Mechanical and dielectric properties of porous lanthanum orthophosphate materials with TiO2 addition via foam-gelcasting

ABSTRACT Lanthanum orthophosphate (LaPO4) powder was prepared by a solid–liquid precipitation reaction of lanthanum carbonate powder with H3PO4 in order to fabricate porous LaPO4 materials. The phase transition and morphology evolution of the reaction products were found to be dependent mainly on the mass ratios of reactants and calcination temperatures. Porous materials using as-synthesized single-phase LaPO4 powders and TiO2 additives via direct foam-gelcasting exhibit large spherical-shaped cells without preferred orientation. The bulk density increases with the increase of sintering temperatures, as evidenced by bigger strut thickness, smaller cell and window sizes and lower inter-connectivity. Porous LaPO4 bulk ceramic with 5 wt% TiO2 addition sintered from 1100°C to 1500°C shows a porosity within 66.80–82.03% and a compressive strength from 4.0 to 17.2 MPa. The compressive strength dependence on the relative density follows a power-law relationship with an exponent value of ~2.45. While imaginary parts of dielectric permittivity of the porous LaPO4 materials measured over the X-band frequencies (8.2–12.4 GHz) depend weakly on the frequency and TiO2 addition, the real parts are affected strongly by TiO2 addition and sintering temperatures and fall within the range between 1.9 and 3.5 in this study.


Introduction
Lanthanum orthophosphates (LaPO 4 ) as advanced ceramic materials possess high thermal stability and excellent oxidation resistance at elevated temperatures [1]. They have a layered crystallographic structure that imparts machinability and toughness to the composites [2]. Its porous counterparts have many additional advantages such as low density, high specific surface area, improved thermal insulation and low dielectric constant. Preparation of such porous materials involves, in general, synthesis of LaPO 4 powder and manufacture of bulk ceramics.
The LaPO 4 powder can be synthesized by the solidliquid precipitation [3,4] or solvothermal reaction [5] using H 3 PO 4 and La 2 O 3 or La(NO 3 ) 3 as the raw materials. The phase transformation and morphology of the final products were influenced by calcination temperatures, synthesis methods, processing conditions and types of raw materials [6,7]. The porous lanthanum orthophosphate materials have been fabricated by template and sol-gel routes so far [8,9]. Porous LaPO 4 ceramics sintered at 1200°C have the micro-hardness less than 350 HV with a porosity of~13% [10]. The porosity of Al 2 O 3 /LaPO 4 composite ceramics with narrow pore-size distribution prepared by cold isostatic pressing was only~40% [11]. The water-based foamgelcasting method possessing the advantages of both direct foaming and gelcasting methods has attracted wide attention as a simple, versatile and low-cost fabrication method for highly porous ceramics [12].
Dielectric properties of dense LaPO 4 materials in the low-frequency range (10 2 -10 7 Hz) [13] and microwave band (8)(9)(10)(11)(12)(13)(14) [14,15] have beeninvestigated. However, detailed studies on dielectric properties of porous LaPO 4 materials are scarce. The present work focuses on the effect of TiO 2 addition on the mechanical and dielectric properties of porous LaPO 4 materials, since TiO 2 addition can be beneficial to the desired mechanical strength and tailored dielectric constants as required in many applications [16,17]. The LaPO 4 powder was prepared by the reaction of lanthanum carbonate (La 2 (CO 3 ) 3 ) powder and diluted orthophosphoric acid. The effects of TiO 2 addition in porous LaPO 4 materials on the phase composition and porosity were also investigated.

Materials
Commercially available rutile powders with an average particle size of 3 μm, lanthanum carbonate powders, and orthophosphoric acid were employed as starting materials for synthesizing lanthanum phosphate powders. N-methylol acrylamide and N, CONTACT Yufu Liu yfliu@seu.edu.cn N′-methylene-bis-acrylamide were used as an organic monomer and crosslinker respectively for direct foam-gelcasting method. Ammonium persulfate was used as a reaction initiator and N,N,N′,N′tetramethylethylenediamine was used as a catalyst during the in-situ gelation processing, Sodium hexametaphosphate was applied as a dispersant and sodium carboxymethyl cellulose was utilized as a rheological additive. Sodium dodecyl sulfate and lauryl alcohol were put into use as a foaming agent and foam stabilizer, respectively. The details of raw materials are listed in Table 1.

Preparation of porous LaPO 4 materials
The lanthanum phosphate powder was synthesized via a direct solid-liquid precipitation reaction. In order to obtain pure lanthanum orthophosphate powders with few or no by-products, 3, 4, 5 and 6 g orthophosphoric acids were mixed respectively with 10 g deionized water before 10 g lanthanum carbonate powders were slowly added into the diluted orthophosphoric acid under magnetic stirring to obtain the white precipitate. The suspension was kept in a water bath (~70°C ) continuously for 4 h under magnetic stirring. At last, the white precipitate was dried at 150°C for 24 h before calcination at various temperatures of 600°C, 1000°C, 1200°C and 1500°C at 5°C/min and hold time of 2 h. The synthesized powders are designated as 10:10:3, 10:10:4, 10:10:5 and 10:10:6 samples, corresponding to the mass ratios of lanthanum carbonate, deionized water and orthophosphoric acid, respectively.
Porous LaPO 4 materials were manufactured by direct foam-gelcasting process [12]. A series of low viscous slurries were obtained by ball-milling for 4 h including 45 g deionized water, and 100 g mixed as-synthesized single-phase LaPO 4 powder and commercial rutile powder (containing 2, 4, 5 and 6 g TiO 2 , respectively). The corresponding samples with various TiO 2 contents are designated as LT2, LT4, LT5 and LT6, respectively. Foamed slurries were stirred mechanically until the foam volume reached~350 mL in a scaled beaker compared to the non-foamed suspension. The adequately stirred and well-foamed slurries were poured into a plastic mold and gelled in~15 min. After demolding, the green bodies were dried at 50°C for 48 h, and then sintered at 1100°C, 1200°C, 1300°C, 1400°C and 1500°C respectively with 2°C/min and hold period of 2 h. The corresponding LT5 porous materials sintered at various temperatures are denoted as LT5-1100, LT5-1200, LT5-1300, LT5-1400 and LT5-1500, respectively.

Characterization
The phase structure of lanthanum phosphate powders after various calcination temperatures and porous LaPO 4 ceramics with TiO 2 addition was identified using X-ray diffraction (XRD, D8 Discover, Bruker, Germany) analysis in the spectra range of 2θ = 10-70°with a step length of 0.15°/s. The powder morphologies and microstructure of porous LaPO 4 samples were investigated with a scanning electron microscopy (SEM, Sirion 6700 F, FEI, Netherlands). The elemental composition of the LaPO 4 powder was measured by an attached energy dispersive spectroscopy (EDS, Genesis 60 S). The weight and volume of a porous specimen are used to obtain its bulk density, and the porosity was established by the following equation: V ¼ ð1 À ρ=ρ 0 Þ Â 100%whereρis the bulk density of the porous sample and ρ 0 (5.12 g/ cm 3 ) is the density of dense LaPO 4 [18]. The compressive strength was tested on a mechanical testing machine (CMT4503, SANS, Shenzhen, China) using the specimens sized about 20 mm×20 mm×15 mm. The crosshead loading speed is 0.5 mm/min. The samples of sintered porous materials were machined with sizes of 10.16 mm × 22.86 mm × 3 mm and dried at 120°C for 0.5 h after ultrasonic cleaning in ethanol for 15 min. Then, the room-temperature dielectric property of the dried samples was measured by the waveguide method over the X-band frequencies (8.2-12.4 GHz) with a networker analyzer (N5230 C, Agilent, USA).

Phases and morphologies of lanthanum phosphate powders
The XRD patterns of lanthanum phosphate powders calcined at 1500°C with various mass ratios of H 3 PO 4 are shown in Figure 1. The monoclinic LaPO 4 (JCPDS # 32-0493) [6] is the dominant phase in all sample powders, while traces of La 3 PO 7 (JCPDS # 49-1023) [19] for a mass ratio of 10:10:3 and La(PO 3 ) 3 (JCPDS file # 33-0718) [20] for 10:10:5 and 10:10:6 are also observed. Relatively pure monoclinic LaPO 4 phase was obtained only for a mass ratio of 10:10:4 ( Figure 1(b)), so powder under this processing condition was used subsequently. Figure 2 illustrates the XRD patterns of the as-synthesized white precipitate calcined at various temperatures. The broad peaks demonstrate an incomplete crystallization of the hexagonal phase LaPO 4 ·0.5H 2 O (JCPDS #46-1439) [6] dried at 150°C because of the following reaction: The XRD spectra of the powder calcined at 600°C shows broad and low-intensity peaks of monoclinic structure LaPO 4 owing to the dehydration of the hydrated lanthanum phosphates [21]: LaPO 4 Á 0:5H 2 OðsÞ ! LaPO 4 ðsÞ þ 0:5H 2 OðgÞ " Such a hexagonal to monoclinic transformation is irreversible. Further elevated thermal treatment at 1000°C leads to sharp diffraction peaks of LaPO 4 phase and additional characteristic peaks of La(PO 3 ) 3 phase [21]: The XRD pattern for powder samples calcined at 1200°C is similar to that of 1000°C. According to Figure 1(b), after calcination at 1500°C, the phase of La(PO 3 ) 3 disappears [19,22]: The reaction and phase transformation mechanisms to generate LaPO 4 from La 2 (CO 3 ) 3 and H 3 PO 4 are believed to be similar to these reported previously, although different reactants were used in the literature [21,23]. The evolution characteristics in morphology of lanthanum orthophosphate powders calcined at various temperatures for 2 h are depicted in Figure 3. The powder calcined at 150°C is agglomerated and composed of many nano-sized needle-like assemblies in Figure 3(a). At 1000°C, the lanthanum orthophosphate particles have undergone significant shrinkage in the form of uniform spherical nanograins (Arrowhead "A" of Figure 3(b)) with diameters of~300-500 nm. The spherical nanograins appear to be connected by low melting-point La(PO 3 ) 3 phases (Arrowhead "B" of Figure 3(b)). At 1200°C, hard agglomerates were generated due to grain coalescence to brick-or rod-like forms (Arrowhead "C" and "D" of Figure 3(c)). After calcination at 1500°C, these rod-like particles showed layered structures. The EDS analysis in Figure 3(e) revealed the mass contents of O, P and La elements are about 63.86%, 10.73% and 25.41%, respectively, which gives the atomic ratio close to 1:1:4 and a direct evidence that the powder is composed of stoichiometric LaPO 4 . Figure 4 depicts the XRD patterns of the porous LaPO 4 materials with different TiO 2 contents sintered at 1500°C for 2 h. Polycrystalline LaPO 4 and rutile phases are detected. With increasing addition of rutile from Figure 4(a-d), the intensity of the rutile phase grows stronger, while the lanthanum orthophosphate phase in the presence of TiO 2 remains stable with no new compounds.

Phase structure and microstructure
The bulk density and porosity of the LT5 specimens sintered at different temperatures are plotted in Figure 5. With increasing sintering temperatures from 1100°C to 1500°C, the porosity of porous LT5 ceramics decreases from 82.03% to 66.80% and the bulk density increases  from 0.92 to 1.70 g/cm 3 . The TiO 2 additive is beneficial to a faster densification rate above 1300°C [24][25][26].
The microstructure of porous LaPO 4 materials sintered at various temperatures is presented in   windows and only a small portion is closed cells (isolated pores) (Figure 6(a)). It is believed that the large spherical cells are generated by mechanical foaming, and the small windows are due to the merger of air bubbles. In the highly individual SDS-foamed slurries, the air bubble may grow in size and coalesces, leading to the porous ceramics with smaller strut thickness, larger cell and window sizes and bigger permeability [12,27]. Very small-sized pores located at the grain boundaries are also observed after sintering at 1100°C (see the inset in Figure 6(a)), which indicates a hierarchical pore structure. With the sintering temperature elevated to 1500°C shown in Figure 6(b), the size of cells and windows is decreased due to the densification. Closely packed spherical/equiaxial LaPO 4 grains are also confirmed. The mean grain size increases in the range from~1 μm to~5 μm. The SEM micrographs of porous LaPO 4 materials sintered at 1500°C for 2 h with respect to various contents of TiO 2 additives distinctly manifest uniformly distributed open porosity in sintered porous materials ( Figure 6 (c,d)). The microstructures of porous LaPO 4 materials sintered at 1500°C indicate that samples with various TiO 2 contents exhibit almost a similar pore structure with no obvious difference. The amount of the TiO 2 additive has no significant effect on the bulk density after sintering at 1500°C [28].  thereby improving the compressive strength. The compressive strength of LT5-1500 sample is as high as 17.2 MPa while the porosity was 66.80% due to the strengthening of sintering struts and less micro-cracked framework. The compressive strength of porous materials can be obtained according to the Gibson and Ashby model [29,30]: σ=σ 0 ¼ Cð1 À VÞ 3=2 ¼ Cðρ=ρ 0 Þ 3=2 where σ and ρ are the compressive strength and bulk density of a porous sample, respectively; C is a constant;σ 0 andρ 0 are the theoretical strut strength and density of dense LaPO 4 , respectively. As shown in Figure 7(b), in the double log plot, the slope of compressive strength versus relative density of porous LT5 ceramics is 2.448. The variation between the exponent value determined in this work and that reported in the Gibson and Ashby model can be owing to the different microstructures. In addition, after sintering at 1500°C, the compressive strength of porous materials with various TiO 2 contents is believed to be close to each other due to the similar pore structures shown in Figure 6. The compressive strength is mainly related to the porosity according to the model. Figure 8 displays the measured real and imaginary permittivity of the porous LaPO 4 samples with different TiO 2 contents sintered at 1500°C for 2 h in the X-band. The mean real part of dielectric permittivity increases slightly in the range from 2.9 to 3.5 as the TiO 2 content was added from 2 wt% to 6 wt% owing to the large dielectric constant of rutile (ε' =~100 [16]). The waveguide method is easy to measure the complex permittivity, but the deviation of the imaginary permittivity is complicated, particularly when the dielectric loss is small. Figure 9 represents the measured real and imaginary permittivity of LT5 porous ceramics sintered at  various temperatures in the X band. The mean real permittivity of LT5 porous materials increased from 1.93 to 3.20 with the decrease of porosity from 82.03% to 66.80% when the sintering temperature is elevated from 1100°C to 1500°C. The real permittivity of porous materials is smaller than that of dense LaPO 4 (ε' =~11 [14]) owing to the high porosity. The real permittivity is influenced by the TiO 2 contents and porosity variation. The imaginary permittivity is similar to the result shown in Figure 8. At some frequencies tested, the imaginary permittivity shows abrupt changes due to the measurement accuracy. Overall, the porous LaPO 4 materials have a low dielectric loss. The complex permittivity tailored by the TiO 2 addition and sintering temperatures will have potential applications for microwavetransmitting materials. The relationship between the effective permittivity and porosity of the LT5 porous ceramics is shown in Figure 10. The real permittivity of the LT5 porous ceramics can be concerned with that of fully dense ceramics and porosity according to the Maxwell-Garnett model and the Bruggeman model [31]. Due to the hierarchical pore structures, the mean real permittivity of LT5 porous ceramics is located below by the form of the Maxwell-Garnett model and above by the form of the Bruggeman model. Obviously, there is a large difference between the effective permittivity predicted by the two models and the experimental data. Therefore, it is necessary to find a suitable model to predict the relationship between the effective permittivity and porosity of the porous lanthanum orthophosphate ceramics in the next work.

Summary
Relatively pure lanthanum orthophosphate powder was synthesized by a solid-liquid precipitation reaction of La 2 (CO 3 ) 3 powder, deionized water and H 3 PO 4 with a reagent mass ratio of 10:10:4 and calcination temperature of 1500°C. The phase is transformed from hexagonal phase LaPO 4 ·0.5H 2 O to monoclinic LaPO 4 phase with typical characteristics of layered structures. Highly porous LaPO 4 materials with controllable porosity and hierarchical pore structures were prepared by direct foam-gelcasting method using the assynthesized LaPO 4 powder and commercial TiO 2 powder as raw materials. The porous materials exhibit approximately large spherical cells with isotropic properties. Open cells show smaller connected windows and thicker strengthening struts with the increase of sintering temperature. The compressive strength varies in the range of 4.0 to 17.2 MPa and the corresponding porosity decreases from 82.03% to 66.80% for porous LT5 ceramics as sintering temperature increases from 1100°C to 1500°C. The power-law relationship of the compressive strength on the relative density fitted in this work has an exponent of~2.45. The real permittivity tailored by the TiO 2 addition and sintering temperatures is ranging from about 1.9 to 3.5 and the change of imaginary part is complicated. The porous LaPO 4 materials prepared in this work will be potential for microwave-transmitting materials.