From the Laser Plume to the Laser Ceramics

*Vladimir Osipov, Vyacheslav Platonov, Vladislav Shitov and Vladimir Solomonov*

### **Abstract**

The main stages of preparation of ceramic active elements of solid-state lasers are considered. The physical principles of laser synthesis of nanopowders are described. The features and processes taking place during compaction and compacts sintering are specified. Also we report on the investigation of characteristics of highly transparent ceramics on the basis of nanopowders synthesized in laser plume. It is shown that this approach enables to increase the "orange peel" formation threshold in the ceramics with strongly disordered crystalline structure. It opens the road to relatively simple synthesis technology from oxide materials and application of this ceramics as the gain media with oscillation efficiency higher than 50% and also leads to simplification of the synthesis technology of magnetoactive ceramics and to production of highly transparent YAG samples without the use of sintering heterovalent additives.

**Keywords:** laser plume, nanopowder, compact, sintering, highly transparent ceramics, laser ceramic elements

### **1. Introduction**

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 [4–6]. One of the key components of high power 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 pioneering work on synthesis of the laser ceramics and obtaining effective generation [7], a large amount of research was carried out in this direction. The requirements [8] are specified to achieve high-efficiency laser generation in ceramics: the thickness of the grain boundaries is of the order of 1 nm, the scattering loss per pass is less than 0.05–0.1% cm−1 (residual porosity at the level of 10−4 vol.%), optical uniformity with wavefront distortion of the λ/19.5. Using

yttrium-aluminum garnet-based ceramics (Y3Al5O12, YAG) with similar characteristics in the geometry of a thin disk (active medium Ø11 × 0.15 mm), an output power of 1.8 kW with a slope efficiency of 74.1% was implemented [9]. Moreover, a record output power of 6.5 kW with a slope efficiency of 57% was achieved in [10]. In a ceramic disc 8.5% Yb:LuAG with a thickness of 0.15 mm, an output power of 1.74 kW with a slope efficiency of 71.2% was demonstrated [11]. 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 × 30 × 3 mm3 , the power of continuous laser generation was 2.44 kW [12], and with increasing dimensions up to 120 × 50 × 3 mm3 —4.35 kW [13]. The cascade of several Nd:YAG ceramic elements sized 100 × 100 × 20 mm3 allowed this value to increase to 67 kW [14], and further to 105.5 kW [15].

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 [16].

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 [17] and 152 fs [18] 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 duration of 53 fs was demonstrated [19].

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 [20]. To meet these requirements, synthesis techniques were developed based on spark plasma sintering [21–23], hot isostatic pressing [24–26], and vacuum sintering with doping of heterovalent ions [27]. The latter option is more attractive due to the less expensive and uncomplicated technology. However, this approach, with a significant content of additives (more than 1 mol.%), is fraught with a significant disadvantage due to the release of heterovalent ions during sintering into the regions adjacent to the grain boundaries. In the synthesis of oxide ceramics, the possibilities of this approach can be expanded by using nanopowders obtained by laser evaporation, where the synthesis of nanoparticles proceeds at high temperatures and rapid cooling. This will ensure high uniformity of nanoparticles and ceramics based on them.

## **2. Preparing nanopowders**

There are many methods for preparing nanopowders: mechanical crushing, precipitation from solutions, sol-gel, self-propagating high-temperature synthesis, physical vapor deposition. However, nanopowders prepared by the method of laser evaporation of a solid target in a gas atmosphere 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 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 plume at high temperature and rapid cooling. This prevents segregation of the dopants and ensures

### *From the Laser Plume to the Laser Ceramics DOI: http://dx.doi.org/10.5772/intechopen.94464*

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.

For the synthesis of oxide nanopowders by this method, a CO2 laser (λ = 10.6 μm) and a 600 W fiber ytterbium laser (λ = 1.06 μm) were used. The average output power of the CO2 laser was 550 W at a repetition rate of 650 Hz pulses with an energy of W = 1.4 J, a peak power of about 9 kW, and a duration of 500 μs at a power level of 0.1.

**Figure 1a** shows a block diagram of the laser complex for preparing nanopowders [28, 29]. Laser radiation was focused on the target with a lens, which also served as the entrance window of the evaporation chamber. The target was made from oxide micro-powder (or a mixture of them) by pressing and sintering it. As a result of the action of laser radiation, a laser plume 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. **Figure 1b** (upper) 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 (lower). 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.

The nanoparticles are formed in a laser plume. A laser plume is a flow of incandescent vapors of a solid target in the form of a weakly ionized plasma from the region of incidence of the laser beam on the target [30, 31]. In visible light, the plume is typically in the shape of a needle directed normal to the target surface, regardless of the angle of incidence of the laser beam (**Figure 2**). This tip is surrounded by a vortex structure, which is clearly manifested in shadow photography [32].

When exposed to single pulse or pulse-periodic laser radiation, the plume appears after the delay time td. During this time, the target substance is heated to

### **Figure 1.**

*Preparing a nanopowder: (a) block diagram of the laser complex for preparing a nanopowder, (b) image of laser target after exposure to radiation CO2 laser (top), ytterbium fiber laser (bottom) [29].*

**Figure 2.**

*Scanning of photographs (exposure 1* μ*s) of a laser plume (CO2 laser pulse duration 200* μ*s, incident angle of 45°). Top row: visible light photography, bottom row: shadow photography. The captions below them indicate the shooting delay time relative to the start of the plume initiation (t) and the peak laser power (P) at the time of shooting [30].*

the evaporation temperature in the area of the laser beam incidence. For a linear leading edge of a laser pulse, the delay time is defined as td = 2Wd/Pd. Here, Wd is the energy required for preheating the target substance to the evaporation temperature, and Pd is the instantaneous power of laser radiation at the moment of the flare appearance. For different substances, due to the difference in Wd, the delay times of td differ. After the appearance of the glow (l = 0), the height of the plume (l) increases at a rate proportional to the square root of the peak power of the laser pulse. The maximum height of the plume (lm) is reached at the moment of the maximum laser pulse. The diameter of the luminous zone of the plume is typically 0.5–1.0 mm, which approximately corresponds to the size of the laser spot on the target surface.

Over the entire length of the plume, its emission spectrum is represented by dominated structured molecular bands of radicals of cations of the target substance [31] against the background of a continuous band of recombination radiation (**Figure 3**). In this case, the short-wave part of the spectrum is well approximated by the Wien's curve, which makes it possible to determine the temperature of

### **Figure 3.**

*The spectrum of the plume glow from a CO2 laser at different distances (l) from the target of yttriumstabilized zirconium oxide (solid curve), recombination radiation (bold curve), and approximation of the Wien's curve (dashed line) [31].*

### *From the Laser Plume to the Laser Ceramics DOI: http://dx.doi.org/10.5772/intechopen.94464*

the luminous gas in the flare. Thus, when irradiated with a pulsed CO2 laser, the maximum temperature close to the boiling point of the target material is reached at the target surface, and the flame temperature decreases nonmonotonically as it moves away from the target. When irradiated with pulses of a fiber ytterbium laser (1.07 μm), the temperature of the plume near the target slightly exceeds the melting point of the substance.

The transverse dimensions of the crater that appears on the target after exposure to a laser pulse almost coincide with the size of the laser spot on the target, and its depth depends on the wavelength of the laser radiation. For example, at the same pulse energy (1.0–1.4 J) after CO2 laser irradiation, the crater depth is 5–10 μm, almost independently of the target substance. After a fiber laser pulse, the crater depth is 6–8 times greater, and with repeated exposure, the target surface becomes needle-like. These features are due to different mechanisms of absorption of radiation by the target of these. Thus, the frequency of a photon at the wavelength of a CO2 laser is comparable to the frequency of optical phonons of oxide crystals. Therefore, in this case, such materials are almost opaque and the depth of penetration of laser radiation into them is only a few micrometers. For fiber ytterbium laser radiation, oxides we used are transparent – absorption is possible only on crystal and mechanical defects of the target. If these materials are single crystals with a minimum content of defects, the characteristic depth of penetration of laser radiation into them is tens of centimeters. This corresponds to the absorption index α~10−2–10−3cm−1. If such defects are located inside the target in the area of the laser beam incidence, the initial heating also occurs inside the target (in the area of these defects). Then, due to the strong temperature dependence of the absorption coefficient, a heat wave is formed [33], which moves along the laser beam from the defect to the target surface, upon reaching which a laser plume is formed. This process is compounded by the fact that after repeated exposure, the surface of the initial target is covered with a layer of transparent melt 100–400 μm thick, in which the defect concentration is much lower than in the initial target made of sintered micro-powder.

This model is confirmed by the fact that the delay time for the appearance of a laser flare from the beginning of the laser pulse exposure has a large spread and on average increases with increasing transparency of the target. In particular, the delay in the appearance of a laser plume on the surface of a semitransparent Nd:Y2O3 ceramic with α = 23 cm−1 (an analog of the fused layer) averaged over several radiation pulses was 5–10 times greater than for the original sintered micro-powder target (α = 1.7 × 103 cm−1) at the same radiation intensity I = 0.4 MW/cm2 [31]. The spread of the delay in the formation of a laser plume during evaporation of the same target is due to the stochastic nature of the depth of defects from the target surface at different points.

When nanopowders are obtained using both lasers, in addition to nanoparticles, spherical particles with sizes from 0.5 to 150 μm are also formed [29, 33], as well as shapeless target fragments of the same size. Fragments are formed after the end of radiation exposure to a specific part of the target due to thermal splitting of the cooling fused layer [30]. Spherical particles are liquid droplets of the melt, is sprayed by the vapor pressure of the laser crater.

Especially many drops are formed when the target is vaporized by continuous ytterbium laser radiation. At the same average radiation power (600 W) and the same intensity on the target as for the CO2 laser (≈1.3 MW/cm2 ), the production capacity of the Nd:Y2O3 nanopowder decreased to 15 g/h, and its output during evaporation of one target to 9 wt.% [34]. High-speed shooting of the laser plume showed that this effect is due to the transition from steam to vapor-drop ablation. The latter becomes dominant ~500 μm after the start of the laser pulse. A similar

pattern is observed in the evaporation of targets from YSZ and FeMgAl2O4. Theoretical analysis [33] allowed us to establish that one of the reasons for the appearance of drops in the laser flare is related to the presence of melt in the crater and 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. This analysis made it possible to establish the characteristic size of the instability:

$$
\lambda\_{\epsilon} = \frac{2\pi\sigma}{\rho\_2 V^2} = \left(20 \div 90\right) \cdot 10^{-6}\,\mathrm{m} \tag{1}
$$

and its development the increment

$$
\tau = \frac{3\sqrt{3}\pi\sigma}{\rho\_2 V^3} \sqrt{\frac{\rho\_2}{\rho\_1}} \approx 100 \text{ } \mu\text{s},\tag{2}
$$

where *ρ*1 and *ρ*2 are the melt and vapor densities, σ is the surface tension coefficient, *V* is the vapor flow rate.

Optimizing the duration (<00 μs) and radiation density, separation and trapping, it was possible to prepare high-quality nanopowders. **Figure 4** shows an example of a photo of YSZ nanopowder, and the distribution of particles of different composition in size is given as an example. Depending on the thermophysical properties of refractory oxides, the pressure and speed of the carrier gas, the productivity of producing a nanopowder using a CO2 laser with an average radiation power of 600 W varies from 10 to 80 g/hour.

The distinguishing feature of nanoparticles synthesized in a laser plume, 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 **Figure 5**. 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.

### **Figure 4.**

*A typical photo of YSZ nanoparticles (a) and the size distribution of nanoparticles of different compositions (b) [29].*

**Figure 5.**

*Results of mapping the elemental composition of Yb:Lu2O3 nanoparticles with the use of SEM and X-ray SMA.*

### **Figure 6.**

*Results of X-ray diffraction analysis of ceramics and nanopowders with different concentrations of HfO2 [35].*

This finding is supported by the results of X-ray diffraction analysis of Nd:Y2O3 nanopowders and ceramics doped with HfO2 (**Figure 6**). 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.

A feature of the above method for producing nanopowders is that they crystallize in a laser plume, as a rule, in metastable phases. For example, yttrium oxide nanopowders crystallize in the monoclinic phase, while alumina nanopowders in the γ-phase. This effect is associated with very rapid cooling and quenching (within ≈1 ms) of the resulting nanoparticles during vortex mixing of the laser plume with air and, possibly, with the resulting oxygen deficiency in nanoparticles formed from trivalent cation radicals.
