RU2416681C2 - METHOD OF PRODUCING OPTICAL MEDIUM BASED ON SiO2 NANOPARTICLES - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: method is implemented through hydrolysis of a colloidal suspension from a tetraether of ortho-silicic acid in an organic solvent - ethanol in the presence of a catalyst - ammonia, in which the tetraether of ortho-silicic acid is preheated at temperature 350-420 K for 150-240 minutes with ratio of the latter to 25% ammonia between 2:1 and 1.2:1.

EFFECT: high beam strength o the optical medium.

2 dwg, 3 ex

Description

The invention relates to the field of production of optical media, including media with selective transmission, with high radiation resistance.

It is known that a number of enterprises, in particular Northrop Grumman Corporation, announced the creation of an aircraft-mounted solid-state laser with a power of about 100 kW (nanosecond range in pulse duration) and the work on creating a similar laser with a power of 200 kW, and a self-propelled laser is being advertised installation capable of hitting unmanned aircraft. At the same time, a number of companies are known that are developing high-power chemical lasers (femtosecond pulse duration range) installed on airplanes and capable of hitting ground, air and space targets. In any case, there have been reports of testing such kilowatt-range lasers, which means that it is possible in principle to create a target on the surface under certain conditions (factors such as range, mirror and focusing efficiency, absorption, etc. must be taken into account), high powers (for several seconds - the duration of quasi-continuous laser operation) from hundreds of MW / cm ² to units of GW / cm ² .

Known technical solutions for the production of optical media and elements with increased radiation resistance using various crystalline materials, for example, patents RU 02288489 C1 from 2006, RU 02112820 C1 from 1998, RU 02345388 C1 from 2009 and RU 01544044 A1 from 1995 d. in which data are presented on the production of optical media and elements with increased radiation resistance using various technological methods for growing crystalline materials or by pressing powders from such materials. In all cases, the resulting effect of increasing radiation strength does not exceed 10-20 GW / cm ² . Technical solutions are also known for producing synthetic materials with a high homogeneity of structure (lattice packaging such as opal) using suspensions of spherical silica nanoparticles (SiO2 _. ) With high monodispersity obtained by hydrolysis of tetraethoxysilane in an alcohol-ammonia medium, for example, patents RU 2162456 and RU 2051864, mainly , for jewelry use. Such materials do not have radiation strength in excess of 50-100 GW / cm ² . A technical solution is also known from the patent US 7063890 B2, cl. 428/402 from 2006 (prototype), namely, an optical medium obtained using SiO ₂ nanoparticles with a diameter of 210-240 nm, manufactured using a colloidal suspension, which provides increased resistance of such a composite to light and mechanical stresses. However, such and other known materials do not have radiation strength in excess of 30-50 GW / cm ² .

An object of the invention is to obtain optical media based on SiO ₂ nanoparticles, including selective transmission media with high radiation strength in the range of 120-600 GW / cm ² .

The problem is solved by hydrolysis of a colloidal suspension of orthosilicic acid tetraester in an ethanol organic solvent in the presence of an ammonia catalyst, in which the orthosilicon tetroester is preheated in the temperature range 350-420 K for 150-240 minutes with a ratio of the latter and ammonia 25% - concentration in the range from 2: 1 to 1.2: 1.

Nanocomposites, like many other materials (thin films, porous media, colloidal aggregates such as opal matrices - regular packings of SiO ₂ nanospheres), under certain conditions, have a structure that is not described by crystallographic groups. Nevertheless, as a rule, such materials possess other types of ordering, in particular of the fractal type, when processes of self-organization of geometric structures with self-maintaining shape occur in the system. As a rule, colloidal aggregate nanocomposites do not have translational invariance in the crystallographic sense of this term, with the exception of lattice packings, when this definition can be extended to the centers of nanospheres that form the pack. The fractal dimension (d) of an aggregate in the form of a nanoparticle - a fractal of smaller particles or minimal (in size) clusters is determined by the ratio between the number (N) of such particles in it and the size (for spherical, in particular colloidal particles - gyroscopic radius) of the aggregate ( R _c ) in the form of the formula: N = (R _c / R ₀ ) ^d , where R ₀ is a constant that determines the minimum distance between honey nanoparticles. The introduced fractal dimension describes, for example, the dependence of the volume of interspherical voids on the linear dimensions of a colloidal particle. It is significant that, unlike periodic systems (d = 3), colloidal media with fractal geometry have d <3 (for example, d≈1.78 for aggregates of the cluster-cluster type and d≈2.5 for aggregates of the type particle-cluster )

To obtain systems from colloidal aggregates of the type of opal matrices — regular packings of SiO ₂ nanospheres containing interspherical voids, an express method is used to produce spherical silica particles based on the hydrolysis of TEOS - orthosilicic acid tetraester (Si (OS ₂ H ₅ ) ₄ ) in an organic solvent - ethyl alcohol (C ₂ H ₅ OH) in the presence of a catalyst - ammonia (ammonium hydroxide) (NH ₄ OH). In the course of the hydrolysis reaction, small branched polymer particles of silica are first formed, which, due to internal polycondensation, turn into spherical amorphous particles. The resulting suspension is an unstable system, prone to coagulation, gelation and precipitation. The main factors affecting the aggregate stability of an organosol are: the quality of the starting reagents, primarily TEOS; homogeneity of the reaction mixture; concentration and amount of input components; constancy of reaction conditions (temperature, humidity). In addition, the degree of monodispersion of the suspension depends on these factors, namely, the size dispersion of spherical particles of SiO _{2 is} controlled by changing the ratio of the components of the tetraester of orthosilicic acid and ammonia (25% concentration) in the range from 2: 1 to 1.2: 1. At the same time, an additional method of changing the degree of monodispersity of colloidal particles in suspension is the heating of TEOS in the temperature range 350-420 K for 1-2 hours.

The methods for filling the inter-spherical voids with materials with different refractive indices are based on the impregnation of the opal matrix with a precursor substance with a certain chemical composition, followed by heat treatment, during which the necessary chemical composition is formed in the inter-spherical voids of the opal matrix. Substances - precursors must have good solubility in water (or other solvents) and pass into oxides (or other compounds) at moderate heat treatment temperatures.

Of particular interest is the excitation of acoustic vibrations in such materials using optical lasers, when the effects of the appearance of unusual states due to the interaction of the states (bands) of the phonon and photon subsystems play a significant role. These effects are possible only in the class of photon-phonon materials with a certain fractal dimension, which include the resulting media with the establishment of a regime of total internal reflection for acoustic vibrations in such a medium, when the energy of high-power optical radiation, for example by laser pulses, is converted into acoustic vibrations with non-destructive medium amplitudes.

To study the radiation strength (at room temperature) of crystalline materials (quartz, ruby), quartz glass, and opal matrices (ordered, namely, regular packings of SiO ₂ nanospheres), a ruby solid-state laser with a generation wavelength of 694 was used as a source of exciting radiation 3 nm, a generation line width of 0.015 cm ^-1 , a pulse duration of 20 ns and a maximum energy of 0.4-0.5 J, which makes it possible to obtain up to 2 GW / cm ² without focusing, and when using focusing, up to 1000 GW / cm ² ( with a beam spot area of 0.2 mm ² ) Similar results were obtained when using a pulsed Nd-YAG laser as a source of excitation (wavelengths for the first and second harmonics are 1.06 μm and 0.53 μm, respectively) with a pulse duration of 20 ns. The threshold value of the laser pulse energy when using a wavelength of 1.06 μm is approximately two times higher than for radiation from a ruby laser or for the second harmonic of an Nd-YAG laser.

The experimental data obtained are as follows. When using crystalline materials or quartz glass, the destruction of samples, including in the form of burning holes or internal breakdown, took place at a power of 15-40 GW / cm ² (hereinafter, depending on the quality of the material), and using focusing on the surface - at 5-20 GW / cm ² (when using a single pulse). In the case of optical media obtained by the proposed method (with similar sizes and conditions of exposure to laser radiation), the necessary exposure parameters for destruction are more than 250-600 GW / cm ² (depending on the quality and fractal characteristics of the sample) when exposed to a single pulse. The highest radiation strength was achieved in opal matrices with a fractal dimension parameter of 2.85, although the effect was observed in the range of this parameter from 2.5-2.95.

The scatter of nanosphere diameters, which makes it possible to determine the fractal dimension by the standard technique, was monitored according to the data (Fig. 1) of transmission electron microscopy (JEM-200 microscope). Calculations based on available experimental data show that an increase in the dispersion (not monodispersity) in the diameters of SiO ₂ nanospheres by 1.5% in the studied samples leads to a decrease in the fractality parameter from 3 by 0.1.

Below are examples of the implementation of the proposed technical solution.

Example 1

To obtain an optical medium from a colloidal suspension of the type of regular packings of SiO ₂ nanospheres containing interspherical voids, an express method is used to obtain spherical silica nanoparticles based on the hydrolysis of TEOS - orthosilicic acid tetraester (Si (OC ₂ H ₅ ) ₄ ) in an organic solvent - ethanol (C ₂ H ₅ OH) in the presence of a catalyst - ammonium hydroxide (NH ₄ OH). To do this, mix 500 ml of alcohol C ₂ H ₅ OH with 10 ml of a 25% aqueous solution of ammonia (NH ₄ OH) and 16 ml of TEOS, preheated at a temperature of 380 K for 180 minutes. A suspension is obtained with nanosphere sizes in the range of 240-250 nm, which corresponds to a fractality parameter of 2.85, which is placed in a quartz glass vessel with a flat bottom for a period of 2 months (from 2 to 3 months depending on the specified thickness of the deposited material ) The obtained sample was subjected to drying and heat treatment at standard parameters (the duration and temperature of heat treatments do not affect the dispersion in the sizes of nanospheres in the obtained precipitate) in an oven (150 ° С for a day) and in a muffle furnace (700 ° С for 10 hours) . Tests of the obtained sample of the optical medium by laser radiation showed that it has a radiation strength of 550 GW / cm ² .

Example 2

To obtain an optical medium from a colloidal suspension of regular packages of SiO ₂ nanospheres, an express method for producing spherical silica particles is used, based on the hydrolysis of TEOS - orthosilicic acid tetraester (Si (OC ₂ H ₅ ) ₄ ) in an organic solvent - ethanol (C ₂ H ₅ OH ) in the presence of a catalyst - ammonium hydroxide (NH ₄ OH). To do this, mix 500 ml of alcohol C ₂ H ₅ OH with 10 ml of a 25% aqueous solution of ammonia (NH ₄ OH) and 20 ml of TEOS, preheated at a temperature of 350 K for 240 minutes. A suspension is obtained with nanosphere sizes in the range of 280-300 nm, which corresponds to a fractality parameter of 2.6, which is placed in a quartz glass vessel with a flat bottom for a period of 2 months (from 2 to 3 months depending on the specified thickness of the deposited material ) The obtained sample was dried and heat treated at standard parameters in an oven (150 ° С for a day) and in a muffle furnace (700 ° С for 10 hours). Tests of the obtained sample of the optical medium by laser radiation showed that it has a radiation strength of 350 GW / cm ² .

Example 3

To obtain an optical medium from a colloidal suspension of regular packages of SiO ₂ nanospheres, an express method for producing spherical silica particles is used, based on the hydrolysis of TEOS - orthosilicic acid tetraester (Si (OC ₂ H ₅ ) ₄ ) in an organic solvent - ethanol (C ₂ H ₅ OH ) in the presence of a catalyst - ammonium hydroxide (NH ₄ OH). To do this, mix 500 ml of alcohol C ₂ H ₅ OH with 10 ml of a 25% aqueous solution of ammonia (NH ₄ OH) and 12 ml of TEOS, preheated at a temperature of 420 K for 150 minutes. A suspension is obtained with nanosphere sizes in the range 220-225 nm, which corresponds to a fractality parameter of 2.85, which is placed in a quartz glass vessel with a flat bottom for a period of 2 months (from 2 to 3 months depending on the specified thickness of the deposited material ) The obtained sample was dried and heat treated at standard parameters in an oven (150 ° С for a day) and in a muffle furnace (700 ° С for 10 hours).

Tests of the obtained sample of the optical medium by laser radiation showed that it has a radiation strength of 450 GW / cm ² .

Considering the transparency windows (submicron and micron regions) for materials based on SiO ₂ (the opal matrix is a regular packing of SiO ₂ nanospheres, with diameters from the range of 200-400 nm, as a rule) and the possibility of controlling the position of the transparency windows for such a photonic crystal, it seems possible to use such materials also as protective filters in a given region of the spectrum from powerful laser influences. The advantage of such filters based on opal matrices is the possibility of tuning them over a wide frequency range, high radiation strength and the presence of a narrower spectral reflection band, which is achieved by introducing optically active substances with a certain refractive index into its inter-spherical voids, which determines the position of the required reflection maximum, filter created on the basis of regular packages of SiO ₂ nanospheres according to the well-known Bragg formula (for normal incidence :)

.λ _max = 2 a n _ef ;

.

- период в направлении измерения для регулярной упаковки; d - диаметр наносфер; n_ef - эффективный показатель преломления опаловой матрицы (регулярной упаковки); n₁ - показатель преломления оптически активного вещества, заполняющего межсферические пустоты опаловой матрицы; n₂=1,33 - показатель преломления аморфного кремнезема; η=0,26 - коэффициент заполнения опаловой матрицы оптически активным веществом.Where

- period in the direction of measurement for regular packaging; d is the diameter of the nanospheres; n _ef is the effective refractive index of the opal matrix (regular packing); n ₁ is the refractive index of an optically active substance filling the interspherical voids of the opal matrix; n ₂ = 1.33 - refractive index of amorphous silica; η = 0.26 is the fill factor of the opal matrix with an optically active substance.

The filter was tested based on regular packages of SiO ₂ nanospheres with a nanosphere diameter d = 245 nm, using an impregnation for 5 hours to introduce an optically active substance with an index of refraction of n ₁ = 1.455 into the inter-spherical voids (refractive index of glycerol at 90 % concentration). The position of the maximum reflection of such a filter is 620 nm.

Testing by laser radiation showed that the filter has a radiation strength of 480 GW / cm ² .

The dependence of the position of the region of maximum reflection of the proposed optical medium on the refractive index of the optically active substance with which its inter-spherical voids are filled is shown in Fig. 2 for the nanosphere diameter d = 245 nm.

The high efficiency of converting high-power laser pulses of one type of energy (optical radiation) into long-term dissipation with the appearance of other types of oscillations in non-crystalline dispersed structures with certain properties makes it possible to fabricate and use optical media based on SiO ₂ nanoparticles as optical media and filters based on them with very high (compared with non-dispersed and non-fractal materials) radiation resistance.

Claims (1)

A method of obtaining an optical medium based on SiO ₂ nanoparticles, characterized in that the process is carried out by hydrolysis of a colloidal suspension of orthosilicic acid tetraester in an organic solvent - ethanol in the presence of an ammonia catalyst, in which the orthosilicon tetraester is preheated in the temperature range 350-420 K in during 150-240 min with a ratio of the latter and ammonia of 25% concentration in the range from 2: 1 to 1.2: 1.

Images