**3. Compacting, annealing and sintering 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 treatment on nanoparticles, magnetic pulse compacting.

In [36], the transparency of laser ceramics was investigated using compacts prepared by slip casting and dry pressing. It was shown that ceramics samples, whose

### *Advanced Ceramic Materials*

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 ultrasonic treatment (UST), magnetic-pulse pressing and cold isostatic pressing. All of them showed rather close relative densities of compacts at the same pressures, which is confirmed by the results presented in [36, 37]. Nominally pure and neodymium-activated yttrium oxide nanopowders, designated by us as Y2O3, 8 NDY, 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 dry nanopowders (without plasticizers) of all these types were pressed as uniaxial static pressing (without UST), 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 2–4 mm. The experimental results in the form of the dependence of the relative density on the compacting pressure at a constant power of UST 3 kW and 0 kW (i.e. without UST) are shown in **Figure 7**.

According to the technique described in [37], 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:

$$b \not\supset \rho\_{\text{thor}}\left(P\right) = b \cdot Ln\left(\frac{P}{P\_{cr}}\right) + \mathbf{1} \tag{3}$$

where *ρ* is the density of the compact, *ρ*theor 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 UST and is determined mainly by the compacting pressure, thereby confirming the findings obtained using other methods.

### **Figure 7.**

*Curves of nanopowder compaction: (a) 8NDY, 3NDY, 1NDY, Y2O3 with UST, W = 3 kW; (b) 8NDY, Y2O3 with UST W = 3 kW and without UST (W = 0 kW) [37].*

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

The effect of nanoparticle size on compacts density is discussed in [38] 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 [38] sought to take into account the most important of these reasons. Their calculations of the dependence of the density of compacts on the axial pressure are shown in **Figure 8**.

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 nanopowders, produced by laser evaporation, - greater solubility of ingredients in each other and particle uniformity, but poorer compressibility. This question remains open in relation to the synthesis of laser ceramics up to this point. Further, the results obtained using a nanopowder, obtained by laser evaporation of a solid target, 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. The prepared compacts with a relative density of 0.46–0.58 are usually air calcined to remove organic matter and to provide additional oxidation and phase transformations.

**Figure 9** 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.

### **Figure 8.**

*Axial pressure as a function of the compact density for systems with a particle size d = 10 nm (1), 30 nm (2), 100 nm (3) and a system without Van der Waals forces (4). Solid lines are isotropic initial configurations; dashed lines are anisotropic configurations [38].*

**Figure 9.**

*Dependence of the grain size, mechanical stresses and densities of compacts on the calcination temperature of compacts from the nanopowder of the monoclinic phase [35].*

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 can be conditionally divided into three stages. The dependencies shown in **Figure 9**, characterize the processes in two of the three stages of sintering. In stage I (700–1200 °C), there is no shrinkage of the compact, 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 1200°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 grain boundaries, then the shrinkage process occurs quite intensively.

When the compacts are compacted, the diffusion processes are decisive. Therefore, an increase in these rates 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 in the unalloyed matrix. 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 isovalent ions Lu3+ or Sc3+ ions or the Zr4+ and Hf4+ heterovalent ions, and also the Al4+ cations by Ce3+ in garnet ceramics. The compacts with a diameter of 15–32 mm, a thickness

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

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.
