Ultra-dense (Bi, V, B)-oxide-added zircon ceramics fabricated by liquid-phase assisted spark plasma sintering (SPS)

Ultra-high-density zircon (ZrSiO4) ceramics were prepared using the spark plasma sintering (SPS) technique of zircon nanopowder with the addition of three different sintering agents, i.e., Bi2O3, V2O5 and B2O3. The effect of each agent and the SPS parameters (temperature and pressure) on phase composition, microstructure, thermal and mechanical properties of the ceramics were evaluated. The identified crystalline phases of the sintered ceramics were zircon and monoclinic zirconia. The addition of a sintering agent affects the structure of zircon ceramics, i.e. the lattice parameter and the crystallite size. The sintered ceramics reached relative densities up to 99.9% of the theoretical one when V2O5 or B2O3 was added. SEM observations confirmed the densification of the zircon ceramics. We found the ceramics exhibited thermal conductivity ranging from 0.39 to 0.61 Wm−1K−1 at 373 K while the coefficient of thermal expansion was 2.3–4.0 × 10−6/°C and the Vickers hardness was obtained to be 9.52–12.66 GPa. The Young’s (E), bulk (B), and shear (G) moduli, Poisson’s ratio ν, Pugh’s ratio B/G, and the ratio of HV3/E*2 of the ceramics are in a range of 240 − 288 GPa, 207 − 267 GPa, 91 − 109 Pa, 1.95 − 2.45, and 0.011 − 0.019 respectively. We found that high-density, quasi-ductile zircon ceramics can be synthesized at a low sintering temperature and short holding time.


Introduction
ZrSiO 4 (or zircon) is one of the proper materials for industrial and structural applications in high temperature. For example, it is used in steel and glass industries owing to its structural stability up to 1680°C [1]. In several applications, zircon needs to be formed into ceramic. A high-density sintered ceramic from pressed powder is challenging because of its high melting point. Advanced sintering methods are conventional ways to obtain a high-density zircon ceramic. The sintering process may be improved by combining it with pressing at an elevated temperature, such as hot pressing. The process promotes inter-particle bonds and reduces their surface energy, consequently closing down porosity. However, the fabrication of high-density materials through the mentioned sintering process is insufficient due to forming the furthest grain growth in the end product [2]. This increased grain growth may be assigned to prolonged sintering time, which is possible because after attaining densification, the grain growth continues considering the elevated temperature.
Recently, we have reported the synthesis of zircon ceramics by an arc plasma sintering (APS) technology [3], i.e., a sintering method that utilizes plasma to densify previously pressed zircon powders. The working principle of the APS was illustrated schematically by Bandriyana et al [4]. Unlike conventional furnaces, the APS uses a shorter sintering time of 8 min to produce zircon ceramics with a maximum bulk density of 93.5% [3]. Unfortunately, the ceramics have non-homogenous characteristics, i.e. graded microstructure and properties according to the layer.
The other advanced sintering method, spark plasma sintering (SPS), allows densification with lower temperatures and shorter dwelling times [5]. Furthermore, SPS permits the densification of sintered ceramic to near the theoretical density values with limited grain growth [6], [7]. Previously, a high-density zircon ceramic could be obtained with 10 min of holding time at 1400°C under uniaxial pressure of 100 MPa by SPS [8]. However, when conventional sintering was used, a fully dense zircon ceramic could only be achieved at a temperature higher than 1400°C. In this work, we attempted to reduce the sintering temperature and improve the density of zircon ceramics without significant detriment to the grain size and thermal and mechanical properties of the sintered bodies.
Several methods can increase bulk density and reduce the sintering temperature, e.g., adding a low melting point additive and using smaller particle sizes of raw powders [9]. The most widely used additive to increase the density of zircon ceramics is SiO 2 [10] and Al 2 O 3 [11]. However, using these additives tends to degrade the mechanical properties of ceramics, such as decreasing the hardness and fracture toughness [12]. On the contrary, other work shows that B 2 O 3 can improve wetting ability [13] and be used as a sintering agent in ceramic synthesis [14]. For example, Pratapa et al [15] added B 2 O 3 to produce forsterite ceramics with low porosity. Furthermore, using 1 mol% V 2 O 5 as a sintering agent was carried out by Liu et al for ZnTiO 3 ceramics to provide densification of 96% at a temperature of 900°C [16]. In addition, the MgO-CaO-Al 2 O 3 -SiO 2 glass density increased proportion to the addition of Bi 2 O 3 at 930°C compared to 1000°C of a sample without the sintering agent [17]. However, no study uses B 2 O 3 , V 2 O 5 , and Bi 2 O 3 as zircon ceramic sintering agents. This study will examine the effect of separately adding B 2 O 3 , V 2 O 5 , and Bi 2 O 3 in the manufacture of high-density zircon ceramics with a low sintering temperature by the SPS method. The presence of a sintering agent in the form of a liquid phase during sintering is expected to facilitate grain rearrangement through the capillary forces between particles. To optimize the properties of the ceramics, the amount of the liquid phase must be carefully controlled. Therefore, the effect of the amount of each added sintering agent in zircon ceramics were also evaluated.
The particle size of raw materials is an essential consideration in ceramic manufacture; it must be guaranteed to accomplish the desired physical, microstructural, and thermo-mechanical properties. The nanomaterial is expected to reveal different properties to coarse-grained ceramics-furthermore, the smaller the particle size, the higher the driving force for sintering. In our previous work [18], we successfully synthesized zircon nanopowder with a crystallite size of 40 nm from natural sand using a relatively simple purification process followed by milling and annealing. The zircon nanopowder was used as the raw powder to synthesize the ceramics in the present study.
To summarise, the principal aim of this work is to examine the influence of the sintering agents (Bi 2 O 3 , V 2 O 5 , and B 2 O 3 ) and the spark plasma sintering parameters on the phase composition, physical, microstructure, and thermo-mechanical properties of zircon ceramics.

Synthesis of zircon nanopowder
Natural zircon sand from Central Kalimantan, Indonesia, as the raw material, was extracted into high-purity zircon nano powder followed by the addition of a sintering agent, then further densified to obtain high-density zircon ceramics using SPS. The purification process of the sand was described elsewhere [18], where three main stages were involved, i.e., magnetic compound removal, HCl soaking, and reaction with NaOH. These steps obtained a pure sub-micron zircon powder. The powder was then milled for 10 h to acquire nano-sized zircon powder, followed by annealing for an hour at 200°C) to reduce the milled powder's residual strain. The crystallite size of the milled-annealed zircon powder was 40 nm [18]. Therefore, the powder was readily used as the raw zircon to produce the ceramics.
2.2. Synthesis of zircon ceramics by SPS with the addition of Bi 2 O 3 , V 2 O 5 , and B 2 O 3 as a sintering agent This study prepared the high-purity reagent grade of Bi 2 O 3 , V 2 O 5 , and B 2 O 3 powder supplied by Sigma-Aldrich as the sintering agent. The zircon nanopowder was mixed separately with 3 and 6% by weight of Bi 2 O 3 , V 2 O 5 , and B 2 O 3 as sintering before being subjected to SPS. A sintering-agent-free powder as a control sample was also prepared. In the synthesis, the powders were poured into a 10 mm diameter graphite die and densified using an SPS machine in a vacuum with a heating rate of 10°C min −1 up to the maximum designated temperature and held for 10 min The SPS was performed under a temperature of 1250 and 1350°C and pressure of 40 and 80 MPa. The sintered body was mirror-polished using a series of diamond slurries up to 1 μm size. The ceramics are designated with N followed by the sintering agent symbol (Bi, V, and B) and the composition (0, 3, and 6) in wt%.

Characterization
Characterization of the sintered zircon ceramics included phase composition, density, microstructure, and mechanical and thermal properties. The XRD data of the ceramic samples were collected using a Cu-Kα x-ray diffractometer (0.0170°step size). The data were analyzed for phase identification, compositions, and zircon crystallite size. A correction for instrument peak broadening has been made for crystallite size estimation. The Archimedes-based method was applied to quantify the density of the ceramics using an electronic densimeter BK-DME300D. The microstructure of the ceramics after polishing and thermal etching at 1000°C for an hour was examined by a scanning electron microscope (SU-8000, Hitachi) at 10 keV. Then, the thermal expansion coefficient, thermal conductivity, Vickers hardness, and elastic moduli were evaluated. The Vickers hardness test of 2 kgf was performed on the polished ceramic surface. The longitudinal and shear sound velocities in the ceramics at room temperature were calculated following V = 2d/t, where t is the time difference between two back-reflected pulses and d is the thickness of the samples. The elastic properties were determined from the calculated velocities. Meanwhile, the shear (G), Young's (E), and bulk (B) moduli, Poisson's ratio ν, and Pugh's ratio B/G were calculated as follows [19]: where V L and V S represent the longitudinal and shear sound velocities, respectively. The plasticity of zircon ceramics is evaluated using the ratio of H E , where H V is the micro-Vickers hardness and is the effective Young's modulus. Meanwhile, the coefficient of linear thermal expansion α of the ceramics was determined using a Mettler Toledo © thermo-mechanical analyzer. Finally, the thermal conductivity λ of the ceramics was determined using a comparative rod apparatus technique by comparing the thermal conductivity of the zircon ceramics with that of a reference. The conductivity was estimated from the heat flow and temperature difference at a given thermal input.

Results and discussion
3.1. Phase analysis of the ceramics The XRD patterns of zircon ceramics with 3 and 6 wt% addition of Bi 2 O 3 , V 2 O 5 , or B 2 O 3 as sintering agents at varied SPS sintering temperature and pressure are shown in figure 1. The control sample is denoted with N0. The XRD data show that the identified phases in the ceramics are zircon (ZrSiO 4 , PDF No. 1-011-265) and monoclinic zirconia (m-ZrO 2 , PDF No. 89-426). SPS exposure on the sintering-agent-added zircon powder does not significantly affect the main phase, i.e., zircon. Here, zirconia formed due to zircon decomposition. Decomposition is the breakdown of chemical compounds into simpler components, such as zirconia and silica, resulting from thermal treatment [20]. The finding of monoclinic zirconia in some of the ceramics is understandable since, thermodynamically, this phase is the most stable form of zirconia polymorphs at room temperature.
Meanwhile, silica peaks do not appear in the XRD patterns implying that it is in an amorphous state [21]. The decomposition mechanism of zircon is related to the ZrO 8 coordination expansion in the ZrSiO 4 structure because of the elevated temperature followed by silica liberation [22]. It is noted that the dissociation is also affected by the presence of an impurity. From the literature, it was found that zircon decomposed at a temperature above 1650°C, but the presence of TiO 2 lowered the dissociation temperature of zircon by about 200°C [22], [23]. In this work, the addition of sintering agents (Bi 2 O 3 , V 2 O 5 , and B 2 O 3 ) also promote the lowering of the decomposition temperature of zircon. The ceramics exhibit a decomposition when sintered at 1250°C, which is about 400°C lower than the typical dissociation temperature of zircon (table 1). Furthermore, figure 1(d) shows the XRD data of the most intense peak of monoclinic zirconia of the sintering agent-free and -added zircon ceramics prepared at a sintering temperature of 1250°C and pressure of 80 MPa. Adding V 2 O 5 more effectively inhibits zircon decomposition than without the B 2 O 3 or Bi 2 O 3 additions.
In conclusion, the addition of 6 wt% V 2 O 5 prevents such a decomposition. The quantitative analyses of the XRD data for all ceramics are presented in table 1. Effects of SPS sintering parameters can also be seen in the table. Two main findings can be extracted from the table, i.e., (a) the decomposition of zircon appears to be more considerable at higher temperatures, and (b) higher pressure inhibits the decomposition of zircon.
The Effect of SPS sintering on the zircon structure in the ceramics can be seen in the Rietveld analysisderived cell parameters and volumes (table 1). They differ reasonably when compared within 2 standard deviations. The difference can be explained as follows. Zircon is a tetragonal crystal with space group of I4 1 /amd, with c/a ratio≈ 0.906 implying that zircon exhibits a possible structural anisotropy. The Young's moduli of zircon are 395 and 387 GPa along the c and a axes, respectively [24]. With respect to pressure effects, the compressibility along the a-axis is greater than that along the c-axis. For example, the sample with no sintering agent addition (N0) sintered at 1250°C exhibits a and c decreases by 0,009 and 0,002% due to higher sintering pressure. The sintering agent-added ceramics also exhibit the same behaviour. The values of zircon's corresponding elasticity and linear compressibility manifest this structural anisotropy. This work confirms that the difference in the lattice parameter changes of zircon in the SPS ceramics can be attributed to the anisotropic crystallographic elastic moduli of the phase.
Meanwhile, zircon exhibits an expansion in lattice parameters due to sintering temperature (see table 1, sample sintered at 1250 and 1350°C at the same pressure of 40 MPa). Higher sintering temperature allows higher thermal energy for the crystal to expand. However, the anisotropy in the expansion also occurs in the crystal. In general, the ceramics exhibit higher expansion in the c direction than in the a direction. This result is in line with previous findings where the expansion was anisotropic with values along the c and a axes of zircon are 5.4 × 10 −6 /°C and 3.2 × 10 −6 /°C, respectively [25].
From table 1, we also found that during sintering, the added sintering agent's liquid phase inhibited the zircon crystal's expansion in all directions. The 'flowing' phases dominate the lattice parameters' expansion. Therefore, the addition of a sintering agent affects the structure of zircon ceramics. The zircon structural anisotropy as the consequences of pressure, temperature, and liquid phase addition can be traced back as due to a heterogeneous bonding nature in the forming polyhedra, i.e. SiO 4 tetrahedra show rigid characteristics and are firmly structured, while ZrO 8 dodecahedra are relatively soft and weak [26]. Table 1. Rietveld-derived phase content, crystal lattice parameters, and crystallite size of zircon ceramics with Bi 2 O 3 , V 2 O 5 , and B 2 O 3 sintering agent addition (referred to figure 1) using Rietica and MAUD softwares. Numbers in parentheses represent estimated standard deviation to associated average value at its least significant level.

Sample
Rietica GoF Phase composition (mol%) Lattice parameters of zircon MAUD GoF We further investigated the Rietveld-output crystallite size data of zircon as shown in table 1. The software used was employed after the correction in instrument line broadening [27]. Note that crystallite size may differ from grain size, mainly if the grain size is larger than 1 μm [27]. Our data shows that, in general, zircon crystallite revealed a sub-micrometric size ranging from 169 to 249 nm. As expected, the zircon crystallite expands with the sintering temperature and contracts with the sintering pressure. A 3 wt% of liquid phase facilitates the expansion of the grain boundary, while 6 wt% of the phase tends to prevent grain growth in the sintering agent-added ceramics. The grain growth mechanism of ceramic materials during liquid-phase-assisted sintering can be described as follows. When the temperature reaches the melting point of the sintering agent, the liquid phase wets the solid zircon grains. Subsequently, the smaller, thermodynamically less stable grains dissolve into the liquid phase and then nucleate to form a larger one. The grain growth rate was affected by the diffusion process of the smaller grains through the liquid phase to the larger grains [28]. As a result, the spark plasma sintering parameters and the sintering agent addition have affected the structure, i.e. the lattice parameters and volumes, of zircon in the produced ceramics and, hence, its crystallite size. Table 2 presents the properties of the zircon ceramics with various SPS sintering conditions and types of sintering agents. Each of the characteristics of the ceramics is discussed below.

Density and microstructure of the ceramics
The relative density values of the ceramics are presented in table 2. Increases in the relative density of zircon ceramics with sintering temperature, pressure, and sintering agent addition are obvious. The higher relative density is due to the development of liquid phases during sintering on account of the lower melting points of Bi 2 O 3 , V 2 O 5 , and B 2 O 3 , i.e. 824, 690, and 480°C, respectively [29], [30] as compared to the sintering temperatures. The sample with no addition of sintering agent (N0) sintered at 1250°C and 80 MPa attained 97.3% of the theoretical density, and it increased up to 99.9% when the sintering agent was added. The formation of a liquid phase by Bi 2 O 3 addition and its effect on ceramic density was also reported in other materials such as ZnO [31].
Meanwhile, the addition of B 2 O 3 as a sintering agent here is also consistent with our previous results, where the addition of B 2 O 3 can increase forsterite ceramic density to 99.7% [15]. Moreover, the role of V 2 O 5 on densifying ceramic bodies was found in CaO [32] and in NdAlO 3 ceramics [33] at 4 and 1 wt% additions, respectively. The liquid-phase sintering mechanism can be considered to follow three stages [34]: (i) in the initial stage, the liquid was formed, and the solid particles were rearranged. A wetting liquid from the sintering agent provides a capillary force between zircon grains thus improves the mass transport and convinces particle rearrangement, (ii) during the intermediate stage, the smaller zircon grains which thermodynamically less stable dissolve into the liquid-phase and then trigger into larger ones, and (iii) the liquid-phase fills the cavity between zircon grains and then promotes the densification of the ceramics. The liquid-phase sintering process in this work appears to promote the lowering of the sintering activation energy. It thus aids the densification of the zircon ceramics at a lower sintering temperature. As a general conclusion, adding Bi 2 O 3 , V 2 O 5 , and B 2 O 3 as sintering agents plays a great role in manufacturing high-density zircon ceramics by SPS at a relatively low sintering temperature and a relatively short sintering time. The densification of the ceramics was confirmed by SEM observation on the polished and etched surfaces, as shown in figures 2 and 3. The effect of sintering parameters on the densification and grain growth of the sintering agent-free samples can be seen in figure 2. At the same sintering pressure of 40 MPa, the surface of the N0 ceramic sintered at 1350°C is significantly denser than the 1250°C ceramic (see figures 2(a) and (b)). Meanwhile, the sintering pressure also plays a crucial role in the densification of the zircon ceramic (compare figures 2(a) and (c) for different sintering pressures). The evolution of the zircon grain can also be detected here. The grain size increases with rising sintering temperature for additive-free ceramics. A higher sintering temperature allows higher thermal energy for the crystallite to expand, while a higher applied pressure during sintering suppresses grain growth.
Moreover, the sintering temperature and pressure affect the zircon grain size distribution. As shown by figure 2(a), generally, a non-uniform grain size distribution is observed-however, samples with higher sintering temperature and pressure exhibit spherical-shaped grains with relatively uniform size. Figure 3 shows the role of different agents on the microstructure of the zircon ceramics. As described previously, the liquid phase of the sintering agents enhanced the densification of the ceramics. The figure confirms that adding Bi 2 O 3 , V 2 O 5 , and B 2 O 3 has successfully assisted the manufacturing of the dense zircon ceramics over the free-additive ones. During sintering, the grains are soluble in the liquid so that the liquid to the solid wetting initiates and then offers a capillary force to pull the grains collectively, further assisting a faster densification with a shorter time or lower sintering temperature. The liquid phase sintering mechanism with its detailed three stages was as discussed before. At the same additive composition of 3 wt%, the surface of the Bi 2 O 3 -added ceramic prepared by SPS of 1250°C − 80 MPa is significantly denser than the B 2 O 3 -and Moreover, the additives promote the grain growth of zircon. The discrepancies in the densification and promotion of grains growth of the ceramics promoted by the different types of sintering agents can be ascribed to their physical properties. The liquid sintering behaviour of each agent depends on the forces between atoms in them. For example, the bond strength of Bi-O (337.2 kJ mol −1 ) is weaker than that of B-O and V-O. Therefore, with the same sintering temperature or pressure, Bi 2 O 3 is easier to wet ZrSiO 4 grains which causes the reduction of the activation energy to allow more densification.
Further, because of the difference in zircon solid solubility in each liquid, different transport rates are responsible for grain growth and densification take up. Moreover, the addition of a sintering agent affects not only the grain growth but also the grain size distribution. It can be seen that, by the addition of 3 wt%, Bi 2 O 3 is more effective in homogenizing zircon grain size than B 2 O 3 and V 2 O 5 .
The influences of sintering temperature and agent content (6 wt%) on the microstructure of the ceramics can be explained as follows. The surface micrographs of the ceramics with the sintering temperatures of 1350°C and pressure of 40 MPa are shown in figure 4. Sample N0 exhibits spherically-shaped grains with a relatively uniform size (about 1 μm). However, when a 6 wt% sintering agent was added, abnormal grain growth appeared, and the grain shape transformed into plate-like structures (for the NB6 sample) or square-like structures (for the NBi6 sample). Generally, the more the additive, the larger the grain size-as shown in figures 4(a) and (b). A similar inhomogeneous development of ceramic grains due to sintering agents and the temperature was also reported by Wu et al [35]. Table 2 also summarizes the mechanical properties of the zircon ceramics. As shown by the table, the Vickers hardness of the ceramics is 9.52 to 12.66 GPa. The spark plasma sintering parameters provide a significant role in enhancing the hardness. For example, the sintering-agent-free N0 sample, at pressure of 40 MPa, shows a hardness increase of 10.9% due to higher sintering temperature. Meanwhile, increasing the sintering applied pressure, 40 and 80 MPa at the same temperature, increases hardness by 19.4%. Here, the increase in the hardness of the ceramics can be explained by the crystallographic nature of zircon.

Mechanical properties of the ceramics
The hardness of a ceramic describes its intrinsic resistance to plastic deformation regarding the formation and movement of dislocations when a force is applied [36]. At the microscopic level, the crystal structure and its degree of ionic and covalent chemical bonding play a dominant role in controlling the material hardness.   [37]. Furthermore, at the macroscopic level, the increasing Vickers hardness of the sintering agent-added zircon ceramics is attributed to the high densification due to the introduction of a liquid phase, as previously explained. Additionally, Pratapa et al [15] described the dependence of the hardness of a material containing a liquid phase on the number of topological constraints between crystalline phases in the network. They argued the slip of the planes of atoms in the crystalline phases results in dislocations and, thus, permanent deformation. Here, the liquid phase located between zircon grains resists the propagation of the planes and permanent deformation, hence constructing a harder zircon ceramic. We found that the deformation of the sintering agent-free ceramics was lower than the high-density sintering agent-containing ceramics indicating the critical role of the sintering agent.
Further, we examined the elastic moduli of the ceramics by sound velocity measurement. As can be seen in table 2, zircon ceramics reveal Young's modulus E, i.e. 240 − 288 GPa. Previous investigators have reported the Young's modulus (E) of zircon ceramics as 210 [38], 275 [39] and 288 GPa [26]. The zircon ceramics in this work are multiphase, i.e., crystalline oxides of zircon and monoclinic zirconia as well as non-crystalline phases of amorphous silica and additive glass. Each phase contributes to the E value of each ceramic. According to the literatures, the E value of the phases are B 2 O 3 of about 17.1 GPa [40], Bi 2 O 3 of 180 − 195 GPa [41]; V 2 O 5 of 20 − 80 GPa [42], and m-ZrO 2 of 233 − 260 GPa [43]. In general, the Young's moduli of zircon ceramics agree with the fundamental 'rule-of-mixture' by assuming that the very low-value porosity provides insignificant effects.
Further, we examine the bulk modulus B and the shear modulus G. In this study, zircon ceramics exhibit relatively high bulk and shear moduli, i.e. 207 − 267 GPa and 91 − 109 GPa, respectively. In zircon structure, the ZrO 8 dodecahedra are strongly incompressible (B ZrO8 − 280 GPa), compared to the SiO 4 tetrahedra (∼225 GPa) [44], resulted in the large values of the bulk modulus. Meanwhile, the shear modulus characters result from the strong covalent Zr-O bonding due to the hybridization effect between the 4d states of zirconium and oxygen-2p states [45]. The ratio of bulk B and shear G moduli (B/G) is also an interesting parameter. A high and low value is related to the ductility and brittleness of the material, respectively, and the critical value is about 1.75. A material with Pugh's ratio higher than 1.75 shows a ductile deformation by plastic creep, while a material with a smaller ratio than 1.75 exhibits brittle fracturing. The Pugh's ratio for the zircon ceramics in this paper is significantly higher than the critical value, i.e. 1.95 − 2.45 at ambient conditions, indicating that the ceramics are 'quasi-ductile' with high resistance to fracturing. Another plasticity parameter of zircon ceramics in this work is the ratio of H E .
The ratio value is associated with the ability of the material to resist plastic deformation [47]. In this study, the ratio H E V 3 2 * / value of zircon ceramics is 0.011 − 0.019, whereas the well-known machinable ceramic LaPO 4 has a relatively smaller value of 0.008 and a typical brittle ceramic such as α-Al 2 O 3 exhibits a value of 0.0441 [48]. Accordingly, the resistance to plastic deformation of the zircon ceramics can be classified as 'quasi-ductile' behaviour, confirming the discussion in the previous passage.

Thermal properties of the zircon ceramics
The thermal expansion coefficient and thermal conductivity of the zircon ceramics are presented in table 3. In general, we found relatively low linear thermal expansion coefficients, i.e., ranging from 2.3 − 4.0 × 10 −6 /°C, as compared to other ceramics [35]. Typically, the designated coefficient is attributed to the interactions anharmonicity in the material following [49]: where C v is the specific heat, γ is the Grüneisen parameter, and B is bulk modulus which represents the system's anharmonicity related to the phonon frequencies' dependence on the crystal volumes. In a pure harmonic system, the Grüneisen parameter is zero, thus resulting in no thermal expansion. The low value of thermal expansion coefficients in the zircon ceramics in this work is due to small specific heat and crystal lattice anharmonicity [50], [51]. Moreover, a material with strong bonds tends to have a low thermal expansion coefficient. Little changes in bond lengths in a material are due to the strongly bonded polyhedra linked in three dimensions [52]. The zircon thermal expansion mechanism at the elevated temperature is due mainly to the expansion of the ZrO 8 dodecahedra and diminutive change in the SiO 4 tetrahedra [53]. The increased dodecahedral volume associates with the longer Zr−O distance, which can be further related with the shared-edges in the SiO 4 tetrahedra.
The thermal expansion coefficient of bulk materials can also be determined based on the composition of the individual oxides in the samples. In this term, the sintering agents (Bi 2 O 3 , V 2 O 5 , and B 2 O 3 ) also play a role in the low coefficient of thermal expansion of the zircon ceramics. An opened structure material with a low packing density would allow the lattice to partially absorb the thermal energy to allow a larger transverse vibration amplitude perpendicular to the directions of the existing bonds [52]. In this work, the liquid phase of a sintering agent is a typical example, but a sintering agent affects the final thermal expansion coefficient of bulk ceramics differently. Furthermore, a small amount of amorphous silica as the zircon decomposition product may also affect the lowering of the ceramics' thermal expansion coefficient. The effect here, however, is diminutive [54] since the little phase composition in the bulk ceramics, as reported in the previous passage. It is worth noting that zircon-reidite transformation [55], [56] is not observed in this work. Table 3 shows the thermal conductivity of the 1250°C-80 MPa SPS sintered zircon ceramics. In general, the ceramics exhibit an ultra-low thermal conductivity, i.e. ranging from 0.39 to 0.61`Wm −1 K −1 at 373 K, compared to the value reported by Nakamori et al [39], i.e. ∼12 Wm −1 K −1 at 373 K. The behaviour can be explained as follows. The major heat carriers in ceramic materials are phonons, i.e. the quantized lattice vibrations. Using the wave-particle concept of phonons and the kinetic theory, the contribution of phonons to thermal conductivity can be expressed as [57] l = C Vl where l is the phonon mean-free-path, C P is heat capacity, and V is average sound velocity. The calculated mean-free-path for zircon was 3.1 nm at 298 K [58]. The mean-free-path l can be determined by several mechanisms of scattering inside the medium, where the most common ones are: (i) scattering of phonons at boundaries, (ii) scattering of phonons at defects (isotopic impurities, vacancies), and (iii) phononphonon scattering [56]. The ultra-low thermal conductivity of the zircon ceramics in this study is mainly due to the heterogeneous bonding in the polyhedra. The rigid Si-O tetrahedra efficiently transports heat, but the softer Zr-O dodecahedra offer 'weak zones' that scatter phonons [26].
Furthermore, the low thermal conductivities can also be attributed to the enhancement of interfacial thermal resistance resulting from the smaller grains. As the grain size decreases, phonons are scattered at surfaces and interfaces and may interfere with the incoming ones. Then, phonon dispersion is altered, affecting the apparent thermal conductivity [59]. The additional collision mechanism between phonons increases the resistance to heat flow and thus reduces the effective thermal conductivity.
Moreover, the role of the sintering agent addition in the ultra-low thermal conductivity of zircon ceramics could be significant. The amorphous sintering agent phase in the ceramics provides an additional heat blocking mechanism and further reduces thermal conductivity. Therefore, different agents would affect the final thermal conductivity of the bulk ceramics differently. Such behaviour was also reported for amorphous silica [60]. Finally, we obtained high-density zircon ceramics that exhibited ultra-low thermal properties in this work, leading to excellent thermal insulating materials.

Conclusion
The paper presented here evaluated the role of the sintering agent (Bi 2 O 3 , V 2 O 5 and B 2 O 3 ) and the sintering parameters (temperature and pressure) in spark plasma sintering (SPS) to obtain high-density zircon (ZrSiO 4 ) ceramics. The identified crystalline phases from the XRD patterns of the sintered ceramics were zircon and monoclinic zirconia. The physical characterization revealed that up to 99.8% of the theoretical density of zircon ceramics could be obtained by sintering with the addition of any of the sintering agents at a low temperature of 1250°C, the pressure of 80 MPa, and a short holding time of 10 min The densification of the ceramics was confirmed by SEM observation. Furthermore, the ceramics exhibit as hard and quasi-ductile materials, with the Vickers hardness H V , Young's modulus E, bulk modulus B, shear modulus G, Poisson's ratio ν, Pugh's ratio B/G, and the ratio of H E