Phase transition,microstructure and properties of (Na<Subscript>0.5</Subscript>Bi<Subscript>0.5</Subscript>)<Subscript>1-x</Subscript>Ba<Subscript>x</Subscript>TiO<Subscript>3</Subscript> lead-free piezoelectric ceramics

In this article, (Na_0.5Bi_0.5)_1-xBa_xTiO₃ lead-free piezoelectric ceramics were prepared by solid-state reaction. The influence of Ba contents on phase structures, compositional distribution and electrical properties of (Na_0.5Bi_0.5)_1-xBa_xTiO₃ ceramics were systematically investigated to further understand the nature of phase transition. It was found that the phase structure of (Na_0.5Bi_0.5)_1-xBa_xTiO₃ transforms from rhombohedral to tetragonal symmetry at x = 0.06 ~ 0.07 and Ba²⁺ segregation forms the coexistence of Ba-rich tetragonal and Ba-deficient rhombohedral phases close to MPB. The electrical properties of prepared samples regularly changed with Ba content, which is closely related to the distribution of rhombohedral and tetragonal phases. The prepared sample near MPB exhibited the largest dielectric constant and the excellent piezoelectric properties (the maximal measuring field reached 78 kV/cm and the piezoelectric constant d ₃₃ = 151pC/N).     

Effects of co-doped CaO/MnO on the piezoelectric/dielectric properties and phase transition of lead-Free (Bi0.5Na0.5)0.94Ba0.06TiO3 piezoelectric ceramics

The piezoelectric properties of (1?x)(Bi_0.5Na_0.5)TiO₃-xBaTiO₃ ceramics were reported and their piezoelectric properties reach extreme values near the MPB (about x?=?0.06). The X-ray analysis of (Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ ceramics for all compositions exhibited a pure perovskite structure without any secondary phase. Within a certain ratio of contents, the co-doped ceramics enhanced piezoelectric coefficient (d ₃₃ ), lowered the dielectric loss, and increased the sintered density. The temperature dependence of relative dielectric permittivity (K ₃₃ ^T ) reveals that the solid solutions experience two phase transitions, ferroelectric to anti-ferroelectric and anti-ferroelectric to relaxor ferroelectric, which can be proven by P-E hysteresis loops at different temperatures. In addition, the specimen containing 0.04/0.01 wt.% CaO/MnO showed that the coercive field E _c was a minimum value of 26.7 kV/cm, while the remnant polarization P _r was a maximum value of 38.7 μC/cm², corresponding to the enhancement of piezoelectric constant d₃₃ of 179 pC/N, electromechanical coupling factor k _p of 37.3%, and relative dielectric permittivity K ₃₃ ^T of 1137. (Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ ceramics co-doped with CaO/MnO were considered to be a new and promising candidate for lead-free piezoelectric ceramics owing to their excellent piezoelectric/dielectric properties, which are superior to an un-doped BNBT system.     

Studies on new systems of BNT-based lead-free piezoelectric ceramics

Several new systems of Bi_0.5Na_0.5TiO₃-based lead-free piezoelectric ceramics were proposed based on the design of the multiple complex in the A-site of ABO₃ compounds. These ceramics were prepared by conventional ceramic techniques. The comparison of the piezo- and ferroelectric properties of these ceramics with those of the best properties of the Bi_0.5Na_0.5TiO₃-based lead-free piezoelectric ceramics published recently shows that these ceramics of the new systems have better ferroelectric and piezoelectric performance, and better temperature characteristic of the properties. Among these materials, Bi_0.5(Na_1?x?y K _x Li _y )_0.5TiO₃ possesses higher piezoelectric constant (d ₃₃?=?230.8 pC/N), higher electromechanical couple factor (k _p?=?0.41), larger remanent polarization (P _r?=?40 μC/cm²) and a better P–E hysteresis loop below 200 °C. Practical devices such as ceramic middle frequency filters and buzzers have been made by using these lead-free piezoelectric ceramics.     

Structure and electrical properties of (Na0.5Bi0.5)1−x Ba x TiO3 ceramics made by a citrate method

(Na_0.5Bi_0.5)_1?x Ba _x TiO₃ (x?=?0–0.12) powders were synthesized by a citrate method, and the structure and electrical properties of the resulting ceramics were investigated. A gradual change of crystal structure with the increase of BaTiO₃ concentration was detected together with a significant evolution in grain size and shape. A rhombohedral-tetragonal morphotropic phase boundary (MPB) near x?=?0.06 at room temperature was ascertained for the ceramics. The dielectric constant (? _r) and dissipation factor (tanδ) attain maximum values at x?=?0.08 and x?=?0.06, respectively. The specimen of x?=?0.06 provides the maximum piezoelectric constant (d ₃₃?=?180 pC/N) and electromechanical coupling factor (k _p?=?0.28), accompanied by a large remanent polarization of P _r?=?37.1 μC/cm² and a low coercive field of E _c ?=?42.7 kV/cm.     

Lead-free piezoelectric ceramics of (Bi1/2Na1/2)TiO3–(Bi1/2K1/2)TiO3–(Bi1/2Ag1/2)TiO3 system

The ternary lead-free piezoelectric ceramics system of 0.90[x(Bi_1/2Na_1/2)TiO₃–(1???x)(Bi_1/2K_1/2)TiO₃]–0.10(Bi_1/2Ag_1/2)TiO₃ (x?=?0.77, 0.79, 0.81, 0.83, 0.85) were successfully synthesized by conventional ceramic sintering technique. The samples were studied by X-ray diffraction, dielectric, ferroelectric and piezoelectric measurements. The MPB composition of the system appears to be near BNKA-79 according to the results of the X-ray diffraction and ferroelectric properties. The sample with x?=?0.79 showed the highest piezoelectric constant d ₃₃?=?160 pC/N. The maximum electromechanical coupling factors, k _p and k _t, are 0.30 for BNKA-79 and 0.42 for BNKA-85, respectively.     

Effect of BiAlO3 concentration on the dielectric and piezoelectric properties of lead-free (Bi0.5Na0.5)0.94Ba0.06TiO3 piezoelectric ceramics

In this paper, lead-free (1-x)(Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃-xBiAlO₃ (BNBT-BA, x?=?0, 0.010, 0.015, 0.020, 0.025, and 0.030) piezoelectric ceramics were synthesized using a conventional solid-state reaction method. The effect of BiAlO₃ concentration on dielectric, ferroelectric and piezoelectric properties were investigated. The ferroelectric and piezoelectric properties of BNBT ceramics are significantly influenced by the presence of BA. In the composition range studied, X-ray diffraction revealed a perovskite phase with the coexistence of rhombohedral and tetragonal phases. The temperature dependence of dielectric properties showed that the depolarization temperature (T _d) shifted towards lower temperatures and that the degree of diffuseness of the phase transition around T _d and T _m became more obvious with increasing BiAlO₃ content. The remanent polarization increased with increasing BA, and reached a maximum value of 30 μC/cm² at x?=?0.020. As a result, at x?=?0.020, the piezoelectric constant (d ₃₃) and the electromechanical coupling factor (k _p) of the ceramics attained maximum values of 188 pC/N and 34.4 %, respectively. These results indicate that BNBT-BA ceramics is a promising candidate for lead-free piezoelectric materials.     

Synthesis and piezoelectric properties of (1 − x)(Na0.5K0.5)NbO3–x(Ba0.95Sr0.05)TiO3 ceramics

(1 ? x)(Na_0.5K_0.5)NbO₃–x(Ba_0.95Sr_0.05)TiO₃ [(1 ? x)NKN–xBST] ceramics were synthesized by the conventional solid-state sintering, and their microstructure and piezoelectric properties were investigated. The sintering temperature of the specimens was 1075 °C in air atmosphere and a morphotropic phase boundary (MPB) was observed in the specimens with 0.03 ≤ x ≤ 0.05. Compared with the piezoelectric properties of the NKN ceramics, the enhanced d ₃₃ value of 136 pC/N and ? ₃ ^T /? _o value of 671 were obtained for the (1 ? x)NKN–xBST specimens with x = 0.03.     

Nonstoichiometric (K,Na)NbO3 ceramics: Densification and electrical properties

In order to clarify the influence of excess ions in A or B sites on perovskite (K,Na)NbO₃ ceramics, various compositions of (K_0.48Na_0.52)Nb_1+x%O₃ (ABO₃) ceramics where x is in the range of ±1 % were prepared by conventional solid state method and their densification, structure, dielectric and piezoelectric properties were investigated. Results showed that a small amount of excess A-site ions could compensate for the deficiency of K and Na ions in A-sites caused by volatilization resulting in good piezoelectric properties. The ceramics with x?=??0.1 exhibited optimum piezoelectric properties with d ₃₃?=?127pC/N and k _p?=?0.41. However, presence of too much alkali elements (x?<??0.5) led to deterioration of density, dielectric and piezoelectric properties, although the crystal structure was not changed. The electrical properties, on the other hand, were not sensitive to the B-site excess ions. These results are expected to be very useful for further designing of (K,Na)NbO₃-based ceramics as lead-free alternatives to piezoelectric materials.     

Study of the electrical properties of Pb(Zn,Ni)1/3Nb2/3O3–PbTiO3 ceramics across the morphotropic phase boundary

Structure and electrical properties of (1???x)Pb(Zn_0.2Ni_0.8)_1/3Nb_2/3O₃–xPbTiO₃ ceramics with x?=?0.24–0.38 were examined in detail. X-ray diffraction measurements reveal that all samples are in pure perovskite phase, and most of them lie within the morphotropic phase boundary (MPB) region. The dielectric behaviors of all compositions are characterized with diffuse phase transition and frequency dispersion. The variable power law and the Vogel–Fulcher relation have been used to describe such dielectric behaviors. The highest dielectric constant and the largest piezoelectric coefficient are simultaneously observed when x?=?0.30. The variation of the ferroelectric property with PT content is also discussed.     

Enhanced piezoelectric properties of 0.55Pb(Ni1/3Nb2/3)O3-0.135PbZrO3- 0.315PbTiO3 ternary ceramics by optimizing sintering temperature

0.55Pb(Ni_1/3Nb_2/3)O₃-0.135PbZrO₃-0.315PbTiO₃ (PNN-PZ-PT) ternary piezoelectric ceramics with excess 1.0 wt.% PbO were synthesized by the conventional solid-state reaction method at 1175–1300 °C for 2 h, respectively. The influence of sintering temperature (T _s) on microstructure, piezoelectric, dielectric, and ferroelectric properties were systematically investigated. The results of XRD and Raman scattering spectra demonstrated that a typical perovskite structure with mainly rhombohedral symmetry near the MPB region were obtained for all the samples. The tetragonal phase content was increased slightly with the increase of sintering temperature. In addition, with increasing T _s the average grain size increases while the density decreases were also found. The results of electrical measurements confirmed that piezoelectric constant, dielectric constant, remnant polarization were firstly increased and then decreased with the increase of sintering temperature. The optimum and remarkable enhanced electrical properties of d ₃₃?=?1070 pC/N, k _p?=?0.69, ε _r?=?8710, tanδ?=?0.026, P _r?=?24.08 μC/cm², and E _c?=?3.2 kV/cm were obtained for the sample sintered at 1250 °C for 2 h. Meanwhile, the sample exhibits a typical relaxor ferroelectric behavior with the maximum dielectric constant ε _m =24541 at Curie temperature T _c?=?113.3 °C at 1 kHz.     

Synthesis and piezoelectric properties of [Bi1-z(Na1-x-y-zKxLiy)]0.5BazTiO3 lead-free piezoelectric ceramics

[Bi_1-z(Na_1-x-y-zK_xLi_y)]_0.5Ba_zTiO₃ lead-free piezoelectric ceramics were fabricated by ordinary ceramic technique and the piezoelectric and ferroelectric properties of the ceramics were studied. The ceramics can be well sintered at 1,100–1,150 °C for 2 h. X-ray diffraction (XRD) analysis shows that K⁺, Li⁺ and Ba²⁺ diffuse into the Bi_0.5Na_0.5TiO₃ lattices to form a solid solution with a single-phase perovskite structure. The introduction of K⁺, Li⁺ and Ba²⁺ into Bi_0.5Na_0.5TiO₃ significantly decreases the coercive field E _c but maintains the large remanent polarization P _r of the materials. The ceramics provide piezoelectric constant d ₃₃ of 205 pC/N, electromechanical coupling factor k _p of 0.346, remanent polarization P _r of 31.7–38.5 μC/cm², and coercive field E _c of 3.18–5.16 kV/mm.     

Morphotropic phase boundary and electrical properties of lead-free bismuth sodium lanthanum titanate—barium titanate ceramics

The important properties of lead-free piezoelectric ceramics have been investigated from Bismuth Sodium Lanthanum Titanate and Barium Titanate system: (1 − y)(Bi_0.5Na_0.5)_{(1 − 1.5x)}La _x TiO₃(BNLT)—yBaTiO₃(BT) where x = 0.017 and y = 0 − 0.2, respectively. The morphotropic phase boundary (MPB) was found to be around y = 0.1 by the x-ray diffraction and dielectric measurement at various amount of BT. The temperature dependence of dielectric constant (ε _r ) at various value of y showed the diffuse phase transition exhibiting the relaxor type ferroelectrics. The degree of diffuseness increased at a high doping content of about y = 0.15 where the second phase transition (T₂) of the ferroelectric to antiferroelectric phase disappeared. Moreover, this sample had the maximum piezoelectric coefficient (d ₃₃) of about 112 pC/N with relatively low dielectric constant. The optimum sintering temperatures and the microstructures of the dense BNLT-BT ceramics were also examined.     

Synthesis and characterizations of the (1 − x)Pb(Sc1/2Nb1/2)O3–xPbTiO3 solid solution ceramics

We report a systematic study of the (1???x)Pb(Sc_1/2Nb_1/2)O₃–xPbTiO₃ (PSN–PT) solid solution in the form of ceramics with compositions at or near the morphotropic phase boundary (MPB) region (0.35?≤?x?≤?0.50). The PSN–PT ceramics have been synthesized by an improved two-step wolframite precursor method. The synthetic process has been optimized in terms of calcining and sintering conditions. Both dielectric permittivity measurements and differential scanning calorimetry (DSC) show a clear peak at T _C, at which the transition from the paraelectric to ferroelectric phase takes place. Interestingly, the solid solution of the MPB compositions displays a T _C?>?200 °C, i.e. higher than the T _C of the Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ and Pb(Zn_1/3Nb_2/3)O₃–PbTiO₃ solid solutions, making the PSN–PT system very promising piezoelectric and ferroelectric materials for high-temperature applications. A dielectric maximum as high as 50,000 is obtained for the 0.65PSN–0.35PT ceramic with losses smaller than 0.05. The values of the remnant polarization and the strain level of the PSN–PT ceramics are comparable to those of the PZT ceramics.     

Preparation,characterisation, and electrical properties of (K0.5Na0.5)NbO3 lead-free piezoelectric ceramics

Na_0.5?K_0.5NbO₃ (KNN) ceramics were sintered at different temperatures (970 °C, 1000 °C, 1030 °C, 1060 °C, and 1090 °C) for 3 h by a pressureless sintering method. The powders had been synthesised by sol–gel method, using citric acid as a coordination agent and ethylene glycol as an esterifying agent. The effects of temperature on the phase, microstructure, dielectric, ferroelectric, and piezoelectric properties of the as-prepared ceramics were analysed. The results revealed that all of the ceramics had a pure perovskite phase with orthorhombic symmetry. The piezoelectric constant (d ₃₃), the relative dielectric constant (ε _r), the planar electromechanical coupling coefficient (K _p), and the remnant polarization (P _r) initially increased and then decreased with increasing of temperature in such KNN ceramics. The volatilization of sodium and potassium increased with increasing sintering temperature. Over the range of temperatures studied, those ceramics sintered at 1060 °C had the following optimal properties: (ρ?=?3.97 g/cm³, d ₃₃?=?119 pC/N, ε _r?=?362.46, tan δ?=?0.05, K _p?=?0.23, P _r?=?11.97 μC/cm², E _c?=?10.35 kV/cm, and T _c?=?408 °C).     

Excellent dielectric constants observed in heterogeneous conduction Ba(Zr<Subscript>0.25</Subscript>Ti<Subscript>0.75</Subscript>)O<Subscript>3</Subscript> ceramics doped with Sr(Fe<Subscript>0.5</Subscript>Nb<Subscript>0.5</Subscript>)O<Subscript>3</Subscript>

The (1-x)Ba(Zr_0.25Ti_0.75)O₃-xSr(Fe_0.5Nb_0.5)O₃ or (1-x)BZT-xSFN ceramics have been fabricated via a solid-state reaction technique. All ceramics exhibit a pure phase perovskite with cubic symmetry. The addition of a small amount of SFN (x?=?0.1) produces an obvious change in dielectric behavior. Very high dielectric constants (ε_r?>?164,000 at 1 kHz and temperature?>?150°C) are observed and the value is obviously higher than dielectric constants for Ba(Zr_0.25Ti_0.75)O₃ and Sr(Fe_0.5Nb_0.5)O₃ ceramics. The ferroelectric measurement data suggests that the unmodified sample exhibited a ferroelectric behavior. However, a transformation from a ferroelectric to a relaxor-like behavior is noted with increasing x concentration. Impedance Spectroscopy (IS) analysis indicates that the presence of excellent dielectric constants is due to the heterogeneous conduction in the ceramics after adding SFN, which can be explained in terms of the Maxwell-Wagner polarization mechanism.     

Dielectric properties and morphotropic phase boundaries in the xPb(Zn1/3Nb2/3)O3−(1−x)Pb(Zr0.5Ti0.5O3 pseudo-binary system

Ceramics in the xPb(Zn_1/3Nb_2/3)O₃−(1−x)Pb(Zr_0.5Ti_0.5)O₃ [xPZN–(1−x)PZT] solid solution system are expected to display excellent dielectric, piezoelectric, and ferroelectric properties in compositions close to the morphotropic phase boundary (MPB). The dielectric behavior of ceramics with x = 0.1−0.6 has been characterized in order to identify the MPB compositions in this system. Combined with X-ray diffraction results, ferroelectric hysteresis measurements, and Raman reflectivity analysis, it was consistently shown that an MPB exists between x = 0.2 and x = 0.3 in this binary system. When x ≤ 0.2, the tetragonal phase dominates at ambient temperatures. In the range of x ≥ 0.3, the rhombohedral phase dominates. For this rhombohedral phase, electrical measurements reveal a profound frequency dispersion in the dielectric response when x ≥ 0.6, suggesting a transition from normal ferroelectric to relaxor ferroelectric between 0.5 ≤ x ≤ 0.6. Excellent piezoelectric properties were found in 0.3PZN–0.7PZT, the composition closest to the MPB with a rhombohedral structure. The results are summarized in a PZN–PZT binary phase diagram.     

Lead-free K<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>NbO<Subscript>3</Subscript>–Bi<Subscript>0.5</Subscript>Li<Subscript>0.5</Subscript>ZrO<Subscript>3</Subscript>–BiAlO<Subscript>3</Subscript> ternary ceramics: Structure and piezoelectric properties

Lead-free perovskite (0.995–x)(K_0.5Na_0.5)NbO₃–x(Bi_0.5Li_0.5)ZrO₃–0.005BiAlO₃ ternary piezoelectric ceramics were projected and prepared by a conventional solid-state method. A research was conducted on the effects of (Bi_0.5Li_0.5)ZrO₃ content on the structure and piezoelectric properties of the ceramics. By combining the X-ray diffraction patterns with the temperature dependence of dielectric properties, a rhombohedral–orthorhombic–tetragonal phase coexistence was identified for the ceramics with 0.02 ≤ x ≤ 0.025, and a rhombohedral–tetragonal phase boundary was determined in the composition x = 0.03. Upon further increasing the (Bi_0.5Li_0.5)ZrO₃ content, the rhombohedral–tetragonal phase boundary transformed to a single rhombohedral structure with x ≥ 0.035. An obviously improved piezoelectric activity was obtained for the ceramics with compositions in and around the rhombohedral–tetragonal phase boundary, among which the composition x = 0.025 exhibited the maximum values of piezoelectric constant d ₃₃, and planar and thickness electromechanical coupling coefficients (k _p and k _t), of 252 pC/N, 0.366, and 0.466, respectively. In addition, the ceramic with x = 0.025 was found to possess a relatively high Curie temperature of 368 °C, suggesting it may have a prospect for applications at elevated ambient temperatures.     

High piezoelectric properties of (Ba,Ca)TiO3-0.04LiF ceramics sintered at a low temperature

Lead-free piezoelectric ceramics are strongly needed to replace the lead-based piezoelectric ceramics with increasing environmental concerns. Barium titanate (BaTiO₃) systems are one of the most promising candidates due to excellent electrical properties. However, the sintering temperature for traditionally sintered BaTiO₃ ceramics are about 1300°C, which restricts the applications of BaTiO₃ ceramics. It is necessary to develop high piezoelectric properties of BaTiO₃ based ceramics which are able to sinter at low temperature. The (Ba_0.94Ca_x)Ti_0.94O_δ-0.04LiF (x?=?0.00?~?0.05 mol) ceramics were synthesized by a conventional sintering method at 1050°C. All the samples show high relative densities over 90%. X-Ray Diffraction pattern indicated that the crystallographic structure of the samples (x?=?0.00 and 0.01 mol) are orthorhombic phase and changes to pseudocubic one with increasing Ca content to x?=?0.03 mol. Two-phases with orthorhombic and pseudocubic symmetries coexisted at x?=?0.02 mol, which contributes the excellent properties, in which the piezoelectric constant d ₃₃?=?361 pC/N, the planar electromechanical coupling coefficient k_p?=?41.2%, the Curie temperature Tc?=?70°C, the temperature of phase transition T _O-PC?=?34°C near the room temperature, the relative permittivity ε _r?=?4028 and the remanent polarization P _r?=?9.39 μC/cm².     

Piezoelectric ceramics of (Bi1/2Na1/2)TiO3-PbTiO3-BaTiO3 system

Piezoelectric and ferroelectric properties of bismuth sodium titanate, (Bi_1/2Na_1/2)TiO₃(BNT)-based solid solution, that is, (Bi_1/2Na_1/2)(_1-x)(Pb_a Ba_b)_xTiO₃(a + b = 1) [BNPB(100x-100a/100b)], are studied from the viewpoint of a new group of lead-free or low-lead content piezo-electric ceramics with a rhombohedral(F_a-tetragonal (F_β) morphotropic phase boundary (MPB). It is evident that the MPB seems to be remarkably efficacious in promoting piezoelectric and pyroelectric activities by electrical poling. X-ray diffraction data, dielectric properties and D-E hysteresis loops show that the MPB exist near (Bi_1/2Na_1/2)TiO₃[BNT] at x = 0.13–0.14, 0.08–0.09 and 0.06–0.07 in the case of b = 0, b = 0.5 and b = 1, respectively. BNPB ceramics are superior for piezoelectric ceramics in high-frequency ultrasonic uses, or special piezoelectric actuator materials with a lower free permittivity ε₃₃Tε₀, and a high electromechanical coupling factor k₃₃, along with a high mechanical strength.     

Enhancement in the physical properties of K0.5Na0.5NbO3 ceramics by the addition of 0.5 Li2O – 0.5K2O – 2B2O3 glass

The addition of powdered 0.5 Li₂O–0.5K₂O–2B₂O₃ (LKBO) glass (0.5 to 2 wt%) to potassium sodium niobate, K_0.5Na_0.5NbO₃ (KNN) powder facilitated higher densification which resulted in improved physical properties that include dielectric, piezoelectric and ferroelectric. The required polycrystalline powders of KNN were synthesized through solid-state reaction route, while LKBO glass was obtained via the conventional melt-quenching technique. Pulverized glass was added to KNN powders in different wt% and compacted at room temperature and these were sintered around 1100°C. Indeed the addition of optimum amount (1 wt %) of LKBO glass to KNN ceramics facilitated lowering of sintering temperature accompanied by larger grains (8 µm) with improved density. The dielectric constant (?_r) measured at room temperature was 475 (at 10 kHz), whereas it was only 199 for the LKBO glass free KNN. The piezoelectric coefficient (d₃₃) was found to be 130 pC/N for 1 wt% LKBO added glass, which was much higher than that of pure KNN ceramics (85 pC/N). Indeed, the LKBO glass added samples did exhibit well saturated P versus E hysteresis loops at room temperature. Though there was no particular trend observed in the variation of P_r with the increase in glass content, the P_r values were higher than that obtained for KNN ceramics. The improved physical properties of KNN ceramics encountered in these studies were primarily attributed to enhancement in density and grain size.