RU2370752C1 - Device for measuring distribution of size and concentration of nanoparticles in liquids and gases - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: device has a laser with an optical channel for moving laser radiation, on which a cell containing the analysed medium is placed, at the opposite output of which there is a light-absorbing screen, and four photodetectors lying at 35, 60, 90 and 145 degrees to the incident laser beam, respectively.

EFFECT: use of several reception channels and improvement of laser radiation characteristics allows for considerable widening of the range of size and concentration of the analysed nanoparticles.

4 cl, 5 dwg

Description

The invention relates to optical diagnostic devices for measuring the distribution of the concentration and size of nanoparticles in liquids and gases. In particular, the proposed device can be used in complexes of technological control of the sizes of nanopowders in their production, in the development of new technologies for producing nanodispersed substances, in biology and medicine for measuring the sizes of biopolymer particles suspended in biological fluids, pharmaceuticals, and other submicron biological objects. In water, for example, the characteristic sizes of measured objects range from 0.2 to 1000 nm.

The prototype of the proposed device is a "device for measuring clusters and microparticles in liquids" (RU 49620, G01J 9/02, 07/11/2005, [L1]). As a result of the operation of the aforementioned device, shortcomings were identified and new tasks arose that required changes to some technical solutions embedded in the aforementioned device.

When using the device [L1], the effect of the back-reflected and scattered radiation on the input face of the cell of the radiation on the laser operation characteristics was detected. Extremely high requirements are imposed on the stability of the laser, especially in the low-frequency region (up to 100 kHz) of changes in its power, which can be met only when conditions are created for the laser to eliminate spurious glare at the laser radiation wavelength. The greatest problems are caused by radiation reflected from the front of the cell. To solve the aforementioned problem, a tilt is introduced (Fig. 1) in the position of the cuvette 7 relative to the beam incident on it from the laser 1.

In this case, the reflected beam is absorbed by a specially installed additional absorber 10.

Another solution to this problem is the use of an optical insulator - a device that does not allow back-reflected or scattered radiation to get into the laser cavity.

To reduce fluctuations in the laser power in the frequency range in the hertz range, we used stabilization of the laser power using a signal from an additionally installed photodetector that monitors the output radiation in the low-frequency region.

In the case of a semiconductor laser, a photodiode is often mounted on which radiation is emitted through a “dull” laser mirror. The signal of this photodiode can be used to control and stabilize the power of the radiation coming out through another output mirror. However, it is impossible to use this photodiode in the power stabilization system in the mentioned device for the following reasons:

a) the semiconductor laser is characterized by a large divergence of the output beam (up to 40 °), therefore, the photodiode built into the laser does not overlap the entire beam exiting through the “blind” mirror; and

b) the optical axis of the laser beam moves in space.

All this leads to the fact that the output signal of this photodiode always contains a variable component, which is an obstacle in the laser power stabilization system.

With this in mind, the stabilization system of the output power of a semiconductor laser should look as follows (figure 2).

The radiation from the semiconductor laser 1 is incident on a beam splitter 2, with which a small part of the energy (a few percent) is sent to a photodetector (FPU) 3, in front of which a collecting lens 4 and aperture 5 are mounted. The signal of the FPU is used to create feedback in the power stabilization system 6 The main part of the radiation enters the cuvette 7 with the test solution, behind which there is a light-absorbing screen 8. With this construction of the optical scheme, the power of the radiation entering the cuvette is proportional to the power radiation emission in the feedback circuit.

As a beam splitter (2), a thick plane-parallel plate can be used (Fig. 3). With a large thickness of the plate 2, only the BC beam, the B ₁ C ₁ , B ₂ C ₂ beams, etc., arising from reflection from the planes of the plate, get to the photodetector (3) on the photodetector (3) of the feedback loop of the power stabilization system do not fall. In the case of a thin beam splitting plate, the beams of the BC, B ₁ C ₁ , B ₂ C ₂ , etc., which will interfere with the formation of low-frequency oscillations of the photocurrent and thus create low-frequency noise in the power stabilization system, will fall on the photodetector.

To divide the output beam of a semiconductor laser, it is possible to use a trihedral prism as a beam splitter (2) (Fig. 4), in which an antireflection coating can be applied to the LM face to reduce radiation reflection.

It is known that the polarization of radiation from a semiconductor laser is subject to significant fluctuations. The photodetector (3) of the feedback loop of the power stabilization system measures the total power of two orthogonal polarizations. In the measurement process, radiation of only one polarization is used (perpendicular to the plane of FIGS. 2-4), the radiation of the second polarization creates additional noise at the output of the photodetector (3) of the feedback circuit. To eliminate this noise, it is necessary between the laser 1 and the beam splitter 2 to install a polarizer 9 (Fig.2-4), which ensures the transmission of radiation only of the polarization that is used in the measurements, while the power of this particular polarization is stabilized.

An increase in the requirements for the accuracy of determining the size of nanoparticles and an increase in the range of measured sizes up to fractions of a nanometer has led to the requirement to increase the range of frequencies of the scattered signal recorded by the photodetector (3). The measures mentioned above made it possible to increase the reliability of the results in the low-frequency region, but were insufficient to expand the frequency band of the received signal. We reached a solution to this problem by setting up several photodetector devices (3) that receive signals at different scattering angles. The fact is that the frequency of the received signal, effective in the sense of obtaining final results, is proportional to the square of the sine of the half scattering angle. For this reason, a receiver located at a large scattering angle allows you to detail the low-frequency region of the signals, and a receiver located at a small scattering angle allows you to detail the high-frequency region of the signals. Using four receivers located at angles of 35, 60, 90 and 145 degrees, we were able to expand the effective band of the received signal by about 10 times. This choice of the number and location angles of the receiving channels seems to us close to optimal in terms of satisfying both constructive and algorithmic (associated with the capabilities of mathematical processing) requirements.

An additional advantage of using several angular channels is the ability to determine the scattering diagram, which gives additional information about the sizes of the particles under study.

The physical principle of operation of a quasi-elastic scattered light spectrometer (CURS spectrometer) is based on the use of a well-known physical phenomenon associated with the interaction of laser radiation with moving particles (as a result of Brownian motion) in a transparent medium. As a result of this interaction, due to the Doppler effect known in classical physics, a small part (10 ^-3 ÷ 10 ^-9 ) of laser radiation with a frequency ν is scattered by these moving (stray) particles, while the scattered radiation changes its spectrum, and a frequency shift Δν. Outside the cell (7), scattered radiation with frequencies ν ± Δν is observed. In aqueous solutions, when observed at an angle of 90 degrees and normal temperature, the characteristic diffuse motion of clusters of the sizes under consideration leads to Doppler shifts of the radiation frequency in the range from 1 to 200000 Hz.

The purpose of the COURSE measuring device is to register these frequency changes against the background of the frequency range ~ 10 ¹⁵ Hz typical of laser radiation, while the necessary resolution of the measuring circuit of the device should be approximately 10 ¹⁴ ÷ 10 ¹³ . Such high resolution requirements (in the best optical instruments it is 10 ⁶ ÷ 10 ⁴ ), combined with the requirement to achieve a high dynamic range recorded by signal receivers (up to 6-8 orders of magnitude), in combination with increased requirements for the quality of measurements, they required new physical solutions, since even the most modern element base of precision amplifiers does not allow receiving signals of the required quality.

The result of the proposed technical solutions is the elimination of these restrictions, namely:

- a significant decrease in the minimum measured frequencies of changes in the scattered signal by improving the quality of laser radiation (1) due to the use of the measures described above;

- a significant increase (by 10 times) of the maximum measured effective frequencies of changes in the amplitude of the scattered signal due to the use of simultaneous measurement of the signal along the receiving channels, which differ in the scattering angle.

The device (Fig. 5) contains a laser 1, at the output of which an optical insulator 11 is installed. On the beam path from the laser 1 there is a cuvette 7 for the test liquid, at the output of which a light-absorbing screen 8 is installed to eliminate spurious glare from the transmitted radiation. At several scattering angles with respect to the incident beam, photodetectors 3 with signal preprocessing units are located, connected to a computer (not shown in the diagram), in which the mathematical processing of signals is carried out.

The claimed KURS-spectrometer is as follows. Laser beam 1 with a power of 1 to 100 milliwatts enters the cuvette 7 with the test fluid. Here, the beam is partially scattered on the nanoparticles contained in the liquid. Most of the beam is not scattered, it leaves the cell and is absorbed by the light-absorbing screen 8. Part of the beam, scattered by nanoparticles, is incident on photodetectors 3. Beats of the scattered optical signal in the photodetector become fluctuations in the photocurrent. Then, in a computer (computer), this fluctuating electrical signal is processed using predetermined mathematical methods and the final results are presented on a monitor screen in a user-friendly form containing the sizes and concentrations of nanoparticles.

The use of several receiving channels in the proposed device, as well as methods for improving the characteristics of laser radiation, can significantly expand the ranges of sizes and concentrations of the studied nanoparticles.

Claims (4)

1. A device for measuring the distribution of the concentration and size of nanoparticles in liquids and gases, containing a laser with an optical path for transporting laser radiation, in the path of which a working cell with a test medium is installed, a light-absorbing screen is installed at the opposite exit, and a photodetector for recording scattered on radiation nanoparticles with a signal pre-processing unit connected to a computer, characterized in that the photodetector contains four photodetectors located at angles of 35, 60, 90 and 145 °, respectively, relative to the incident laser beam. 2. The device according to claim 1, characterized in that the working cell is tilted with respect to the incident laser beam at an angle of 2-5 °. 3. The device according to claim 1, characterized in that an optical isolator is placed between the laser and the cuvette — a nonreciprocal optical element, not allowing the radiation reflected from the obstacle with the same polarization to fall back into the laser output window. 4. The device according to claim 1, characterized in that a laser power stabilization system is introduced comprising an external photodetector, a beam splitter and a polarizer, the system allowing to stabilize the radiation power of only that polarization that is necessary for measurements.

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