Crystallization processes, compressibility, sinterability and mechanical properties of La-monazite-type ceramics

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Crystallization processes, compressibility, sinterability and mechanical properties of La-monazite-type ceramics C. Babelot a,b , A. Bukaemskiy a,∗ , S. Neumeier a , G. Modolo a , D. Bosbach a a b

Institut für Energie- und Klimaforschung, IEK-6, Forschungszentrum Jülich GmbH, Germany Zentralinstitut für Engineering, Elektronik und Analytik, ZEA-1, Forschungszentrum Jülich GmbH, Germany

a r t i c l e

i n f o

Article history: Received 6 October 2016 Received in revised form 25 November 2016 Accepted 28 November 2016 Available online xxx Keywords: Monazite Rare-earth elements Thermal analysis Crystal structure Mechanical properties

a b s t r a c t Lanthanide orthophosphate ceramics with monazite structure gained broad interest for several industrial applications. The crystallization processes, compressibility and sinterability of monazite-type lanthanum orthophosphate powder hydrothermally synthesized at 200 ◦ C as well as mechanical properties of the sintered compacts were investigated. Based on a combination of thermo- and surface area analyses, X-ray diffraction as well as scanning electron microscopy studies it was found that the crystallization process occurs at ∼500 ◦ C and the final crystallization of LaPO4 monoclinic phase takes place at 1400 ◦ C. The sintered pellets are characterized by a density of 98% of theoretical density, a Vickers hardness of 5.7 ± 0.1 GPa and fracture toughness of 1.4 ± 0.1 MPa m0.5 . © 2016 Elsevier Ltd. All rights reserved.

1. Introduction Monazite-type monoclinic lanthanide orthophosphates (LnPO4 ; Ln = La − Gd) have gathered considerable attention in the last decades. Natural monazite is a widely distributed mineral in granitic igneous and metamorphic rocks, and alluvial sands [1,2] and the most efficient source for the commercial mining of rare earth elements [3,4]. Synthetic lanthanide orthophosphates and particularly LaPO4 based ceramics came into consideration for several applications due to their outstanding physical and chemical properties, such as high thermal stability, chemical inertness, optical emissivity, and radiation damage resistance. So, they are being considered as promising material as phosphors [5,6] and photonic materials [7,8], as proton conductors [9,10], heterogeneous catalysts in green chemistry [11], high efficient light traps/photon convertors for solar cells [12] and potential nuclear waste forms for the conditioning of tri- and tetravalent actinides [13–18]. In recent years crystalline synthetic monazites have been subject to extensive investigations regarding structure [19–23], sintering [24–26], physical properties [27], chemical stability [28,29] as well as radiation resistance [30]. However, the importance of crystallization processes, compressibility and sinterability

∗ Corresponding author. E-mail address: [email protected] (A. Bukaemskiy).

as well as mechanical properties is often neglected. These aspects play an essential role for the fabrication of highly densified ceramic materials with respect to industrial applications. Here we present detailed investigations of processing and mechanical properties of monazite-type LaPO4 ceramics. In particular the crystallization processes, the compressibility and the sinterability of the powders as well as Vickers hardness and fracture toughness of the obtained sintered materials were studied. 2. Experimental details 2.1. Hydrothermal synthesis of LaPO4 La(NO3 )3 ·6H2 O (Sigma-Aldrich, 99.99% p.a.), NaOH (Merck, ≥98% p.a.), (NH4 )2 HPO4 (Merck, ≥99% p.a.), HNO3 (Merck, 65% p.a.) were used for the synthesis without any additional purification. LaPO4 was prepared by hydrothermal synthesis according to a two-step-method by Meyssamy et al. [5]. This method was chosen because it provides high crystalline materials with welldefined morphology and particle size distribution directly after synthesis [5,31]. The synthesis route can be described in two steps as follows: 1) La(NO3 )3 + 3 NH4 OH → La(OH)3 + 3 NH4 NO3 2) La(OH)3 + (NH4 )2 HPO4 → LaPO4 + 2 NH4 OH + H2 O First insoluble La(OH)3 was precipitated by pouring a 3 mol L−1 aqueous solution of La(NO3 )3 ·6H2 O into 1 mol L−1 NaOH. Subse-

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quently an aqueous solution of 2.7 mol L−1 (NH4 )2 HPO4 was added. The mixture was adjusted to pH = 12.5 using NaOH (4 mol L−1 ), poured into a Teflon beaker and heated up to 200 ◦ C in a stainless steel autoclave for 2 h. The resulting suspension was centrifuged. In order to dissolve excessive lanthanum hydroxide, the precipitate was suspended in HNO3 (100 mL, 0.1 mol L−1 ) and stirred for three days at pH 1. The white suspension was centrifuged at the same conditions again. Next, the precipitate was washed with deionized water in order to remove nitrates coming from the starting materials and dried at 95 ◦ C for 24 h. 2.2. Compressibility and sinterability Prior to pelletization the LaPO4 powder was thoroughly handground in agate mortar and calcined at 350 ◦ C, 500 ◦ C or 950 ◦ C for 2 h, then pressed into cylindrical pellets of 10 mm diameter by cold uniaxial pressing (64–765 MPa) using an Oehlgass, Hahn & Kolb press. The green pellets were sintered at 1400 ◦ C for 3 h in air atmosphere. Detailed investigation of the sintered density s as a function of green density g has been performed in order to determine the optimal pressure. The green density g of the pellets was determined geometrically and the sintered density s by hydrostatic weighing in water (Archimedes method). For further investigations (microstructure and mechanical properties) the sintered pellets were polished carefully with diamond paste (1 ␮m). Prior to investigation of the samples’ microstructure the pellets were thermally etched in air atmosphere at 1000 ◦ C for 3 h. 2.3. Sample characterization

 



 

=

sin  1 + 4ε · L 

The microhardness HV (GPa) of sintered pellets was measured by a diamond Vickers indenter (Anton Paar MHT 10) and determined according to Munz & Fett [37]. The fracture toughness KIc (MPa m0.5 ) was determined applying the indentation crack length method using the Niihara equation for Palmqvist-type cracks [37]. The value of Young’s modulus of LaPO4 (139 GPa) was taken from our investigations recently published by Thust et al. [27] because the value is between other reference data available in the literature: E = 133 GPa [38,39] and 146–156 GPa [40]. 3. Results and discussion

The thermal behavior of the dried powder was studied from room temperature to 1300 ◦ C by thermogravimetry (TG) coupled with differential scanning calorimetry (DSC) (Netzsch, model STA 449C Jupiter) in air atmosphere at a heating rate of 10 ◦ C min−1 . Powder X-ray diffraction (XRD) patterns were collected at room temperature (Bruker D8). The diffractometer is equipped with a Lynx-eye detector adopting the parallel geometry (reflection mode) and using Cu K␣1,2 radiation (␭ = 1.54184 Å) in the 10◦ ≤ 2 ≤ 120◦ range with a step size of (2) = 0.03◦ . The phase compositions of investigated powder were analyzed with the software “Match!” (phase identification from powder diffraction; Crystal Impact). Scanning electron microscope (SEM) investigations were conducted on powder after synthesis and on sintered pellets, using a FEI Quanta 200 environmental scanning electron microscope (ESEM). The values of the grain size and the average grain size were estimated by the linear intercept method [32]. Additionally the particle size was estimated from BET measurements assuming that the particles are spherical. The theoretical density of LaPO4 (5.081 g cm−3 ) was taken from Bregiroux et al. [33]. The mean nanocrystallite size (L) and the lattice distortions (ε) were determined from the XRD data (ˇ, ) using the HallWilliamson method [34] by Eq. (1) ˇ · cos 

Fig. 1. TG-DSC plot of LaPO4 powder from RT to 1300 ◦ C. Inserted grey lines define characteristic temperatures of thermochemical effects occurring during thermal treatment.

(1)

where  is the Bragg angle,  is the wavelength of incident X-rays, and ˇ is the peak half-width corrected to instrumental widening. In order to determine the values of the Bragg angle and the half-width of reflections each Bragg peak were fitted as a sum of Gaussian functions. It is found to be the preferable function for fitting XRD-patterns with narrow particle size distribution [35] that is characteristic for precipitated powder [36].

3.1. Thermal behavior of La-monazite-type powder by means of TG-DSC measurements After the hydrothermal synthesis step the sample is mainly composed of La-monazite (LaPO4 ) and its hydrated form Larhabdophane (monoclinic, LaPO4 ·0.667H2 O) [41,42], which is likely to be present as a minor secondary phase. The thermal behavior of phosphate materials were investigated in [24,43–46]. In this study, the thermal behavior of the LaPO4 powder was investigated from RT up to 1300 ◦ C by TG coupled with DSC. Fig. 1 shows the mass loss (%) (dotted line) and the DSC signal (solid line) as a function of temperature. The thermogram consists of two large regions; an endo-effect region from RT to ∼850 ◦ C and an exo-effect region from ∼850 ◦ C to 1300 ◦ C. A precise definition of the region boundaries is impossible because of a superposition of these effects. For a comprehensive study of the effects caused by thermal treatment a detailed analysis of the TG-DSC thermogram was performed. The temperatures defining effects or areas of interest are illustrated by the grey vertical lines T1 –T6 in Fig. 1. The first endo-effect at 20–130 ◦ C (Tmin = 73 ◦ C) corresponds to the desorption of adsorbed water and gas. The mass loss of these desorption effects is 1.68 %. The second endo-effect at 130–225 ◦ C (Tmin = 173 ◦ C) and the following exo-effect at 290–315 ◦ C (Tmax = 312 ◦ C) are caused by the decomposition of ammonia salts residues [41]. These effects are accompanied by an additional mass loss of 1.26 % (in the thermal region from 130 ◦ C to 520 ◦ C). In the temperature region between T4 = 520 and T5 = 805 ◦ C a weak and wide exo-effect is evident. This effect is associated with a mass loss of 0.67 %. In this region the phase transition from rhabdophane to monazite structure is known to occur [44,46]. The rhabdophane phase was not observed by our XRD phase analyses (see section 3.2 below). Hence, it can be assumed that the rhabdo-

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Table 1 Lattice distortion (ε) and mean nanocrystallite size (L) determined by the HallWilliamson method, mean particle size determined from BET measurements (DBET ) and from SEM investigations (DSEM ) after calcination at different temperatures.

Fig. 2. XRD patterns of LaPO4 powders after thermal treatment at different temperatures.

phane phase in our samples was of very small amount and/or of low crystallinity degree in comparison to the monazite phase. The final mass loss of 0.16 % is observed between T5 = 805 ◦ C and T6 = 1100 ◦ C due to residual dehydroxylation of the material. At temperature above 1100 ◦ C the mass is constant. 3.2. Effects of thermal treatment on structure of La-monazite-type powder studied by X-ray diffraction In order to study the evolution of the crystal structure as a function of temperature, LaPO4 samples were calcined at five different temperatures. The temperatures (Tx1 to Tx5 ) were selected from the TG-DSC study of the La-monazite-type sample at which thermal effects are evident: • Tx1 = 90 ◦ C: drying temperature after synthesis. • Tx2 = 350 ◦ C: elimination of adsorbed water and residues. • Tx3 = 500 ◦ C: before the rhabdophane to monazite phase transition. • Tx4 = 950 ◦ C: after the rhabdophane to monazite phase transition and initial stage of material crystallization. • Tx5 = 1400 ◦ C: sintering temperature of LaPO4 pellets. Fig. 2 shows the XRD patterns of LaPO4 powder calcined at Tx1 to Tx5 , respectively. All diffraction patterns confirm monazite structure for all samples. Even for samples after synthesis characteristic reflexes of the rhabdophane phase (e.g. at 2 ∼15◦ ) are not evident. The XRD patterns of the samples calcined at Tx1 , Tx2 and Tx3 do not differ significantly. The broadened reflexes indicate a low crystallinity of the material. After calcination at Tx4 (950 ◦ C), the reflexes are sharpened as a result of crystallization. Finally calcina-

Tc (◦ C)

ε (%)

L (nm)

SBET (m2 g−1 )

DBET (nm)

DSEM (nm)

90 (Tx1 ) 350 (Tx2 ) 500 (Tx3 ) 950 (Tx4 ) 1400 (Tx5 )

0.038 ± 0.002 0.034 ± 0.003 0.034 ± 0.002 0.014 ± 0.005 0.005 ± 0.003

18.3 ± 0.1 19.2 ± 0.1 19.3 ± 0.1 58 ± 2 71 ± 2

79 ± 0.5 72 ± 0.5 63 ± 0.5 5 ± 0.5 3 ± 0.5

14.9 ± 0.1 16.3 ± 0.1 18.5 ± 0.2 240 ± 25 350 ± 50

23 ± 3

tion at 1400 ◦ C yields a highly crystalline monazite structure of the LaPO4 material. The mean nanocrystallite size L and the lattice distortion ε have been determined by the Hall-Williamson method and serve as an additional information about the crystallization process. The Hall-Williamson plots for the samples calcined at different temperatures are presented in Fig. 3. For all studied samples the values of ˇ·cos() were plotted versus sin(), which in the case of observed linear relationship allows determination of nanocrystallite size and lattice distortion. Finally, the interception point of ordinate and its slope were used to calculate L and ε, respectively. The results are summarized in Fig. 4 and Table 1. The nanocrystallite size L and the lattice distortion ε of the investigated powder are nearly constant at ∼19 nm and ∼0.036%, respectively after drying at 90 ◦ C and calcination at 350 ◦ C and 500 ◦ C (Table 1). After calcination at 950 ◦ C L significantly increases to 58.0 nm while ε decreases to 0.014 %. Further calcination of the material at 1400 ◦ C results in a small increase of the nanocrystallite size up to 71 nm and the lattice distortion is reduced to nearly zero (0.005 %). Based on these results it can be concluded that the crystallization process already starts at temperatures above 500 ◦ C. A more precise determination of crystallization starting temperature requires more detailed investigation in this temperature region. The mean nanocrystallite size L derived from Hall-Williamson method is compared to the particle size D obtained from the surface area (BET) measurements (DBET ) and SEM investigations (DSEM ). The data are included in Table 1. For the powder that was calcined at 90 ◦ C, 350 ◦ C and 500 ◦ C a very good correlation between the nanocrystallite size L and the particle size DBET was found. After calcination at 950 ◦ C and 1400 ◦ C a significant increase of DBET to ∼240 nm and ∼350 nm, respectively is observed, as a result of sintering of initial particles. The nanocrystallite size also increases but not as strong as the particle size. The morphology and particle size (DSEM ) of powder calcined at 90 ◦ C was investigated by SEM. As can be seen in Fig. 5a particles are of spherical shape. They are interconnected and form dense packed aggregates. From SEM images the size of 50 particles was obtained and type of size distribution was determined. It has been shown that the experimental data (bars in Fig. 5b) can be well fitted by a Gaussian function (black curve in Fig. 5b) with following parameters: average particle size DSEM = 23 ± 3 nm and  = 2.9. DSEM value excellently correlates with the values derived from BET (DBET ) and from HallWilliamson (L) methods. Based on the correlation between L and DSEM , in other words the correlation between the visualized particles size and size of nanocrystallites, it can be concluded that we have monocrystalline particles. According to the classification of Dzisko [36] and Van De Graaf et al. [47] these particles are the primary particles. The absence of facets is related to the nanoscale of particle size.

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Fig. 3. Hall-Williamson plot for the studied samples calcined at different temperatures; a) 90 ◦ C, 350 ◦ C and 500 ◦ C and b) 950 ◦ C and 1400 ◦ C.

Fig. 4. Mean nanocrystallite size L (solid line) and lattice distortion ␧ (dashed line) of studied powder as a function of calcination temperature.

3.3. Compressibility and sinterability of La-monazite-type powders Taking into account that LaPO4 is investigated in view of its application as a matrix for the immobilization of nuclear waste,

specific methods of powder processing such as activation milling prior to sintering were not used to prevent dust formation and reduce contamination risks. Traditionally the activation milling is used to increase the powder activity to sintering [48,49]. In our work we used a powder after low temperature calcination (350 ◦ C, 500 ◦ C and 950 ◦ C). This powder is characterized by a substantial level of microdeformation of ε = 0.03, 0.03 and 0.014% respectively. It is expected that the residual lattice distortion should improve the ability of the material to sintering. Moreover, the green density of powder fabricated by wetsynthesis routes is relatively low compared to standard powder as a result of morphological features of these materials. Therefore, the detailed study of powder compressibility and its correlation with sinterability is a very important task. Based on this investigation the optimal compaction pressure Popt to reach the maximal sintered density can be determined. The compressibility of LaPO4 powder was investigated after treatment at various calcination temperatures, Tc = 350 ◦ C, 500 ◦ C and 950 ◦ C. For each Tc , 10 pellets were fabricated by cold-pressing in a pressure range of 64 MPa to 765 MPa with an increment of ∼60 MPa. The relative green (g /t ) and sintered (s /t ) densities as a function of compaction pressure are presented in Fig. 6 for a representative case of LaPO4 powder calcined at Tc = 500 ◦ C. The

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Fig. 5. a): SEM-micrograph of LaPO4 particles after drying at 90 ◦ C and b): particle size distribution determined from the size of 50 particles of the SEM image.

Fig. 6. Relative green ␳g /␳t and the sintering ␳s /␳t densities as a function of compaction pressure P for pellets fabricated from LaPO4 powder calcined at Tc = 500 ◦ C. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

experimental data of relative sintered density were fitted most accurately by a quadratic polynomial function. The values g /t increase monotonically with increasing P. In contrast the values of s /t show a defined maximum at Popt ≈ 450 MPa. This behavior was investigated in detail by traditional utilized logarithmic scale of P for the investigation of compressibility of ceramic powders [49,50]. This type of scale was theoretically justified by Niesz et al. [51]. The experimental data of relative green density for powder calcined at different temperatures are presented in Fig. 7a. Each compressibility curve consists of three linear regions with two characteristic break points at P1 and P2 . The presences of these regions are characteristic for wet-synthesized ceramic powders and determined by their morphological features [47,52]. According to Van De Graaf et al. [47], the wet-synthesized powder consists of primary particles which tend to form dense aggregates and subsequent formation of soft agglomerates. In the first region at a pressure below P1 partial fragmentation and reorganization of the soft agglomerates take place. In the second region between P1 and P2 the soft agglomerates are crushed and rearranged. In the third region beyond P2 the crushing of dense aggregates of primary particles occurs [52]. The pressures P1 and P2 are denoted as the average compression strength of the soft agglomerates [51] and dense aggregates [52], respectively. The characteristic values of P1 , P2 , Popt and (s /t )opt are summarized in Table 2.

Fig. 7. Relative green ␳g /␳t (a) and sintering ␳s /␳t (b) densities as a function of logarithmic compaction pressure for pellets fabricated from powder calcined at different temperatures. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Table 2 Characteristic pressures P1 , P2 , Popt and maximal sintered density (s /t )opt obtained for pellets prepared from powder calcined at different temperatures. Tc (◦ C)

P1 (MPa)

P2 (MPa)

Popt (MPa)

Popt /P2

(s /t )opt

350 500 950

152 168 226

445 480 595

350 450 520

0.79 0.94 0.87

97.8 98.0 97.8

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Fig. 8. Representative SEM-micrographs of La-monazite-type pellet prepared applying the optimized conditions (Tc = 500 ◦ C, cold-pressing at Popt = 450 MPa and sintering for 3 h at 1400 ◦ C): a) pellet surface and b) surface in the location of Vickers indentation.

As can be seen the values of P1 , P2 and Popt significantly increase with increasing the calcination temperature and Popt is close to the break point P2 (Popt /P2 = 0.8–0.94). The optimal relative sintered density (s /t )opt for all studied samples is about 0.98 (98.0 % TD) and only slightly depends on the powder calcination temperature. In [52] suggested to apply special coordinates (/g,2 and log(P/P2 )) for the description of compressibility of wet-synthesized powder. In this case a set of compressibility curves for the powder produced at different conditions can be described with one universal compressibility curve. For investigated LaPO4 powder the three experimental data sets of compressibility presented in Fig. 7a are transferred to one universal curve independent of the calcination temperature (Fig. 7b). For the second and third region which are relevant for the fabrication of a high quality ceramic the correlation factor R2 is very high and equals 0.99 and 0.98, respectively. The pellets prepared from dried powders are sensitive to underand over-pressing [52]. A low compaction pressure yields low densities of green bodies and of pellets after subsequent sintering. At high compaction pressure the “elastic springback” phenomenon occurs [53]. As a consequence cracks are formed in the green bodies. Finally these cracks cause the formation of additional porosity in the microstructure of the pellets after sintering. This effect starts near to the break point P2 and is significantly more pronounced at a compaction pressure that substantially exceeds P2 . Therefore, we can assume that the crack formation due to crushing of dense aggregates (pressed at P > P2 ) and the subsequent formation of additional porosity during sintering is the origin of the observed decrease of relative sintered density of pellets. Apparently at Popt the optimal conditions of powder pressing for subsequent sintering i.e. the most densified packing of dense aggregates before its fracture are realized. 3.4. Vickers microhardness and fracture toughness of La-monazite-type pellets Fig. 8a shows representative SEM micrographs of a LaPO4 pellet prepared at the optimized conditions. The pellet is characterized by a very dense microstructure with typical polygonal grain morphology. The grain size varies from 0.5 ␮m to 5 ␮m, and an average grain size equals 1.7 ± 0.2 ␮m. The mechanical properties of LaPO4 pellets were investigated in terms of Vickers microhardness (HV ) and fracture toughness (KIc ). Fig. 8b shows a representative SEM micrograph of Vickers indentation. In the bottom left corner of the image a typical indenter imprint (square with diagonals) is clearly evident. From the right top edge of indentation square (white arrow) the beginning of a crack can be observed. The crack starts from the indentation edge

and propagates further almost linearly across the grains. This type of crack can be classified as Palmqvist crack according to Munz & Fett [37] and therefore the fracture toughness can be estimated utilizing Niihara equation. In the first step the appropriate load of the micro-indentation was determined. The applied load (Pind ) was varied from 50 g to 350 g with 50 g increments. The value of HV and K1c was calculated as the average of five measurements. The results are presented in Fig. 9d together with representative optical images of indentation formed at different loads (Fig. 9a–c). It is noticeable that the large holes evident in Fig. 9 a–c are caused by the dislodgement of grains from the pellets surface as a result of polishing method prior to HV and K1c measurements and do not represent the porosity of the samples. For the measurements solely pellets have been selected with a theoretical density of about 98% TD confirmed by Archimedes method. The value of HV monotonically decreases with increasing the applied load which is typical for microhardness measurements [54]. On the other hand K1c is characterized by a maximum at ∼150 g. At low values of applied load (Pind = 50 g) the correct measurement of HV and K1c is difficult due to the small size of indentation and length of cracks (Fig. 9a). High values of applied load (Pind > 250 g; Fig. 9c) cause a large damage around the indentation (bright zone in Fig. 9c). In contrast at medium load (150 g < Pind < 200 g; Fig. 9b) the indentations and cracks are well defined. At these conditions the values of HV and K1c are almost constant with minimum value of experimental error. For the precise measurement of HV and K1c the optimal load of Pind = 150 g was chosen. Averaging the results of 12 indentations gives HV = 5.7 ± 0.1 GPa. These values are in very good agreement with the literature data: 5.6 ± 0.4 GPa [38] and 5.0 ± 0.5 GPa [40]. Applying the Niihara equation a K1c value of 1.4 ± 0.1 MPa m0.5 was calculated. The values of reference data 1 ± 0.1 MPa m0.5 [38,40] are slightly lower probably because of different methods of data analysis (Chantikul et al. [55] and Evans & Charles [56] model, respectively). 4. Conclusions The crystallization processes, compressibility and sinterability as well as mechanical properties of La-monazite-type powder were studied. The material has been synthesized by hydrothermal method and from XRD measurements it turns out that the powder crystallizes directly after synthesis in the monoclinic monazite structure. The crystallinity of the material increases during thermal treatment. Based on the combination of thermogravimetric analyses with XRD measurements the material crystallization process

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Fig. 9. The magnified photographs from light microscope showing the indentation imprints in dependence of the applied load: a) 50 g; b) 150 g; c) 300 g. The dark holes are caused by dislodgement of grains from the pellets’ surface during polishing procedure and do not represent the porosity of the pellets. d) Values of Vickers hardness and fracture toughness as a function of applied indentation pressure.

starts at 500 ◦ C and is accompanied significantly by an increase of particles and nanocrystallite size and a decrease of lattice distortion. The parameters for the fabrication of maximal dense pellets have been optimized by an exhaustive compressibility and sinterability study. The compressibility of LaPO4 powder was investigated after calcination at different temperatures (Tc = 350 ◦ C, 500 ◦ C and 950 ◦ C). The relative green density strongly depends on the calcination temperature and increases monotonically with increasing the pressure. By utilizing special logarithmic coordinates the green density of all investigated powders can be described successfully as three linear curves with two characteristic break points which correspond to the average compression strength of soft agglomerates and dense aggregates, respectively. In contrast the values of the relative sintered density show a defined maximum which represents the optimal pressure (Popt ) for the fabrication of pellets with highest sintered density. A further increase of pressure leads to crushing of dense aggregates of the powder and subsequent formation of cracks and as a consequence to the formation of additional porosity after sintering. Therefore, Popt represents the optimal conditions of powder pressing for subsequent sintering to yield highly densified LaPO4 pellets. The optimal relative sintered density is about 0.98 (98% TD) for all samples. The mechanical properties have been analyzed on LaPO4 pellets prepared at optimized parameters (Tc = 500 ◦ C, Popt = 450 MPa) in terms of Vickers microhardness (HV ) and fracture toughness (K1c ) applying the Niihara equation. A special procedure was used for the determination of the optimal applied load (150 g) at which an averaged value for HV and K1c of 5.7 ± 0.1 GPa and 1.4 ± 0.1 MPa m0.5 ,

respectively was determined. These data are in good agreement with the literature data.

Acknowledgments This work was supported by the Ministerium für Innovation, Wissenschaft, Forschung und Technologie (MIWFT) des Landes Nordrhein-Westfalen; AZ: 323-005-0911-0129.

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