changes in the direction of gravity. Moreover, a decrease in the distance between the bands indicates an increase in Δn between the bands and results in an increase in the change in refractive index per unit length in the direction of gravity.
For example (see Table 3), in the centre of sample 2 a phase change of 2π occurs at Δz = 5 mm, and at the upper edge of the sample at Δz = 0.2 mm. It is the value Δn/d (d – distance between two interference fringes) that characterizes optical inhomogeneity of the sample which effective index of refraction, when determined by volume averaging.
Figure 2 shows the change of Δn/d value in section A-A of the investigated samples. It is important to note that for NPSM-17 samples obtained with the specified geometry of the chemical treatment of blanks, only small (a few millimetres) sections of the sample may have a given value of optical homogeneity (e.g. Δn<10–4).
CONCLUSIONS
This study demonstrates a significant influence of gravity on the formation of the porous structure of NPSM silicate matrices and on the distribution of a free pore volume over the sample area. It is shown, that the nature of this effect does not depend on the type of etching: in the direction of gravity the distance between the interference fringes decreases, which leads to an increase in the effective refractive index in the lower part of the sample located vertically in the working volume.
The optical heterogeneity of the samples is due to the effect of chemical etching and is associated with the removal of the readily soluble boron and sodium oxide-rich phase during acid etching and the removal of "secondary" silica from the borate phase breakdown area during alkaline etching.
The analysis shows that the quantitative data on the change in refractive index per unit length is quite high and that only parts of the working area of the sample can be considered optically homogeneous.
The practically important result is in use of the digital holographic interferometry for studying the optical quality of nanoporous silicate matrices. The developed method of investigation of optical characteristics of NPSM can be applied to control samples when developing the technological regimes of production of NPSM with specified values of optical homogeneity. ▪
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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