Water: its influence on the formation and crystallization of amorphous calcium carbonate

Nature produces CaCO3-based materials that display fascinating mechanical properties. These properties are a result of the exquisite control over the formation of CaCO3 crystals that nature possesses. Such a high level of control is likely achieved, at least in parts, by controlling the crystallization of CaCO3 via a transient phase, amorphous CaCO3 (ACC), with certain soluble additives and insoluble organic scaffolds present in living organisms. Inspired by the excellent properties of natural CaCO3-based materials, a lot of research has been devoted to the fabrication of synthetic counterparts with similar properties. Yet, we are far from obtaining the same degree of control over the properties of these synthetic materials. A contributing reason for this inferior control is our incomplete understanding of the influence of the processing conditions, soluble additives, and insoluble scaffolds on the formation of CaCO3 and thereby the composition, structure and properties of resulting materials.

This thesis is devoted to gaining a better understanding of the early stages of the CaCO3 formation. To achieve this goal, we introduce a new, organic solvent-free method to quench this process with a high temporal resolution. We produce ACC particles using a microfluidic spray-dryer and characterize them with different techniques. This method allows us to demonstrate that the amount of mobile water contained in ACC particles increases during their growth. As a result of the higher amount of mobile water, larger particles display a lower kinetic stability against the temperature-induced crystallization than smaller counterparts. We also reveal that certain additives reduce the amount of mobile water contained in ACC particles, thereby increasing their kinetic stability if exposed to elevated temperatures. To study the influence of additives on the kinetic stability of ACC against crystallization in more detail, we expose ACC particles functionalized with different additives to a humid environment or elevated pressures. We monitor the evolution of the structure and degree of hydration of these ACC particles during their crystallization. We show that the humidity-induced crystallization of ACC follows a distinctly different pathway than the pressure-induced one. In both pathways, the amount of water and its mobility that depends on its interaction strength with additives can influence the crystallization kinetics of ACC. The resulting CaCO3 crystals display a difference in their size, morphology, orientation, and structure. To process ACC particles into bulk CaCO3-based materials that possess similar mechanical properties to natural ones, insoluble organic scaffolds that offer a control over the local composition and structures of these materials across multiple length scales are typically required. To open up new possibilities of achieving this goal, we develop a new method to fabricate two dimensional (2D) structured hydrogel sheets. These 2D structured hydrogel sheets are composed of self-assembled, crosslinked hexagonal prismatic hydrogel particles. Using this method, we can vary the microscale structure and composition of these hydrogel sheets with the size, composition and arrangement of individual particles, their surface morphology with the polymerization conditions, and their mechanical properties with the crosslink density.

Additives: Their Influence on the Humidity- and Pressure-Induced Crystallization of Amorphous CaCO3

Esther Amstad, Alan Böhlen, Camelia Borca, Clémence Courrégelongue, Huachuan Du, Mathias Steinacher

Nature possesses a unique control over the formation of CaCO3 crystals that imparts fascinating mechanical properties to many CaCO3-based biomaterials. This high level of control is, at least in part, achieved through the use of certain soluble additives that influence the crystallization of amorphous CaCO3 (ACC). Inspired by nature, excellent work has been performed to elucidate the influence of additives on the crystallization of ACC that is dispersed in bulk aqueous solutions or subjected to elevated temperatures. By contrast, very little is known about the influence of additives on the crystallization of ACC when exposed to a humid environment or elevated pressures. This incomplete understanding restricts the range of properties that can be obtained in synthetic CaCO3-based biomaterials. To address this shortcoming, we study the influence of additives on the humidity- and pressure-induced crystallization of ACC. We find that the humidity-induced crystallization of ACC follows a distinctly different pathway than the pressure-induced one. As a result, the influence of additives on the crystallization kinetics of ACC, and hence, the size, morphology, structure, and orientation of the resulting CaCO3 crystals, differs considerably. These insights offer new opportunities to design CaCO3-based biomaterials whose mechanical properties more closely resemble natural ones.

AMER CHEMICAL SOC2020

Fabrication of Hexagonal-Prismatic Granular Hydrogel Sheets

Esther Amstad, Alice Cont, Huachuan Du, Mathias Steinacher

Natural soft materials are often composed of proteins that self-assemble into well-defined structures and display mechanical properties that cannot be matched by manmade materials. These materials are frequently mimicked with hydrogels whose mechanical properties depend on their composition and the type and density of cross-links. Protocols to tune these parameters are well established and routinely used. The mechanical properties of hydrogels also depend on their structure; this parameter is more difficult to control. In this paper, we present a method to produce hexagonal-prismatic granular hydrogel sheets with well-defined structures and heterogeneous cross-link densities. The hydrogel sheets are made of self-assembled covalently cross-linked 40–120 μm diameter hexagonal-prismatic hydrogel particles that display a narrow size distribution. The structure and microscale surface roughness of the hydrogels sheets can be tuned with the polymerization conditions, their chemical composition with that of the individual hydrogel particles, and their mechanical properties with the cross-link density. Remarkably, the hydrogels composed of hexagonal-prismatic microparticles are significantly stiffer than unstructured counterparts. These results demonstrate that the stiffness of hydrogels can be tuned by controlling their micrometer-scale structure without altering their composition. Thereby, they open up new possibilities to design soft materials whose performance more closely resembles that of natural ones.

2018

Process monitoring of alumina nanoparticle synthesis by inductively coupled RF thermal plasma

Jong-Won Shin

Since the first inductively coupled plasma (ICP) torch was developed in the 60's, RF thermal plasmas have found a variety of applications in material processing such as crystal growth, thermal spray coatings, spheroidization and vaporization of refractory materials. In recent decades, inductively coupled thermal plasmas have been used for the synthesis of high purity nanoparticles, thank to their remarkable advantages such as high energy density, variable operating pressures and low product contamination. The formation of nano-sized particles in thermal plasmas particularly using solid refractory precursor is a complex process. Hereby the injected precursor powders are heated by the high temperature of the thermal plasma. The heated powders are vaporized and decomposed into vapor species. During the following cooling, the vapor species are condensed to nano-sized particles. In this synthesis process, characterized by evaporation-condensation, the properties of the synthesized particles such as particle size, size distribution, phases and morphology are affected by various process parameters. Gas pressure, flow rates of the different torch gases, size of the precursor powders and powder loading in the plasma are considered as the influencing process parameters. In order to optimize and control the process for tailor-made nanoparticle synthesis, it is necessary to understand the effects of the process parameters on plasma properties and on the particle-plasma interactions resulting in final properties of the synthesized powders. In the present work, firstly, the enthalpy probe technique, a well-established plasma diagnostic tool for high pressure thermal plasma, has been applied to characterize the inductively coupled Ar-H2 thermal RF plasma used for alumina nanoparticle synthesis at different operating conditions. The plasma enthalpy has been determined, and the plasma temperature under a given gas pressure could subsequently be calculated assuming local thermodynamic equilibrium (LTE), once the gas composition has been determined by mass spectrometry. The gas velocity of the plasma jet could also be obtained by using the Bernoulli equation and stagnation pressure. Secondly, the influence of flow rates of central gas and precursor feed rates on the Al2O3 precursor evaporation has been investigated by process monitoring. Optical emission spectroscopy (OES) and laser light extinction (LE) measurements have been carried out to monitor in-situ the injection of the alumina precursors and to obtain information on the ongoing precursor evaporation. The emission line intensities of the aluminum vapor were used to determine the dependence of process parameters on precursor evaporation. Furthermore, the laser light extinction was used to calculate the number density of non plasma-treated powders, from which the number fraction of evaporated powders could be deduced. The size and morphology of the synthesized particles have been characterized ex-situ. Finally, the influence of quenching conditions on the condensation and formation of the nano-sized alumina particle has been studied by process analysis of the gas phase reactions of alumina. Optical emission spectroscopic was performed at different axial positions to monitor the gas phase reactions of the vaporized particles. Axial profiles of the emission line intensities of the main vapor species of alumina, Al(g) and AlO(g), were compared with the results of the enthalpy probe measurements and the thermal decomposition reactions of alumina found in literature. The results allow explanation of the alumina reactions in the thermal plasma as well as gives indications for the optimal axial position of the quenching gas injection. In addition to the determination of the optimal quenching position, the influence of the flow rates of quenching gas on the nanoparticle formation has been investigated. The synthesis of the submicron alumina using vapor-condensation principle predominantly leads to various thermodynamically metastable crystal structures called transition alumina. Therefore, the occurrence of different transition aluminas is discussed with regards to the flow rates of the quenching gas. The results presented in this thesis show that the synthesis process of the nanoparticle can be described on the basis of experimental results and basic information on the powder materials. The explanation of the influence of process parameters on powder evaporation and nanoparticle formation shows the usefulness of the in-situ plasma process monitoring, and the large potential for optimizing and controlling the nanopowder synthesis process in an inductively coupled RF thermal plasma.

EPFL2006

Process monitoring of alumina nanoparticle synthesis by inductively coupled RF thermal plasma

Jong-Won Shin

EPFL2006