Direct observation of micro-particle porosity changes in solid-state

Article

Direct observation of micro-particle porosity changes in solid-state vaterite to calcite transformation by Coherent X-ray Diffraction Imaging Oxana Cherkas, Thomas Beuvier, Dag W. Breiby, Yuriy Chushkin, Federico Zontone, and Alain Gibaud Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b00476 • Publication Date (Web): 12 Jun 2017 Downloaded from http://pubs.acs.org on June 19, 2017

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Crystal Growth & Design

Direct observation of micro-particle porosity changes in solid-state vaterite to calcite transformation by Coherent X-ray Diffraction Imaging O. Cherkas1‡, T. Beuvier1,2‡*, Dag W. Breiby3, Y. Chushkin2, F. Zontone2, and A. Gibaud1* 1- LUNAM, IMMM, UMR 6283 CNRS, Faculté des Sciences 72085 Le Mans Cedex 09, France 2- ESRF, The European Synchrotron 71, avenue des Martyrs, 38043 Grenoble Cedex 09, France 3- Department of Physics, Norwegian University of Science and Technology (NTNU), Høgskoleringen 5, 7491 Trondheim, Norway *Author

to whom correspondence should be addressed: Alain Gibaud, Thomas Beuvier

E-mail: [email protected], [email protected] Abstract: The simplest route to synthetize porous calcium carbonate in large quantity is to mix concentrated aqueous solutions containing Ca2+ and CO32- ions. The formed vaterite microspheres have a porous structure, but are not thermodynamically stable. Heating above 350°C induces a solid-state transformation of vaterite into the most stable phase, calcite, while maintaining an unusual spheroidal morphology. Here, by using three-dimensional coherent X-ray diffraction imaging, the morphological evolution associated with the thermally induced phase transition is studied. We observe that despite an overall similar pore volume, the pore geometry differs markedly before and after annealing. Before annealing, the microspheres display elongated and nanometer sized pores while after annealing they exhibit large and open pores. During transition, the specific surface area decreases from 7 m²g-1 for vaterite to 3 m²g-1 for calcite. The general trend resulting from 3D observations is that the solid state phase transition is not only governed by the decrease of the Gibbs bulk free energy change (∆Gbulk ~ -3 kJmol-1) but is largely influenced by the surface energy change (∆Gsurf ~ -0.1 kJmol-1m-²). The porous calcite microspheres produced by this facile two-step process may have potential use as low-density filler in paint, paper, pharmaceutical and plastic industries.

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Direct changes

observation in

of

solid-state

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micro-particle vaterite

to

porosity calcite

transformation by Coherent X-ray Diffraction Imaging O. Cherkas1‡, T. Beuvier1,2*‡, Dag W. Breiby3, Y. Chushkin2, F. Zontone2, and A. Gibaud1* 1- LUNAM, IMMM, UMR 6283 CNRS, Faculté des Sciences 72085 Le Mans Cedex 09, France 2- ESRF, The European Synchrotron 71, avenue des Martyrs, 38043 Grenoble Cedex 09, France 3- Department of Physics, Norwegian University of Science and Technology (NTNU), Høgskoleringen 5, 7491 Trondheim, Norway *Author

to whom correspondence should be addressed: Alain Gibaud, Thomas Beuvier

E-mail: [email protected], [email protected] KEYWORDS: Calcium carbonate, solid-state transition, coalescence, sintering, porosity


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ABSTRACT

The simplest route to synthetize porous calcium carbonate in large quantity is to mix concentrated aqueous solutions containing Ca2+ and CO32- ions. The formed vaterite microspheres have a porous structure, but are not thermodynamically stable. Heating above 350°C induces a solid-state transformation of vaterite into the most stable phase, calcite, while maintaining an unusual spheroidal morphology. Here, by using three-dimensional coherent X-ray diffraction imaging, the morphological evolution associated with the thermally induced phase transition is studied. We observe that despite an overall similar pore volume, the pore geometry differs markedly before and after annealing. Before annealing, the microspheres display elongated and nanometer sized pores while after annealing they exhibit large and open pores. During transition, the specific surface area decreases from 7 m²g-1 for vaterite to 3 m²g-1 for calcite. The general trend resulting from 3D observations is that the solid state phase transition is not only governed by the decrease of the Gibbs bulk free energy change (∆Gbulk ~ -3 kJmol-1) but is largely influenced by the surface energy change (∆Gsurf ~ -0.1 kJmol-1m-²). The porous calcite microspheres produced by this facile two-step process may have potential use as low-density filler in paint, paper, pharmaceutical and plastic industries. INTRODUCTION

Calcium carbonate is one of the most abundant materials on earth. It is found in the earth crust as karstic concretions, in caves as stalactites or stalagmites, and in the sea bed. It is presently considered a key material in the remediation of anthropogenic CO2 by oceans.1 In natural systems, the control exerted on biomineralisation has led to an extraordinary variety of microstructures offering specific functions. The calcite photonic microlenses encountered in the


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ossicles of a brittlestar species, Ophiocoma wendtii, confer optical properties by focusing the light towards photoreceptors.2 Other functions are reported such as the high resistance to fracture of nacre.3 Understanding how these organisms achieve such a control on the nucleation and growth represents a major challenge in the field of biomineralization. In parallel, a lot of attention has been paid to biomimetic synthesis. The use of organic additives and/or templates with complex functionalization patterns allows developing calcite porous superstructures, such as spherical-4, helical-5, and pyramidal-shaped6 microparticles. These complex forms are fascinating, as they tend to mimic biomineralization and exhibit high porosity. However, for industrial applications they have the drawback of being synthesized in low quantities. The reason is that the precipitation takes place at low saturation degree, i.e. at low precursor concentrations, generally with a Ca2+ initial concentration lower than 0.02mol/L, limiting the amount of produced material. Calcium carbonate is extensively employed as a filler, notably in plastics, paint and paper industries. In such applications, calcium carbonate is used mainly to reduce the production costs and/or to improve the optical properties. It is thus of great interest to be able to efficiently produce low-density porous calcite microparticles with a high yield of synthesis. One facile route consists in mixing two aqueous solutions, one containing Ca2+ ions, and the other containing CO32- ions. By using highly concentrated precursors and no additives, the precipitation of CaCO3 leads to the formation of vaterite microparticles at ambient temperature. The formation of vaterite is preceded by the precipitation of amorphous calcium carbonate (ACC) which exhibits several hydration states.7 The stability of ACC depends on the particle size and hydration level.7-9 The total enthalpy change during the hydrated ACC (with composition close to CaCO3:H2O) to


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vaterite transition through dehydrated ACC is quite large (∆H ~ - 21kJ/mol).9 Without additives and close to room temperature, spherical ACC nanoparticles (NPs) aggregate rapidly forming assemblies of NPs up to a few micrometer in diameter and then transform into vaterite microspheres (denoted MS)

as shown in in situ synchrotron-based small-angle X-Ray

scattering10 and energy dispersive X-ray diffraction11 analysis. The transformation of ACC into vaterite may be initiated by a local dissolution/reprecipitation process, mediated by surface water followed by a solid-state transformation operating via a dehydration/ordering process.12-13 The resulting polycrystalline vaterite usually exhibits a spherical morphology with high porosity14 and may even have a hollow core.15,16 Vaterite is thermodynamically the least stable variety of the three crystalline forms of CaCO3. In aqueous media it transforms into calcite in the course of a few hours at ambient temperature through a dissolution/recrystallization process.11 The driving force of the transformation is the change of Gibbs free energy, estimated to be ∆G = - 2.7 to -3.4 kJ/mol at 25 °C.17,18 This transformation does not take place through a solid-state process as its activation energy is between

250

and

580

dissolution/recrystallization

kJ/mol19-21 process.22

compared In

to

55

the solvent

kJ/mol mediated

estimated

for

the

transformation,

the

recrystallization of calcite does not occur immediately after the dissolution of vaterite. The dissolved matter is generally transported out of the vaterite microsphere by ion diffusion and/or convection which leads to the nucleation and growth of non porous (compact) calcite monocrystals outside the polycristalline vaterite microspheres. Hence, the shape and the pore geometry of the vaterite particles after recrystallizing into calcite are totally changed. Alternatively, vaterite can be transformed in dry conditions above 350 °C through a solid-state transition.21,23-25 This transformation proceeds by atomic transport through the vaterite/calcite


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interface characterized by short-distance internal rearrangements of atoms. The formed calcite particles are thus polycrystalline and attain similar morphological features as the vaterite primary particle, i.e. the calcination of polycrystalline vaterite microspheres leads to the formation of polycrystalline calcite microspheres. Under such conditions, vaterite may be considered a template precursor for calcite formation. Hence, it may be possible to obtain porous calcite particles through a two-step approach, i.e. 1) synthesis of vaterite microparticles in aqueous media and 2) calcination into calcite under dry conditions above 350 °C. The aim of this paper is to answer two questions. First, what is the 3D architecture of primary vaterite microspheres produced in high initial saturation degree? What are the pore geometry, the pore volume and the specific surface area of these particles?

Second, how does this 3D

architecture change after a thermally activated solid-state transformation into calcite and what is the evolution of the Gibbs surface free energy compare to the total Gibbs free energy change during the transition? To answer these questions, we used three dimensional coherent X-ray diffraction imaging (CXDI).26 This technique is a powerful tool for visualizing the outer and the inner structures of microparticles with a resolution of a few tens of nanometers.27,28 CXDI has the great advantage of providing a 3D image in real space, thus giving access to a full description of the morphology of the particles including the porosity and the specific surface area. Contrary to small-angle X-ray scattering (SAXS) and N2 absorption methods, no assumptions of a structural model are necessary to estimate the porosity. CXDI on isolated microspheres shows that a short annealing time leads to a vaterite-to-calcite phase transition without modifying the total pore volume of the microspheres. However, the specific surface area drops significantly. A profound reorganization of the pore geometry takes place from small and anisotropic/elongated pores for vaterite to a bimodal pore geometry for calcite with many small/closed/spherical pores


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and a large/open/unshaped pore. These results clearly demonstrate that the formation of porous calcite microparticles is also driven by the change in the surface free energy during the solidstate transformation.

EXPERIMENTAL SECTION

Synthesis of calcite microparticles. First step: the formation of vaterite microspheres. The crystallization of vaterite particles was performed by rapidly mixing equal volumes (500 ml) of molar solution of calcium chloride (0.5 M, CaCl2.2H2O) and sodium carbonate (0.5 M, Na2CO3.H2O) at ambient temperature. The time of mixing under vigorous stirring was 300 s. After mixing the suspension was filtered and the residual powder washed with the ethanol. The solids were kept dry in oven at 50 °C for 3 h. Second step: the solid-state transformation. The solid-state transformation of vaterite to calcite was achieved by heating the previously prepared powder in an oven at 420 °C for 1h and by cooling down in the oven progressively to ambient temperature. This temperature was low enough to avoid the decomposition of CaCO3 into CaO. For the kinetics study shown in Fig. 2b, the particles were annealed at 420 °C for 1, 2, 3, 5 and 10 min and cooled down outside the oven. Diffraction analysis. The polymorphism of the calcium carbonate particles was analyzed by Xray powder diffraction using a Panalytical Empyrean diffractometer equipped with a copper anode (CuKα, λ = 1.5418 Å) working at 40 kV and 30 mA and a Pixel 2D detector. The diffractograms were analyzed using the MAUD software.29


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Scanning electron microscopy (SEM). In order to check the reliability of the CXDI reconstruction method, we performed SEM on the same particles by using a LEO 1530 scanning electron microscope with 20 keV acceleration, 9 mm working distance and in the secondary electron detection mode. Samples were metalized with ~5 nm of gold. Note that CXDI was done before the SEM observations, i.e. on non-metalized microparticles. SEM images of ensembles of particles were made with a JEOL JSM-7100F with 5 kV acceleration, 10 mm working distance, using the secondary electron mode of detection. These samples were metalized with platinum. Brunauer-Emmett-Teller (BET). N2 adsorption (BET)30 was carried out at 77 K using Coulter SA3100 in order to compare the average specific surface area of a large number of microparticles with the one determined from CXDI on individual microsphere. CXDI measurement. The CXDI experiments were performed at the ID10 beamline at the European synchrotron radiation facility (ESRF) in France using 8.1 keV X-rays from an undulator.28 The coherent primary beam from a Si(111) monochromator was selected to 10x8 µm HxV (FWHM) by rollerblade slits 50 cm up-stream of the sample. The scattering patterns were collected by a Maxipix detector (256×256 pixels) with a 55 µm × 55 µm pixel size located at 2.54 m from the sample.31 Hence, the voxel size in the real space is 13.8×13.8×13.8 nm3. The intense direct beam was blocked by a beamstop to prevent damage to the detector. Complementary technical information about the CXDI experimental design and results treatment explanations are provided in the Supporting Information (S1). Methodology of experiment: particles for CXDI were dispersed on the surface of a Si3N4 membrane (thickness 100 nm) under dry conditions. Isolated particles on the membrane surface were then selected using an optical microscope. The position of the chosen particle was carefully


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adjusted to the center of the coherent X-ray beam. The small angle X-ray scattering measurements were done at incident angles ranging from -80° to 80° in steps of 0.5 degree with respect to the normal of the membrane surface. At each angle, the scattering pattern was collected on the 2D detector using an exposure time of 16 s. A sketch of the CXDI experiment is shown in Figure 1(a).

Figure 1. a) Sketch of the CXDI set-up. The measurements were done in far-field conditions. b) 2D slices of the 3D real space matrix before (at left of the arrow) and after (at right of the arrow) the subtraction of a 3D Gaussian function for vaterite microsphere and for calcite microsphere. Scale bar = 1µm. In far-field conditions, the intensity distribution I(q) is proportional to the modulus square of Fourier transform F(q) of the 3D electron density of the particle ρ(r), I(q)~|F(q)|2 with F(q)=|F(q)|exp(iφ(q))=FFT(ρ(r)).32 To obtain the image of an object in real space, we must know the phase φ of the wave at the detector plane, but this information is lost since we measure


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intensities. However, it is possible to recover the missing phase by an iterative retrieval algorithm (e.g. the Hybrid Input-Output algorithm) if speckles are oversampled (Figure S1, S2 see SI).33 The 3D electron density distribution of the object is directly obtained by phasing the 3D Fourier space assembled from the 2D I(q) measurements at each tilt angle.28 The convergence of the iterative algorithm was reached after ~ 1000 iterations and several 3D reconstructions were averaged to reduce noise and smooth small random high frequency variations (SI Figure S3). A problem encountered in the reconstruction of particles by CXDI was the presence of low-frequency artifacts in the electron density with densities that either are over or under estimated. For instance, pores exhibited in some cases electron densities values that differ from zero and density variations could be seen in materials that are known to be homogeneous. As discussed elsewhere34, these density variations are artefacts most likely arising from the fact that the scattered intensities near the direct beam, corresponding to the lowest spatial frequencies, could not be measured. In real space, the intensity variations strongly resemble ‘unconstrained modes’ as reported by Thibault et al.34 To remedy these density variations, a simple spatial flattening of the electron density was done for each reconstruction by subtracting in real space a 3D Gaussian distribution centered at the mass center (Figure 1b). After subtracting the 3D Gaussian, voxels that had acquired a negative density value, were set to zero. This procedure (3D Gaussian subtraction and zero setting) was important to accurately determine the mass of CaCO3, the porosity and the specific surface area from Chimera35 software. RESULTS

The synthesis of vaterite from 0.5 M initial solutions allowed producing large quantities of CaCO3 (~ 20 g per synthesis) with a very high yield (~85 %). X-ray diffraction revealed that the


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product contained 96 % of vaterite (Figure 2a). The remaining 4 % of calcite microparticles had a

rhombohedral

morphology

(see

SI,

Figure

S5)

which

came

from

a

partial

dissolution/recrystallization process of the vaterite microspheres. After annealing at 420 °C for 1h in dry conditions, vaterite was totally transformed into calcite during the precipitation. X-ray diffraction of particles heated for 1 to 10 min at 420 °C revealed that vaterite was totally transformed into calcite after 3min at 420 °C (Figure 2b). The transformation was thus far faster than the one reported by Rao21 who synthesized vaterite by using metaphosphate. The absence of additives in our synthesis may explain the fast kinetics.

Figure 2. (a) Calculated (full line) and measured (open circles) X-ray diffractograms at ambient temperature for the vaterite and the calcite microparticles. Bragg reflections are indicated with vertical markers below the profile for vaterite (ICSD 15879) and calcite (ICSD 73446). (b) Calcite fraction as a function of the annealing time at 420 °C. The dashed line is a guide to the eye. (c,d) Size distribution extracted from SEM images for (c) vaterite and (d) calcite with SEM images in inset.


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SEM images show that the vaterite and calcite microspheres have an average diameter of 1.6±0.5 µm (Figures 2c,d). The specific surface area determined by nitrogen adsorption at 77 K is SBET = 7.0 m²/g for vaterite and 2.7 m²/g for calcite. Even though the thermally activated phase transformation decreased the specific surface area by a factor ~2.6, the one of calcite is high enough to reveal an inner porosity. Indeed, the specific surface area of a smooth and full sphere of size 1.6 µm and density 2.7 g/cm3 was estimated to be 1.4 m²/g, almost twice lower than the one measured by BET measurements. In addition, the SEM images reveal that calcite microspheres displayholes at its surface, suggesting the existence of an open porosity (Figure 2d). To analyze the inner porosity, CXDI was used on isolated microspheres of vaterite (Figures 3a-d) and calcite (Figures 3f-i). The diameters (1.7-1.8 µm for vaterite and 1.5-1.6 µm for calcite) are close to the average sizes determined from SEM analysis (Figures 2c,d). So the chosen particles are representative of the particle population. The CXDI reconstructed particle exterior surfaces were compared to SEM images of the same particles. The agreement between CXDI and SEM is excellent as shown for vaterite (Figures 3a,b) and calcite (Figures 3f,g). Figure 3c shows a 3D morphology of vaterite after artificially cutting away one fourth of the particle. It reveals the presence of meso- and macropores for a total pore volume of 17±5 %. This value does not include the pores smaller than ~20nm due to the resolution limit. The calculated specific surface area of this particle determined from Chimera software was found to be 7.3±0.8 m²/g, which is close to the value determined from BET measurements (SBET=7 m²/g) on the overall particles. A careful observation reveals that most of the pores tend to be radially elongated, i.e. the pores tend to be directed toward the center. This trend can be seen in Figure 3d where three 2D slices show the inner porosity in white color. The porous network contains


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mainly open pores, i.e. most of the pores of the particles are connected to the exterior. In addition, the porosity is not homogeneously distributed in the microsphere. The central part of particle reaches a local porosity of 35 % and the porosity decreases with the radial distance from the center (curve in yellow on Figure 3e). These observations are in agreement with previous SEM analysis on broken particles which usually reveals that the core appears more porous that the periphery of the microspheres.14 We have not found a clear explanation for this selforganized porous network. The pseudo-radial organization may originate from specific interactions between the vaterite nanoparticles inside the microspheres leading to ordered agglomeration (i.e. ordered self-assembly) as encountered in the formation of mesocrystals in many systems implying the growth of vaterite.36-39 The porosity may have different origins. The porous channels could provide a conduit for the loss of water during dehydration of ACC and its transformation into vaterite. They may also result from a partial Ostwald ripening process (dissolution of the smallest nanoparticles of vaterite at the benefit of the bigger ones), and/or from the partial dissolution/recrystallization into calcite rhombohedra. The latter hypothesis does not explain the total porosity (~17 %) because the percentage of calcite rhombohedra (4 % from XRD measurements) is far lower than 17 %.


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Figure 3. Images of (a-d) vaterite and of (f-i) calcite microspheres . (a,b,f,g): Images of (a,b) vaterite and of (f,g) calcite microspheres observed by (a,f) CXDI and (b,g) SEM. (c,h) CXDI images of the microsphere of (c) vaterite and (h) calcite after cutting one fourth of the volume. (d) 2D slices of the vaterite microsphere showing the inner porous structure in white color. The dotted lines represent the contour of the particle. (e) Distribution of porosity as a function of the radial distance from the center of the microspheres. Porosities were determined from azimuthal averaging. (i) CXDI of the calcite microsphere showing the open (in blue) and closed (in yellow) porosity. After annealing at 420 °C for 1h in dry conditions, the phase transformation led to a microsphere of calcite with a smooth surface. This is a clear indication of the sintering of the vaterite nanograins into a macroscopic calcite particle. From Chimera software, the calculated specific surface area for one particle is S = 2.9±0.2 m²/g and is thus close to the one measured by


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BET (SBET = 2.7 m²/g) on the overall particles. As for vaterite, the chosen calcite particle is thus representative of the average particle population. The total pore volume is 16±2 %, which is quite similar to the one of the vaterite precursor. However, Figure 3h reveals that the pore geometry has totally changed with the presence of two kinds of pores: the open and the closed pores. The open porosity depicted in blue color in Figure 3i represents 86±2 % of the total porosity. It is made of only one large pore with low curvature radii. The closed pores (shown in yellow in Figure 3i) account for 14±2 % of the total porosity. They are polydisperse in size and exhibit an almost spherical morphology. This spherical morphology that was not present in the vaterite precursor is a clear proof that the system tends to reduce its surface free energy. In addition, the excess of porosity at the center of the vaterite microparticle is also visible on the calcite microsphere (red curve on Figure 3e). DISCUSSIONS

Figure 4 summarizes the free energies of the different CaCO3 phases and displays the free energy difference between particles with high and low specific surface areas. The bulk contributions were extracted from literature.9,18 The surface components can be estimated from the relation GSurf = γAM,40 with γ being the surface energy, A the surface of the crystal and M the molar mass of CaCO3. The microparticles with a high specific surface area have a high total free energy compared to their bulk phase. Vaterite microspheres with high specific surface area can transform in three ways: (i) they can be converted in solution into non-porous vaterite by an Ostwald ripening process (denoted OR in Figure 4, from state (1) to (2)). This case is usually encountered when additives are used;6,36,41,42 (ii) without additives they can transform into calcite by a dissolution/recrystallization process (denoted DR in Figure 4, from state (1) to (4)). These two transformations take place in solution and lead to a large decrease of the surface free energy


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(by decreasing the specific surface area); (iii) the solid-state transformation in calcite denoted SS in Figure 4, enabled by heating, might be divided into two steps probably separated in time. A crystallographic phase transition may occur (from state (1) to (3)) followed by a surface-energy driven mechanism (from state (3) to (4)) leading to a decrease of the specific surface area with time.

Figure 4. Gibbs free energy G relative to calcite at 25 °C for ACC, vaterite and calcite. The free energies of ACC and vaterite are extracted from the studies of Radha et al.9 (assuming that the entropic contribution is negligible) and Wolf et al18, respectively. The black arrows indicate the process pathways for the transformations. OR, SS and DR denote Ostwald ripening, the thermally activated solid-state transformation and the dissolution/recrystallization mechanism, respectively. "Bulk + surface" means that both the bulk and the surface components have to be considered to determine the Gibbs free energy, while "Bulk" implies that the bulk contribution dominates G.


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Even though the crystallographic structure of vaterite is not well resolved,43-46 estimations of the surface energies in air are available.47 They are between 0.6 and 1.6 J/m² for vaterite and 0.6 to 1.4 J/m² for calcite depending on the crystallographic faces. Hence, considering an isotropic surface energy of 1 J/m² at the interface CaCO3/air, the surface free energy change is

∆Gsurf ~ - 0.1 kJ/mol per m²/g lost. Here, a decrease of the specific surface area from 7.3 m²/g for vaterite to 2.9 m²/g for calcite corresponds to a surface free energy change of ∆Gsurf ~ - 0.44 kJ/mol. Compared to the change of the total free energy ∆G estimated to be ranging from - 2.7 to -3.4 kJ/mol at

25 °C for the vaterite to calcite solid state transformation,17,18 ∆Gsurf with

γ = 1 J/m² contributes 15±2 % of the total free energy change. According to these energetic considerations, the system will tend to change its porosity in order to minimize the surface contribution for the vaterite to calcite phase transition. The pathway of the pore reorganization leads to a deep pore geometry change from elongated and small pores for vaterite to open micrometer size pores and closed spherical nanometer size pores for calcite. During sintering, the large open pore at a central position inside the microsphere may act as an attraction center for the coalescence of the smaller spherical closed pores. The presence of spherical pores exhibiting curvature radii ranging from

Direct observation of micro-particle porosity changes in solid-state

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