2.2. Piezoceramic Material Development and Investigation

PZT ceramics is a unique ferroelectric system, where “smeared” phase transitions can be realized in order to increase the morphotropic region [67]. Such systems are called ferroelectric relaxors [68]. By doping the PZT system with combined additives of lead magnoniobate, which in turn is a prominent representative of ferroelectric relaxors [69], as well as barium–strontium titanate, it was possible to obtain a new material: PKP-12 with a “smeared” phase transition. It has been established that the unique properties of relaxors are due to the formation and growth of nanoregions (nanodomains) in a crystal due to disorder in the environment of different ions located in crystallographically equivalent positions [70]. From the point of view of the use of these materials in the creation of multilayer actuators for wavefront correctors of the piezoactuator type, they make it possible to accurately control some properties of piezoceramics. For example, the value of the longitudinal piezoceramic coefficient d₃₃ can be increased, which will lead to an increase in the displacement amplitude [65], however, this results in a decrease in the value of the Curie temperature.

The manufacturing of piezomaterials could be exploited two widely known ways: conventional [71] and colloidal-based techniques [72]. The main goal of both techniques is to obtain a homogeneous mixture of initial components—oxides or metals (Pb, Zr, Ti, etc.)

The main steps of these processes are shown in Figure 2 and it follows:

Scheme of piezoceramic material manufacturing process.

After obtaining a homogeneous mechanical mixture, the synthesis stage occurs. Synthesis with a method using solid-state reactions is uniform heating (with a constant rate of temperature rise) to a target temperature of 800–900 degrees, at which the synthesis reaction of a compound with necessary composition occurs.

The synthesized compound is re-grinded in a drum or planetary mill, dried to a residual moisture content of no more than 0.2 percent, after which the powder is ready for formation.

Formation is carried out by uniaxial isostatic hot pressing (with preliminary plasticization in a spray dryer, or the addition of a plasticizer). Such method of manufacturing of the piezomaterial allows for obtaining the samples with high density and reduced quantity of the inner hollows that decrease the probability of the electrical breakdown and increase the local deformation of the piezoelements [73].

To determine the main parameters of the material, a number of samples were made using conventional ceramic technology. Silver electrodes were deposited on the surface of all developed samples, except for elements with longitudinal polarization, by firing a silver-containing paste. When applying an electric field of 8 kV/cm for 10 min at a temperature of 120 °C, followed by direct natural cooling to room temperature, the samples were polarized in air.

To obtain samples with longitudinal polarization, the sintered block was divided into separate elements with electrodes deposited by chemical deposition [74].

Using dynamic measurement methods, the main electromechanical properties of the material were studied: relative permittivity, dielectric loss tangent tgδ, electromechanical coupling coefficients, piezoelectric modules, ultrasonic waves velocities, mechanical quality factor Q_m, Poisson’s ratio δp, and density ρ.

To obtain the electrical capacitance, resonance frequency, and antiresonance of the polarized piezoceramic samples, a Wayne Kerr Electronics WK 6510B precision impedance meter was used.

Figure 3 shows the temperature–frequency dependences of the relative permittivity (ε′ = ε⁄ε₀) where ε is the permittivity of the material, ε₀ is the dielectric constant, and tgδ is the dielectric loss tangent of unpolarized ceramics at frequencies of 0.1, 1, 10, and 100 kHz.

Dependence of the dielectric loss tangent tgδ (T) (a) and the relative permittivity ε′ (T) (b), and of the unpolarized composition under study from the temperature and frequency.

In the frequency range under consideration, a local temperature shift is clearly observed, where a maximum of the permittivity (T_m) is observed as the frequency changes (Figure 3a), which is explicitly characteristic of relaxor ferroelectrics. In addition, the appearance of the dispression effect is observed (situation when curves start to split at the various frequencies) at a temperature significantly below T_m. The maximum dispression of the permittivity is observed at a temperature of about 130 °C. It should also be noted that the maximum of the dielectric loss tangent tgδ (T) has an asymmetric form at different temperatures (Figure 3b), which is also a distinctive feature of ferroelectric materials with a diffuse phase transition [75].

In our case, the possibility of the existence of a relaxor state in the material under study is also confirmed by the behavior of the elastic properties (Figure 4). So, from the temperature dependences of the piezomodulus d₃₁ (T) and the velocity of the longitudinal sound wave V^E₁ (T) in a polarized sample, it follows that the anomalies d₃₁ (T) in the form of a plateau-like maximum and V^E₁ (T) in the form of a very smeared minima are located significantly below T_m. In this case, a sharp drop in the values of d₃₁ (T) at T = 120 °C and a jump in V^E₁ (T) at T~110 °C are associated with the onset of depolarization of the sample.

Temperature dependence of the longitudinal ultrasonic waves velocity V^E₁ (T) (a) and piezomodulus d₃₁ (T) (b) of the developed material.

Taking into account that the temperature of the velocity minimum both in classical ferroelectrics and in relaxor ferroelectrics [76] corresponds to a phase transition, the temperature range from 20 to 150 °C can be the region of phase coexistence. In the material under study, this can first be the region of coexistence of the low-temperature rhombohedral phase R3c and the high-temperature rhombohedral phase R3m, and then the region of coexistence of the rhombohedral and tetragonal phases, and in the T_m region of the tetragonal and cubic phases.

The behavior of the polarization of the resulting material was also studied depending on the applied external field. Figure 5 shows the dielectric hysteresis loop of the material under study, from which the value of the coercive field E_C ≈ 4 kV/cm was determined. The hysteresis loop is saturated and close in shape to the loop characteristic of ferrosoft ceramics.

Hysteresis loop of the developed material.

The values of elastic compliance coefficients were also obtained to be used for the numerical modeling of piezoelements created on the basis of this material. Elastic compliance determines the amount of deformation that occurs under the influence of applied mechanical stress (m²/N).

Here, SijE is the elastic compliance for stress in direction i (perpendicular to the direction in which the ceramic element is polarized) and accompanying strain in direction j, under a constant electric field (short circuit), and SijD is the elastic compliance for stress in direction i (parallel to direction in which the ceramic element is polarized) and accompanying strain in direction j, under constant electric displacement (open circuit).

After that, the electrophysical characteristics of this composition were measured under standard conditions (T = 20 ± 5 °C). The measured data are presented in Table 1. Based on the data obtained, it can be established that the material under study has high values of dielectric permittivity, piezoelectric coefficients, and electromechanical coupling coefficients, which ultimately allow for the use of this material in the manufacturing of the devices for the tasks of actuator technology. Moreover, in comparison with piezomaterials from other manufacturers, this ceramic has a high value of the piezoelectric coefficient d₃₃ and, at the same time, a low value of the dielectric loss tangent. A comparative analysis of the key parameters of widely known piezoelectric materials is shown in Table 2.

Electrophysical characteristics of the developed material.

Electrophysical characteristics of the developed material in comparison with piezoceramics available in the market.