High-Transparent Ceramics Prepared Based on Nanopowders Synthesized in a Laser Torch. Part I: Preparation Features
In recent years, much attention has been paid to the developments aimed at creating solid-state lasers with a high average and peak power. This is primarily due to the wide range of applications of such laser systems: in the industry for remote cutting, welding, quenching, heat treatment and labeling of various materials [1–3], as well as in basic scientific research: to initiate and maintain thermonuclear fusion (laser complex National Ignition Facility, NIF, USA, European projects of the European High Power Laser Energy Research facility, HiPER and Laser Megajoule, LMJ, France, ISKRA‑6, Russia) [4–7], gravity wave studies (LIGO observatory, USA, and Virgo, Italy; GEO600 telescope, Germany) [8–10].
One of the key components of powerful continuous and pulsed-periodic lasers is the active medium, where an inverse population of levels is created. In recent years, increasingly greater attention has been paid to the researches aimed at developing a technology to produce ceramic active elements for high-power laser systems. This is due to many advantages of optical ceramics over traditional media from single crystals and glasses: larger sizes, improved thermomechanical characteristics, the ability to synthesize composite samples, quick production, lower energy costs and price.
After synthesis of the laser ceramics and obtaining effective generation , a large amount of research was carried out in this direction. The requirements of  are specified to achieve high-efficiency laser generation in ceramics: the thickness of the intercrystalline boundaries is of the order of 1 nm, the scattering loss per pass is less than 0.05–0.1% cm1 (residual porosity at the level of 10–4 vol.%), optical uniformity with distortion of the wave the front λ/19.5. Using yttrium-aluminum garnet-based ceramics (Y3Al5O12, YAG) with similar characteristics in the geometry of a thin disk (active medium Ш11 Ч 0.15 mm), an output power of 1.8 kW with a differential efficiency of 74.1% was implemented . Moreover, a record output power of 6.5 kW with a differential efficiency of 57% was achieved in . In a ceramic disc 8.5% Yb:LuAG with a thickness of 0.15 mm, an output power of 1.74 kW with a differential efficiency of 71.2% was demonstrated .
The most impressive output power values were achieved when using active elements of a sufficiently large volume. For example, in a ceramic plate of 1% Nd: YAG with a size of 89 Ч 30 Ч 3 mm3, the power of continuous laser generation was 2.44 kW , and with increasing dimensions up to 120 Ч 50 Ч 3 mm3–4.35 kW . The cascade of several Nd: YAG ceramic elements sized 100 Ч 100 Ч 20 mm allowed this value to increase to 67 kW , and further to 105.5 kW . From the point of view of energy characteristics, the impulses with an energy of 105 J for a duration of 10 ns and an average power of 1 kW at a repetition rate of 10 Hz and cryogenic cooling of a Yb:YAG/Cr:YAG element of ceramics have been implemented to date .
One should also note the progress in the field of implementation of ultrashort laser pulses in ceramic active media. In this direction, laser pulses of 188 fs duration  and 152 fs  were demonstrated using Yb: Y2O3 ceramics. The shortest duration was achieved using composite ceramic Yb:Y2O3/Yb:Sc2O3 media with a total width of the amplification band of 27.3 nm, where a record low pulse width of 53 fs was demonstrated .
When developing the technology to produce ceramic active elements, the main attention is paid to the formation of a nonporous microstructure of the material while maintaining the characteristic grain size in the range from several hundred nanometers to micrometers, which is important for reducing the local depolarization of laser radiation . This is achieved by using modern methods of nanoparticle consolidation, such as sintering in a spark plasma [25–29] and post-processing by hot isostatic pressing [30–34].
The detailed reports on the results of research in the field of the creation of high-transparent ceramics are given in the reviews [12,35–39]. In this paper, the focus is on the synthesis of laser ceramics using nanopowders prepared by laser target evaporation, indicating important features of this approach, and which has not been adequately reflected in the literature. As practice has shown, this approach deserves special attention especially for the synthesis of ceramics with disordered crystalline structure.
The sequence of operations for the preparation of laser ceramics in different laboratories is quite similar. Production of nanopowder → its compaction → sintering of compacts → machining of ceramic samples → quality control. At the same time, each of the stages can differ significantly from each other. The synthesis technology of high-transparency ceramics can be conditionally divided into three main stages: preapring nanopowders, their compacting and sintering of compacts.
This stage is extremely important in the creation of high-transparency ceramics. The following requirements are imposed on nanopowders:
• Small dimensions, since the homogeneous packing of nanoparticles into compacts with a density of 0.5 from the theoretical one to the pores has a capillary pressure p = 2σ/r, where σ is the surface energy, r is the radius of the nanoparticle. For σ = 1 J/m2 and r = 10 nm, p = 200 MPa. This huge pressure stimulates the collapse of the pores;
• High purity (> 99.99%);
• Weak agglomeration of nanoparticles, which helps their homogenous packing during compacting;
• Cubic crystal lattice of nanoparticles, which is necessary to eliminate differences in the propagation of radiation in different directions.
There are many methods for preparing nanopowders: mechanical crushing, precipitation from solutions, sol-gel, self-propagating high-temperature synthesis, physical vapor deposition. A detailed analysis of these methods is carried out in . However, nanopowders prepared by the laser synthesis method meet the above requirements to the fullest extent possible. Indeed, the radius of such particles (5–10 nm), the range of particle size distribution is rather narrow (5–40 nm), their purity is similar to the purity of the starting material, they usually have a spherical shape. The agglomeration of the nanopowders thus produced is due to the action of weak van der Waals forces, so when compacted, such agglomerates break easily. The large capillary pressure and the significant surface energy due to the large surface of such nanopowders allow, under otherwise equal conditions, to reduce the duration or the sintering temperature.
However, the most important advantage of the nanopowders thus prepared is that the doping takes place directly in the laser torch at high temperature and rapid cooling. This prevents segregation of the dopants and ensures high homogeneity of the ingredients in the nanoparticle, in the compact and, as will be shown, in samples of synthesized ceramics.
In this connection, let us consider the process of laser nanopowder synthesis in more detail. Fig. 1 shows a block diagram of the laser complex for preparing nanopowders [40, 41]. Laser radiation was focused on the target with a lens, which also served as the entrance window of the evaporation chamber. As a result of the action of laser radiation, a laser torch consisting of target vapors appeared on the target near its surface. Mixing with ambient air or other buffer gas, the steam was cooled. The cooled vapor was condensed in the form of nanoparticles, which were in the evaporating chamber in a suspended state. A special drive rotated the target and moved it linearly in a horizontal plane so that the laser beam scanned the surface of the target at a constant linear velocity, thereby achieving uniform evaporation of the material from the surface. After evaporation of the surface, the target moved in a vertical direction. The fan pumped air through the chamber and transferred the powder to the cyclone and further into the electric filter where it was assembled. The air was cleaned additionally in a mechanical filter and returned to the chamber. The gas flow rate above the target surface was 15 m/s.
Fig. 1b shows photographs of the laser target before and after exposure of the CO2 laser radiation for which the target material is opaque and the ytterbium laser radiation for which the target is semitransparent.
It can be seen that if the target is translucent for laser radiation, then it evaporates non-uniformly. Its surface consists of a number of needle formations 8 mm high and up to 1 mm thick. To explain this destruction, a mechanism was proposed , according to which radiation is absorbed on target defects. In this case, the front part of the defect heats up more intensely and due to the strong dependence of the absorption coefficient on the temperature, a thermal wave is formed which moves along the beam from the defect to the surface of the target. If the temperature in the heat wave does not reach the melting point, a crater of the usual shape is observed, and otherwise – anterior deflection of the target surface is observed. This leads to the deformed target in the powder.
The appearance of micron spherical particles in nanopowder is due to other causes. In our experiments , it was shown that droplets in the torch during the evaporation of the target from Nd:Y2O3 appear after 200 µs, and after 500 µs it mainly consists of droplets. A similar picture is observed with the use of targets from YSZ and FeMgAl2O4.
It is shown that one of the reasons for the appearance of droplets in the laser torch is due to the presence of a melt in the crater. Such ablation, in our opinion, is due to the development of the Kelvin-Helmholtz instability that is formed between the liquid wall of the crater and the flow of expiring vapor.
The theoretical analysis  made it possible to establish the characteristic size of the instability:
and its development the increment
мкс , (2)
where ρ1 and ρ2 are the melt and vapor densities, σ is the surface tension coefficient, V is the vapor flow rate.
Optimizing the duration (<200 µs) and radiation density, separation and trapping, it was possible to prepare high-quality nanopowders. Fig. 2 shows an example of a photo of a YSZ nanopowder, and the distribution of particles of different composition in size is given as an example. Depending on the thermophysical properties of the refractory oxides, the pressure and the speed of the carrier gas, the productivity using a 600 W laser is from 10 to 80 g/h with an average spherical particle size (5–20) nm and a particle size distribution range (2–40)
The distinguishing feature of nanoparticles synthesized in a laser torch, i. e. at a high temperature and rapid cooling, is a high homogeneity of the distribution of components in the volume. This is confirmed by the results of a study of the distribution of the concentration of dopant (Yb) in the Lu2O3 matrix, carried out in the scanning electron microscope (SEM) mode using the X-ray spectral microanalysis (X-ray SMA) method. The results of mapping the elemental composition of individual nanoparticles are shown in Fig. 3. It follows from these images that the dopant is distributed uniformly over the Lu2O3 matrix, and there is no increased Yb concentration on the particle surface.
This finding is supported by the results of X-ray diffraction analysis of Nd: Y2O3 nanopowders and ceramics doped with HfO2 (Fig. 4).
It can be seen that the dependence of the parameters of the crystal lattice on the HfO2 content is linear. This indirectly indicates a homogeneous occurrence of Hf in a Y2O3 matrix and the absence of second phases, both in a nanopowder and in ceramics.
COMPACTING OF NANOPOWDERS
The requirements that are imposed on the methods of "cold" pressing are, first of all, maximally possible compact density and uniformity of stacking of powders. To produce highly transparent ceramics, the following compacting methods are most often used: slip casting, slip casting under pressure, cold isostatic pressing, static pressing, static pressing with ultrasonic action on nanoparticles, magnetic pulse compacting.
Among the casting methods, the method developed in  is of particular interest. For the preparation of aqueous slip, the authors used ammonium polyacrylate as deflocculant. This allowed them to prepare a slip with a solid concentration of up to 40% by volume with the Newtonian nature of the slip. To improve the quality of the packing of nanoparticles and increase the compact density, the authors used casting of the slip into a porous mold. As a porous medium, a porous plate was used with a thickness of 3–5 mm made of Al2O3 powder with a relative density of ~40%. At a pressing pressure of 200 MPa, the authors achieved a relatively high relative density of the compactum ~ 60%.
In , the transparency of laser ceramics was investigated using compacts prepared by slip casting and dry pressing. It was shown that ceramics samples, whose compacts were prepared by cold isostatic pressing, have greater transparency than with slip casting. This difference is attributed to the high viscosity of the slip using nanoparticles, which prevented tight packaging. At the same time, when using hot pressing at 1750 °C and a pressure of 200 MPa, the samples prepared by slip casting have better characteristics than those based on the compacting of dry powders.
However, the use of hot pressing is a complex and expensive step, therefore, there is a strong desire to create a technological chain of preparation of samples with theoretical transparency, without the use of hot pressing.
Given the above, the most studies are conducted using dry cold pressing of nanopowders. For these purposes, we have tested the method of static pressing of nanoparticles with and without ultrasound, magnetic-pulse pressing and cold isostatic pressing. All of them showed rather close relative densities at the same pressures, which is confirmed by the results presented in [44, 45]. Nominally pure and neodymium-activated yttrium oxide nanopowders, designated by us as Y2O3, 8NDY, 3NDY, and 1NDY (the number before the letter symbol NDY denotes the content of neodymium oxide in mole percent in nanopowder) were used in the experiments. For comparison, the compacts of dry nanopowders (without plasticizers) of all these types were pressed as uniaxial static pressing (without USV), and under the influence of ultrasonic vibrations. The pressures were 240, 480, and 720 MPa. The diameter of the pressed samples was 14 mm, the height of the samples was 2 ч 4 mm. The experimental results in the form of the dependence of the relative density on the pressing pressure at a constant power of USV 3 kW and 0 kW (i. e. without USV) are shown in Fig. 5.
According to the technique described in , the parameters of the pressing equation b and Pcr for each type of nanopowder were determined from the experimental compaction curve. The compression curves of the samples were described by the logarithmic compression equation in dimensionless form :
ρ / ρtheor ( Р ) = b Ln ( P / Pcr ) + 1, (3)
where ρ is the density of the compact, ρtheor is the theoretical density, b is the compaction rate, Pcr is the design pressure at which the theoretical density is reached.
The results obtained show that the relative density of the compacts of the studied nanopowders is slightly dependent on the USV and is determined mainly by the pressing pressure, thereby confirming the findings obtained using other methods.
The effect of nanoparticle size on compacts density is discussed in  using the above method, the granular dynamics. The calculations were carried out for nanopowders with particle sizes from 10 to 100 nm. Typically, deterioration of compressibility with decreasing particle sizes is associated with adhesion of the individual particles, which results in the formation of strong aggregates. As possible causes of the size effect are called van der Waals forces of attraction, the absence of plastic deformation of nanoparticles, the formation of chemical bonds, electrostatic interaction, etc.
The authors  sought to take into account the most important of these reasons. Their calculations of the dependence of the axial pressure on the density of compacts are shown in Fig. 6.
Under the initial anisotropic configuration, the distribution of particles with the presence of vertical chains and a coordination number exactly equal to two accurately was adopted. It can be seen that as the particle size increases at the same pressing pressures, the density of the compacts increases substantially. We should also pay attention to the important role that the van der Waals forces create (curve 4). Of course, there is no exact agreement with the experimental data, but the trend can be traced unequivocally.
This fact raises the question of which nanopowders are most preferable for the synthesis of laser ceramics. On the one hand, small particles due to high surface energy provide high sinterability, and in the case of laser nanopowders, greater solubility of the ingredients in each other and homogeneity of the particle, and on the other hand, a worse compressibility. This question remains open in relation to the synthesis of laser ceramics up to this point.
Further, the results obtained using a nanopowder with an average particle size of 10–20 nm and uniaxial static pressing will be presented for the preparation of compacts with dimensions less than 30 mm. Cold isostatic pressing was used for compacts of larger diameter.
ANNEALING OF COMPACTS
The prepared compacts with a relative density of 0.46–0.58 are usually calcined to remove organic matter, dooxidation, and sometimes to provide phase transformations. The latter case is the most complex and interesting. Let us consider the processes occurring upon calcination in compacts of Nd3+:Y2O3 monoclinic phase . The results of analysis by the differential scanning calorimetry method of such a compact are shown in Fig. 7.
The initial endothermic peak of the heat flux is well known; it characterizes the removal of mechanically bound water and carbon dioxide. The next endothermic peak, according to the literature, appears to be due to the decomposition of yttrium hydroxide and yttrium hydrogen carbonate, which are formed on the surface of nanoparticles when interacting with air moisture. Their decomposition at about 327 °C is accompanied by the liberation of water and CO2.
A weak exothermic peak at about 465 °C is also known and is associated with the burnout of organic matter inevitably falling into nanopowders under ordinary laboratory conditions.
A prolonged exothermic process is superimposed on these processes at a temperature above 200 °C. We associate it with the oxidation of yttrium oxide. This is indicated by the following. If we divide the extracted energy in this process ~350 J/g by the enthalpy of yttrium oxide formation (–8432 J/g), then we find the coefficient of non-stoichiometry in oxygen δО = 4.1 · 10–2, which agrees well with the known measurements of the nonstoichiometry of monoclinic yttrium oxide. The oxidation of yttrium oxide is followed by an avalanche-like exothermic transition from the monoclinic phase to the cubic phase.
Then follows a protracted endothermic process, which in our opinion is due to a reoxidation reaction, when excess molecular oxygen ions are incorporated into the anionic sites of the crystal lattice. This is indicated by the appearance in the luminescence spectrum of compacts, calcined at a temperature above 950 °C, the bands of the molecular oxygen ion. And as the intensity of the bands increases with the temperature raise, indicating an increase in the concentration of the molecular oxygen ion in the crystal structure of Y2O3.
Fig. 8 shows the dependence of the grain size on the calcination temperature. Each point on the graph corresponds to its own pattern. It can be seen that the grain sizes grow reasonably from 24 to 77 nm with an increase in temperature from 715 °C to 1300 °C, and the last point, apparently, is caused by a measurement error. The dependence of the mechanical stresses and density of compacts on temperature is also given there: after transformation at 715 °C into a cubic phase which parameters are greater than in the monoclinic one, mechanical stresses increase with the temperature raise, followed by a certain decrease, accompanied simultaneously by a shock of condensation of compacts, that we also interpreted as a mechanical ordering of grains. Further, the behavior of the curves is logical: the density of compacts increases, mechanical stresses decrease.
SINTERING OF COMPACTS
Sintering can be conditionally divided into three stages. The dependencies shown in Fig. 8, characterize the processes in two of the three stages of sintering.
In stage I, there is no shrinkage of the compact (700–1100 °C), but mass transfer from convex to concave surfaces occurs, mainly by near-surface diffusion. This leads to a decrease in the free surface of nanoparticles, which means that they smooth out, spheroidize and increase the size of contact spots between nanoparticles. In the case of nanopowders, the latter process leads to an increase in the dimensions of the nanoparticles, which is not observed for particles with dimensions of ~ 1 µm. After 1100 °C, a second stage is observed, characterized by rapid shrinkage of the sample. This is due to the diffusion sliding of the grains and the diffusion adjustment of their shape, as well as the "evaporation" of vacancies from the pore surface in the bulk of the particles, with their subsequent exit to the crystallite boundaries and displacement in the boundary layer. Since the particle sizes in our case are small, there are many intercrystalline boundaries, then the shrinkage process occurs quite intensively.
This is confirmed by the data in Fig. 9, which shows the dilatometric results of shrinkage measurements of a compact. It can be seen that in the range from 1200 °C to 1540 °C their density increases from 0.48 to 0.97 of the theoretical density.
The main growth of crystallites occurs at the third final stage of sintering after the porosity has decreased to 8–10%. Its driving force is increased energy content and extent ("tension") of boundaries. The boundaries move to the center of their curvature. Large crystallites grow at the expense of smaller ones, whose atoms, crossing the boundary, reduce the free surface energy.
When the compacts are compacted, the diffusion processes are decisive. Therefore, an increase in their rate by introducing hetero- and isovalent additives that form solid solutions can significantly accelerate the compaction. In this case, heterovalent additives lead to the formation of vacancies that are much higher than their thermodynamic content. The introduction of isovalent additives leads to lattice distortion. Both these additives lead to an acceleration of mass transfer, release and filling of pores. When sintering with such additives, a situation may occur where the removal of pores outstrips the growth of crystallites. In this case, these processes are separated, and the crystallites grow non-porous, which facilitates the synthesis of high-transparency ceramics. Moreover, the introduction of additives changes the conditions for the transition of an atom across the boundary, which can significantly affect the final dimensions of the crystallites.
We have investigated the replacement of the Y3+ cation in Nd3+ : Y2O3 with Lu3+ or Sc3+ ions or the Zr4+ and Hf4+ heterovalent ions, and also the Al4+ cation in garnet ceramics.
The compacts with a diameter of 15–32 mm, a thickness of 0.5–3.5 mm with a relative density of ~0.5 were sintered. The parameters of sintering varied over a wide range: the sintering temperature T = 1550–2050 °C; sintering time ts = 1–30 h; the rate of temperature rise vT = 0.75 and 5.0 K/min. The influence of these factors on the characteristics of high-transparency ceramics will be discussed in the next section.
The main stages and processes taking place in the preparation of high-transparent ceramics, including laser ones. The optimal conditions (pulse duration, power density) at which the productivity of nanopowder production is realized, depending on the thermophysical properties of the material, were found to be 10–80 g/h. It is shown that the nanoparticles obtained are weakly agglomerated, have a spherical shape and an average size of ~10 nm. A distinctive feature of such nanoparticles is the high homogeneity of the composition even at a high level of doping.
It is shown that when compacting nanopowders the density of compacts does not depend on the method of dry pressing and is determined by pressure, although the level of residual mechanical stresses differs. Pressing was carried out at pressures of 250–300 MPa, at which compact densities were ~50%.
After sintering such compacts at optimum temperatures, the samples were suitable for use as active elements of solid-state lasers.
The work was carried out within the framework of the theme of state task No. 0389-2014-003 (2016–2017) and with the support of RFBR project No. 17-08-00064.