Investigation of structural, dielectric, impedance, and mechanical properties of hydroxyapatite-modified barium titanate composites for biomedical applications

Khalid Elfaki Ibrahim; Hamoud A. Kassim

doi:10.1515/chem-2023-0140

Article Open Access

Investigation of structural, dielectric, impedance, and mechanical properties of hydroxyapatite-modified barium titanate composites for biomedical applications

Khalid Elfaki Ibrahim and Hamoud A. Kassim

Published/Copyright: November 6, 2023

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From the journal Open Chemistry Volume 21 Issue 1

Abstract

This report presents the synthesis of pristine barium calcium titanate (BCT) and composite samples of [x(HA)–100 − x(BCT)]; (0 ≤ x ≤ 20) through solid-state reaction with microwave sintering. Hydroxyapatite (HA)–BCT composites have been developed to minimize grain growth, thereby boosting the material’s physical, mechanical, and electrical properties. The evaluated samples were examined for the Fourier transform-infrared spectra, and the results showed a correlation with the X-ray diffraction patterns. The real and imaginary dielectric permittivity was applied to determine the AC conductivity, and the findings indicate a drop in the frequency exponent values (S) from 1 to 0.67 for samples with x = 0. Similarly, for samples with x = 5, the value of S decreases from 0.90 to 0.55. For samples with x = 10, the value of S reduces from 0.7 to 0.54. Lastly, for samples with x = 20, the value of S decreases from 0.63 to 0.45. The exponent S and temperature relationship may be attributed to a thermal activation mechanism. The grains and grain boundary resistivity were estimated using a Cole–Cole plot, and the results showed that the grain boundary resistivity is higher in comparison to the resistivity exhibited inside the grains. This demonstrated the distinct electrical conductivity at the interfaces between the grains in comparison to the interior of the grains. The analysis of hardness indicates that the average hardness of the samples ranges from 5.22 to 4.77 GPa, which is maintained at different HA concentrations. The data suggest that this composite may have the potential to be a biomedically helpful substance.

Keywords: modified BT; microstructure; dielectric constant; AC conductivity; FTIR spectra; hardness; grain resistance

1 Introduction

Several disorders, including osteoporosis and trauma, need artificial implants for bone replacement. Current implant materials that substitute natural bones include metals, ceramics, and polymers that can induce a favorable cell-implant response by mimicking the bone extracellular matrix [1]. When developing a bone implant, it is crucial to consider the material’s strength, fracture toughness, hardness, biocompatibility, and electrical properties. Better bio-integration can be achieved by enhancing the mechanical characteristics and electrical stimulation of the cell–material interface of implants [2]. The electrical properties of human bones are essential to their healing and remodeling [3]. The piezoelectric skeleton generates an electric field through ionic displacement and cell signaling in response to mechanical stress [4]. Researchers and clinicians are very interested in piezoelectric materials for bone repair and regeneration [5] because of their ability to imitate the electrical properties of natural bones. Past research has demonstrated that mechanical stress on piezo ceramics can create an electric field [6]. However, lead is often used in piezo ceramics, which limits their applicability due to the metal’s potential cytotoxicity [7]. Alternatives to lead-free piezo ceramics have been studied and proved effective [8,9,10]. One of these lead-free materials is barium titanate BaTiO₃ (BT) and composites. Piezoelectric materials may promote cellular development and differentiation by simulating the electric potential created by stress in tissues like bone and cartilage [6]. Among piezoelectric ceramics, the (BZT-barium calcium titanate [BCT]) composite has the giant piezoelectric coefficient d33 at room temperature, making it biocompatible [8–12]. This group of piezoelectric ceramics has mechanically created electric surface potentials, which could be the key to functionalizing existing implant designs. Therefore, they can be used to simulate the bone’s ability to generate electrical prospects in response to mechanical stress without needing an external power source. Piezoelectric materials in hydroxyapatite (HA) composites can further enhance the material’s electrical capabilities, cell growth nature, and bone regeneration mechanism. Researchers have found that cell proliferation is enhanced by HA/BT composites [13–19]. In a different investigation, Dubey et al. [20] found that adding BT particles improved an SPS-sintered HA–BT composite’s mechanical (hardness and fracture toughness) and physical properties. According to Wang and Singh [21], the HA matrix’s fracture toughness was enhanced by the addition of BT. The presence of BT particles directly impacts the crack propagation in this composite. The composition of the HA–BT composite and its piezoelectric and dielectric characteristics were compared by Bowen and Topolov [22]. Their findings show that the shape and quantity of porosities in the HA matrix, as well as the volume percentage and dimensional ratio of the BT particles, all influence the electrical characteristics of the HA–BT composite [22]. In actuality, the alignment of BT particles in the polarization direction and the reduction in matrix stiffness (by introducing porosity) cause a decrease in the piezoelectric coefficient. Additionally, Bowen et al. [23] showed that adding BT with strong electrical qualities enhanced the electrical characteristics of pure HA in SPS-ed HA–BT composite. Researchers Tang and Dubey et al. [3,24] have investigated how composition affects the mechanical aspects of HA–BT composites. They demonstrated improving the mechanical characteristics of these composites by adding more BT with a high bonding energy. As a result, novel techniques, such as microwave technology, are currently being devised to decrease energy consumption while retaining or even enhancing the characteristics of the end ceramic product. This innovative and imaginative technology holds the potential to aid various industrial sectors in diminishing their manufacturing expenses and curbing their ecological impact. Conversely, comprehending and examining the fundamental principles of microwaves is intriguing and indispensable for advancing the production of materials indispensable to numerous industrial sectors daily [25,26].

Although a sizable number of articles have been published over the past few years, no thorough study has been done on the creation of x%HA–100 − x%Ba_0.96Ca_0.04TiO₃; [x(HA)–100 − x(BCT)]; (0 ≤ x ≤ 20) composites by a microwave sintering process. Furthermore, the mechanical and electrical characteristics of the [x(HA)–100 − x(BCT)]; (0 ≤ x ≤ 20) composites have not been described. Due to this, [x(HA)–100 − x(BCT)]; (0 ≤ x ≤ 20) composites were created in this study using solid-state reaction with assisting microwave sintering technique. Investigations are conducted on how [x(HA)–100 − x(BCT)]; (0 ≤ x ≤ 20) composites affect the structural, dielectric, impedance, and mechanical properties.

2 Experimental

Products of HA powder (Sigma-Aldrich – USA, 10 µm 98%; BaCO₃ 99.98%, CaO powder over 99.9% purity) and TiO₂ powders with >99% purity have been used as starting materials to prepare the (HA)–(BCT) composite. An energy ball milling technique (Retsch) was employed using zirconia balls in acetone media for 10 h at a rotation speed of 200 rpm to induce nanoparticles of HA powder. The raw powder of Ba_0.96Ca_0.04TiO₃ was weighted and ball-milled for 10 h at the same conditions used in HA powder milling. After drying each milled powder separately in an oven furnace for 1 h, HA powder was calcined at 900°C for 20 min, and the BCT compound was calcined at 1,000°C for 20 min using a microwave furnace with a heating and cooling rate of 50°C/min to induce a phase structure. Consequently, the composite of [x(HA)–100 − x(BCT)]; (0 ≤ x ≤ 20) has been weighed and ball milled for 10 h with the same rotation speed mentioned above. Polyvinyl alcohol was used as a binder to create green pellets that were 10 mm in diameter and 1 mm in thickness. The green pellets’ binder was evaporated by heating them to 500°C at 2°C/min using a muffle furnace (Lindberg/Blue M, USA). The pellets were heated to 1,250°C using a microwave sintering system (Microwave Sintering Furnace. 2.45 GHz, 3 kW Microwave Applicator) with a heating and cooling rate of 50°C/min and held there for 30 min before being cooled to room temperature. Bruker D8 powder X-ray diffraction (XRD) (Bruker D8 Advance) confirmed the phases of the sintered samples. FE-SEM (Carl Zeiss, Ultra 55) has been utilized to analyze the morphology of the sintered samples. For electrical properties, the pellets were heated to 200°C for 1 h with silver electrodes coated on both polished sides. These paints require curing to achieve optimal conductivity. This can involve heating the painted surface to 200°C for a certain duration to promote adhesion and conductivity. The intended composites’ effective creation was confirmed by using Fourier transform-infrared (FTIR) spectroscopy within the wavenumber range of 1,000–4,000 cm⁻¹, using a Bruker alpha-II FTIR spectrometer. Using an impedance analyzer (Agilent E4294A), we studied the frequency dependence of dielectric permittivity from 1 kHz to 2 MHz at different temperatures. Impedance measurements were also measured at room temperature using an impedance analyzer (Agilent E4294A). Utilizing a TriboScope nanomechanical test instrument, hardness measurements were made on a rough surface.

3 Results and discussion

3.1 XRD analysis

Figure 1 depicts the XRD pattern of BCT- and HA–BCT-sintered samples using microwave sintering technology at room temperature. No secondary phase was identified in the perovskite BCT peaks that matched the conventional BT tetragonal structure (JCPDS file number 83-1880) [27,28]. The splitting of the peak at 2θ ∼ 45°–46° confirms the ferroelectricity of BT powder. The XRD pattern of commercial BT agrees with the findings of prior investigations [16]. Figure 1 further shows that the pattern of the characteristic peaks is determined by the standard XRD pattern (JCPDS, 00-024-0033) [29]. The XRD pattern also shows the distinctive peaks of the HA phase at 2θ∼26° and 32°. The peaks in Figure 1 are HA crystalline phases generated with a suitable degree of crystallinity [16]. Figure 1 depicts the XRD pattern of HA–BCT composites, which displays the XRD patterns of (HA) and (BCT) coexisting and indicates different symbols. With increasing HA content, the strength of the distinctive peak of the BCT phase at 2θ ∼ 45° drops. This phenomenon is caused by the coexistence of cubic and tetragonal phases of BT, as per literature reports [16]. As a result, single phases can emerge in (HA)–(BCT) composites, and the coexistence of the (HA) and (BCT) phases during the sintering process.

Figure 1

The XRD pattern of BCT and [x(HA)–100− x(BCT)] samples sintered at 1,250°C for 30 min using a microwave sintering system.

3.2 FTIR analysis

FTIR transmittance analyses were conducted on both the pure BCT and composite samples to ascertain the existence of OH– ions in the sintered samples by microwave sintering system. Figure 2 illustrates the FTIR spectrum of the sintered samples consisting of a combination of (100 − x) HA and (x) BT. Based on the analysis of the FTIR spectrum, it can be deduced that all the peaks seen belong to the original powder used for densification, hence indicating the absence of any discernible loss of OH− or other noteworthy chemical elements within the composites. HA is distinguished by including anionic groups such as PO 4 3 − , OH⁻, and CO 3 2 − [30]. On the other hand, BT is defined by the presence of groups corresponding to Ti–O in conjunction with Ba [31]. The peaks observed in the 1,050–1,110 cm⁻¹ range are associated with the triply degenerate anti-symmetric P–O stretching [31]. The presence of water molecules adsorbed onto the surface is characterized by a relatively broad spectral range spanning from 2,300 to 3,300 cm⁻¹ [32]. Specifically, the high peak detected at 3,570 cm⁻¹ corresponds to the stretching vibration of the hydroxyl (OH⁻) groups. The identification of CO 3 2 − groups is accomplished by observing intense peaks within the spectral range of 1,465–1,480 cm⁻¹, with additional peaks occurring at 1,542 cm⁻¹ [33]. The spectral peaks at about 1,450 cm⁻¹ are not considered reliable indicators for identifying carbonated apatite. This is because these bands might arise from carbonate absorption occurring on the surfaces of apatite crystals or from a distinct carbonate phase separate from the apatite crystals [34]. There is no observable alteration in the band structure of the other bands when comparing the pure sample to the sintered composite. This suggests the sintered composite contains OH− ions inside its lattice structure [35]. As previously mentioned, the FTIR spectra were consistent with the XRD patterns.

Figure 2

The FTIR spectra of (BCT) and [x(HA)–100− x(BCT)]; (0 ≤ x ≤ 20) samples sintered at 1,250°C for 30 min using a microwave sintering system.

3.3 Microstructure analysis

Figure 3(a)–(d) displays SEM images of sintered x%HA–100− x% Ba_0.96Ca_0.04TiO₃; (0 ≤ x ≤ 20) samples sintered at 1,250°C for 20 min using MWS. This image depicts the dense microstructure of pure BCT and the homogeneous distribution of porosities within the grains. Additionally, domains within the grains with various orientations confirm the BCT sample’s ferroelectricity. Figure 3(b–d) displays cross-sectional micrographs of pure HA–BCT composites. This figure also shows a porous microstructure with tiny grains (grains 400 nm). Figure 3 illustrates how the composite with a high HA content exhibits larger porosities, smaller pores, and a low-density microstructure, which can affect the dielectric and mechanical properties of the composite, as we can see in the following discussions. The coexistence of BCT and HA phases can be attributed to more porosities in this composite than in the pure BCT, as shown by EDS analysis in Figure 3(c) and (d). Small and large porosities are scattered across the grains of the composite samples, as seen in Figure 3(c) and (d). The HA phase is more likely to disintegrate, and liquid phase development is probable at high temperatures. The amount of porosity in the microstructure is expected to increase with the addition of HA, as shown by XRD in Figure 1 and EDS in Figure 4(c) and (d). A comparison of the SEM images of the composite samples supports this. The above discussion is also supported by literature reports [18,36,37].

Figure 3

SEM microstructures of (a) (BCT), (b) 5%(HA)–95%(BCT), (c) 10% (HA)–90%(BCT), and (d) 20%(HA)–80%(BCT) samples sintered at 1,250°C for 30 min using a microwave sintering system.

Figure 4

EDS analysis of (a) (BCT), (b) 5%(HA)–95%(BCT), (c) 10% (HA)–90%(BCT), and (d) 20%(HA)–80%(BCT) samples sintered at 1,250°C for 30 min using a microwave sintering system.

3.4 Frequency and temperature dependence of the dielectric permittivity

Figure 5(a)–(d) displays the dielectric constant as a function of frequency for x%HA–100 − x(Ba_0.96Ca_0.04TiO₃); (0 ≤ x ≤ 20) composites measured over the frequency range 1 kH to 2 MHz. The dielectric constant of pure BCT is greater than that of composite samples in Figure 4(a). Results from this investigation show that the dielectric constants of composites are greater than those reported by literature. As can be seen in the SEM image of Figure 3(a), the reason for this is the large grain size, high density, and low number of porosities. The composite sample has the lowest dielectric constant. As the temperature is raised, the value of the dielectric constant rises because of an increase in the number of charge carriers. As a result, all composite samples exhibit a dramatic drop in capacity and dielectric constant compared to pure BCT upon coupling the low-dielectric-constant HA with the high-dielectric-constant BT. A BCT sample has a giant dielectric constant at different examined temperatures and a low frequency of 1 kHz compared to composite models. This may be due to the development of parallel polarization in the presence of two phases with distinct weight percentages and electrical conductivity [38]. Microstructure, the distribution of crystalline phases next to one other factor that can play an essential role in tuning the dielectric properties, and the density of the sample are all known to affect the dielectric constant [39]. Bowen et al. [23] found a lower value for the dielectric constant in a pure HA sample than what was measured. The percentage of BCT, the number of pores in the microstructure, and the sample density all play a role in this variation in the dielectric constant [39].

Figure 5

Dielectric constant of (a) (BCT), (b) 5%(HA)–95%(BCT), (c) 10% (HA)–90%(BCT), and (d) 20%(HA)–80%(BCT) samples sintered at 1,250°C for 30 min using a microwave sintering system as a function of frequency measured at four different temperatures.

3.5 Frequency and temperature dependence of the dielectric loss

Figure 6(a)–(d) illustrates the variation in the dielectric loss of BCT, (b) 5%HA–95%BCT, (c) 10% HA–90%BCT, and (d) 20% HA–80%BCT sintered samples, as a function of frequency measured at four different temperatures. The data in the figure demonstrate a positive correlation between temperature and dielectric loss, indicating that the dielectric loss increases when temperature rises. Researchers in references [40–42] investigated several relaxation phenomena and categorized dielectric loss into three distinct types: conduction, dipole, and vibration. Conduction loss pertains to ions migrating across considerable distances, resulting in a loss. During the process of ion movement, a part of their energy is transferred to the lattice in the form of heat. The amount of heat lost every cycle is directly proportional to a particular variable. The smallest value of conduction loss occurs at lower temperatures. When the temperature rises, there is an increase because of the heightened conduction losses. Consequently, the dielectric loss also increases in tandem with the temperature. The dielectric loss exhibits larger values in the lower and intermediate frequency zones. The vibrations of ions may become ion jump and conduction loss caused by ion migration, in addition to the phenomenon of ionic polarization. In the higher frequency range, the vibrations of ions may become the only contributor to the loss tangent, resulting in a reduction in dielectric loss within this frequency band.

Figure 6

Dielectric loss of (a) (BCT), (b) 5%(HA)–95%(BCT), (c) 10% (HA)–90%(BCT), and (d) 20%(HA)–80%(BCT) samples sintered at 1,250°C for 30 min using a microwave sintering system as a function of frequency measured at four different temperatures.

3.6 Frequency and temperature dependence of AC conductivity

AC conductivity seems to be a valuable approach to getting insight into the dynamics of ionic motion inside ionically conducting materials. The investigation focused on examining the frequency-dependent AC conductivity of (a) (BCT), (b) 5%(HA)–95%(BCT), (c) 10% (HA)–90%(BCT), and (d) 20%(HA)–80%(BCT) sintered samples. The AC electrical conductivity of all samples was obtained by calculating the real and imaginary components of the complex dielectric constant as a function of frequency and temperature using the following equation [43].

(1) σ ac = ω ε o ε r tan δ .

In the given context, ω represents the angular frequency, ɛ _o denotes the electrical permittivity of free space, ɛ _r means the relative permittivity, and tanδ signifies the loss tangent. The conductivity was measured as a function of angular frequency (ω) in the frequency range of 1 kHz to 2 MHz in the logarithmic scale at various temperatures. The experimental findings are graphed as a function of temperature using equation (1), as seen in Figure 7(a)–(d). All the materials’ AC conductivity measurements exhibit low-frequency plateaus, characteristic of electronic DC conductivity [44]. A dispersive zone with frequency-dependent conductivity is beyond this plateau at higher frequencies. The data shown in the picture demonstrate a strong positive correlation between frequency and AC conductivity. This suggests that the relationship between AC conductivity and frequency follows a linear pattern, according to the principles of a power law. The frequency exponent values (S) were derived from the gradients of the linear regressions shown in Figure 7(a)–(d). Figure 8 shows the temperature dependency of the frequency exponent (S) for all the samples. The data demonstrate that the frequency exponent drops as the temperature increases. The relationship between temperature and the frequency exponent offers valuable insights for determining the specific mechanism behind AC conduction. The observed phenomenon in alternating current conductivity has similarities to the documented behavior found in ionic glasses [45]. The Jonscher equation [46,47] has been seen to govern the AC conductivity of highly disordered materials, including amorphous semiconductors and ionic conducting glasses. As reported in reference, ionic conducting materials have a range of values for the parameter S, often falling between 0.6 and 1 [48]. Figure 8 illustrates a decreasing trend in the value of S as temperature increases; specifically, for samples with x = 0, frequency exponent values (S) drop from 1 to 0.67. Similarly, for samples with x = 5, the value of S reduces from 0.90 to 0.55. For samples with x = 10, the value of S decreases from 0.7 to 0.54. Lastly, for samples with x = 20, the value of S decreases from 0.63 to 0.45. The exponent S and temperature relationship may be attributed to a thermal activation mechanism [49]. The exponent frequency (S) within a specific range identifies the low-frequency zone. This region is associated with grain boundary conductivity, which is attributed to the ion hopping process of mobile charge carriers crossing barriers between two sides. Based on the jump relaxation hypothesis, it can be inferred that within the lower frequency range, the conductivity is attributed to the occurrence of efficient electron hops. As the frequency increases, the likelihood of unsuccessful hopping is correspondingly heightened. The observation of increased hopping frequency leading to dispersive conductivity with frequency indicates a lack of effectiveness in achieving the desired outcome. The presence of charge traps inside the band gap of perovskite materials is anticipated. The jump relaxation model proposes distinct activation energies linked to successful and failed hopping events [50].

Figure 7

AC conductivity of (a) (BCT), (b) 5%(HA)–95%(BCT), (c) 10% (HA)–90%(BCT), and (d) 20%(HA)–80%(BCT) samples sintered at 1,250°C for 30 min using a microwave sintering system as a function of frequency measured at four different temperatures.

Figure 8

The temperature dependence of the frequency exponent S in Jonscher’s relation for (BCT) and x(HA)–100− x(BCT) samples was sintered at 1,250°C for 30 min using a microwave sintering system.

3.7 Impedance analysis

The current study aims to analyze the contribution of the grain and grain boundary components based on the observed spectra for pure body-centered tetragonal and composite samples. Figure 9 illustrates the complex plane’s pure BCT impedance behavior and the HA–BCT composite. The shown images demonstrate that the centers of the semicircular arcs are positioned below the X-axis, suggesting that the samples exhibit relaxation behavior that is not characteristic of the Debye type. The point of intersection between the impedance spectra and the actual X-axis signifies the total resistance of the sample, including both the resistance of the individual grains and the grain boundaries. The shown graphic demonstrates a positive correlation between the concentration of HA and the resistance of both the grain and grain border. This finding provides empirical evidence that the conductivity of the sample decreases as the HA concentration increases, corroborating the results reported by Dubey in the existing literature [51]. A Cole–Cole plot exhibiting two semicircles may be effectively represented by an equivalent circuit with a series connection between two capacitors, C1 and C2, accompanied by two constant phase elements (CPEs), Q1 and Q2. This circuit model yields a satisfactory match to the experimental data. Figure 10(a)–(d) shows the optimal fitting of the observed impedance data for the examined samples at varying concentrations of HA, together with the corresponding equivalent circuit used for the simulation. Table 1 displays the values of the circuit components and various parameters used for the simulation. A cylindrical pellet specimen was produced with a consistent thickness of 0.6 mm and an estimated surface area represented by A = 54 mm². The use of silver was employed in the construction of electrodes. The estimation of the bulk resistivity of the samples was conducted by using the grain resistance obtained from Cole–Cole plots and the geometric properties, employing the methodology described in the references [52,53].

ρ = R g × A L .

Figure 9

Impedance analysis of the investigated samples sintered at 1,250°C for 30 min using a microwave sintering system.

Figure 10

Optimized fitting using an equivalent circuit consisting of two RQ elements as shown in the inset for (a) (BCT), (b) 5%(HA)–95%(BCT), (c) 10% (HA)–90%(BCT), and (d) 20%(HA)–80%(BCT) samples sintered at 1,250°C for 30 min using a microwave sintering system.

Table 1

Summarized the Cole–Cole plot results of (BCT) and x[(HA)–100 − x(BCT)] samples sintered at 1,250°C for 30 min using a microwave sintering system obtained from experimental data fitting using an equivalent circuit with a series connection between two capacitors, C1 and C2, accompanied by two CPEs, Q1 and Q2

Sample/parameter	0(HA)–100(BCT)	5(HA)–95(BCT)	10(HA)–90(BCT)	20(HA)–80(BCT)
R _g (Ω)	10.44 × 10⁶	8.35 × 10⁶	6.95 × 10⁶	5.96 × 10⁶
R _gb (Ω)	12.36 × 10⁶	9.96 × 10⁶	7.21 × 10⁶	6.51 × 10⁶
C _g (F)	20.6 × 10⁻⁵	1.61 × 10⁻⁵	5.54 × 10⁻⁶	1.18 × 10⁻⁸
ρ _g (Ω cm)	9.39 × 10⁸	7.51 × 10⁸	6.25 × 10⁸	5.12 × 10⁸
ρ _gb (Ω cm)	11.12 × 10⁸	8.96 × 10⁸	6.84 × 10⁸	5.85 × 10⁸

The parameters that were acquired are shown in Table 2. The observed trend indicates a reduction in bulk resistivity as the concentration of Li-doping increases, which may be attributable to an augmentation in the presence of defects. The impedance spectrum of a semicircle may be associated with relaxor-like phenomena [54]. Based on the findings derived from ceramics and other polycrystalline materials, it has been shown that grain borders can potentially elevate resistivity compared to the resistivity exhibited inside the grains themselves. This phenomenon arises due to the distinct electrical characteristics demonstrated at the interfaces between grains in comparison to the interior of the grains. Grain boundaries refer to the interfaces between different crystalline grains inside a material that exhibits a polycrystalline structure [55]. The borders may exhibit unique structural and compositional characteristics in contrast to the interior of the grains. Within the field of ceramics, it is seen that boundaries can confine charges, impurities, and flaws, resulting in limited regions characterized by elevated resistivity [56]. The presence of obstacles inside the material might impede the flow of electrons or charge carriers, resulting in an elevation of its resistivity. On the contrary, the atomic structure inside most of each grain exhibits a higher degree of organization, with fewer defects and impurities than the grain borders. Consequently, there is a decrease in resistivity at the intra-grain level. Notably, the resistivity of ceramics is subject to the effect of several elements, including the specific kind of ceramic material, its microstructural characteristics, the properties of the grain boundaries, and the potential existence of dopants or impurities [57]. The impact of grain boundaries on resistivity may vary in different scenarios, with some examples exhibiting a more prominent influence while others demonstrating somewhat less significance [58]. In brief, the disparity in resistivity between grain borders and the inside of grains is prevalent in ceramics and other polycrystalline substances, and it can potentially impact the material’s total resistivity.

Table 2

Hardness results of [x(HA)–100 − x(BCT)]; (0 ≤ x ≤ 20) samples sintered at 1,250°C for 30 min using a microwave sintering system, calculated from nanoindentation testing analysis of the experimental results in Figure 11

Sample/parameter	0(HA)–100(BCT)	5(HA)–95(BCT)	10(HA)–90(BCT)	20(HA)–80(BCT)
Hardness/Gpa	5.22	5.02	4.85	4.77

3.8 Mechanical properties

The resistance of a material against plastic deformation or the strength of a substance against the indentation at the surface is example of what is meant when we talk about hardness. The load–displacement curve that is representative of a nanoindentation test performed on a (BCT) sample and (HA–BCT) composites is depicted in Figure 11(a). Nanoindentation was carried out on these materials up to a load of 9.5 mN, and the load–displacement curves that were generated as a result demonstrated that the typical plastic deformation had been eliminated. The amount of work required fell as the concentration of HA increased. The increasing porosity in the microstructure is to blame for this weakening of the material. The mass per unit volume of composites is typically relatively low. This is due to the subsequent phases of barium phosphate, as indicated by reference number [16]. Porosities and the coexistence of (HA) and (BCT) phases are desirable for lowering the mechanical stress that the sintered samples experience. Because of the presence of HA, increasing the amount of HA leads to an increase in the number of pores that form [16]. The high bonding energy between oxygen and barium ions is said to be why materials based on BT have the highest degree of Vickers hardness compared to other samples [59]. These findings are based on research that was conducted in the past. Therefore, with higher bonding energy, more significant activation energy is required for bond failure, and most likely, in the presence of BT, even with a low wt%, the composite hardness increases. This is because increased bonding energy requires increased activation energy. On the other hand, the Hall patches equation states that an increase in yield stress will occur if the grain size of the material undergoes a reduction [60,61]. Consequently, plastic deformation will occur at more significant pressures during the hardness test. As a result, the combination of (BCT) with the (HA) matrix can increase the HA’s hardness. On the other hand, it is essential to note that there has not been much research done on the microstructure and the physical and mechanical properties of the composite contenting, and the information presented in this study is for reporting purposes. Figure 11(b) shows the load–displacement curves analysis of the 0(HA)–100(BCT) sample acquired by nanoindentation testing and the formulas mentioned in reference [62]. The image illustrates the presence of variations in the maximum indentation depth. At the same time, the inclination of the unloading segment indicates the hardness of the materials being indented. In the interim, Table 2 presents the recorded hardness values of samples ranging from 5.22 to 4.77 GPa, along with their corresponding average. Based on the graphical representation in Figure 11(b) and the numerical values provided in Table 2, it is evident that the hardness of the material exhibits certain features that are dependent on the orientation of the HA content. From this, we conclude that material hardness refers to the resistance of a material to deformation, indentation, or scratching. In biomedical applications, material properties are crucial because they can influence the performance and durability of medical devices, implants, and tools. From the obtained results, here are a few ways hardness could potentially provide advantages in specific biomedical applications: (i) Biocompatibility: while hardness itself might not be a primary factor in biocompatibility, materials used in biomedical applications should still be biocompatible and not induce adverse reactions within the body. Finding a balance between hardness and biocompatibility is essential. (ii) Tissue interaction: in some cases, materials with specific hardness properties might be designed to interact with biological tissues in a certain way. For instance, scaffolds with controlled hardness could influence cell adhesion, growth, and differentiation in tissue engineering. (iii) Finally, we can note that the effectiveness of a material in a biomedical application depends on various factors beyond hardness alone. These factors include biocompatibility, corrosion resistance, fatigue resistance, and more.

Figure 11

(a) The typical load–displacement curves caused by a nanoindentation test on BCT and xHA–100 − x(BCT) samples sintered at 1,250°C for 30 min using a microwave sintering system. (b) A load–displacement curve analysis for 0)HA)–100(BCT) sample h _f is indicated to the final depth, h _i is indicated to intercept displacement, and h _max is referred to maximum displacement.

4 Conclusion

The synthesis of pure (BCT) and composite samples of [x(HA)–100 − x(BCT)]; (0 ≤ x ≤ 20) via solid-state reaction with microwave sintering has been done. (HA–BCT) composites have been developed to reduce grain growth, enhancing the material’s physical, mechanical, and electrical properties. The FTIR spectra were investigated in the examined samples, and the results were consistent with the XRD patterns. Using the real and imaginary dielectric permittivity, the AC conductivity has been investigated, and the results show that for samples with x = 0, frequency exponent values (S) drop from 1 to 0.67. Similarly, for samples with x = 5, the value of S reduces from 0.90 to 0.55. For samples with x = 10, the value of S decreases from 0.7 to 0.54. Lastly, for samples with x = 20, the value of S decreases from 0.63 to 0.45. The exponent S and temperature relationship may be attributed to a thermal activation mechanism. Grains and grain boundary resistivity have been estimated using a Cole–Cole plot. The results showed that grain borders could potentially elevate resistivity compared to the resistivity exhibited inside the grains. This phenomenon arises due to the distinct electrical characteristics demonstrated at the interfaces between grains in comparison to the interior of the grains. The findings of this investigation thus closely resemble those of the spark plasma sintering technique. The bulk density and mechanical characteristics of composite samples are reduced due to increased porosity caused by HA incorporation. The hardness analysis shows that the average hardness of samples is from 5.22 to 4.77 GPa, which is approximately maintained at different HA concentrations. The data may suggest that this composite has the potential to be a biomedically helpful substance.

Acknowledgments

The authors are grateful to the Researchers Supporting Project No. RSPD2023R759, King Saud University, Riyadh, Saudi Arabia.

Funding information: This article was financially supported by King Saud University, Riyadh, Saudi Arabia (RSPD2023R759).
Author contributions: Khalid Elfaki Ibrahim: investigator and resources. H. Kassim: synthetization, methodology, formal analysis, and approval of the final version.
Conflict of interest: The authors declare no conflict of interest.
Ethical approval: The conducted research is not related to either human or animal use.
Data availability statement: All data generated or analyzed during this study are included in this article.

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Received: 2023-07-16

Revised: 2023-09-14

Accepted: 2023-09-26

Published Online: 2023-11-06

This work is licensed under the Creative Commons Attribution 4.0 International License.

Articles in the same Issue

https://doi.org/10.1515/chem-2023-0140

Keywords for this article

modified BT; microstructure; dielectric constant; AC conductivity; FTIR spectra; hardness; grain resistance

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