Onset Of Homogeneous Nucleation Research Articles

Heterogeneous Nucleation onto Monoatomic Ions: Support for the Kelvin-Thomson Theory.

In this study, the process of heterogeneous nucleation is investigated by coupling a high‐resolution differential mobility analyser (DMA) to an expansion‐type condensation particle counter, the size‐analyzing nuclei counter (SANC). More specifically, we measured the activation probabilities of monoatomic ions of both polarities by using n‐butanol as condensing liquid. All seed ions were activated to grow into macroscopic sizes at saturation ratios well below the onset of homogeneous nucleation, showing for the first time that the SANC is capable of detecting sub‐nanometer sized, atomic seed ions. The measured onset saturation ratios for each ion were compared to the Kelvin‐Thomson (KT) theory. Despite the fact that certain dependencies of activation behaviour on seed ion properties cannot be predicted by the KT theory, it was found that with a simple adjustment of the n‐butanol molecular volume (9–15 % lower compared to bulk properties) good agreement with experimental results is achievable. The corresponding density increase may result from the dipole‐charge interaction. This study thus offers support for the application of the KT model for heterogeneous, ion‐induced nucleation studies at the sub‐nanometer level.

Containerless Undercooled Melts: Ordering, Nucleation, and Dendrite Growth

Electromagnetic and electrostatic levitation are applied to containerless undercool and solidify metallic melts. A large undercooling range becomes accessible with the extra benefit that the freely suspended drop is accessible directly for in situ observation. The short-range order in undercooled melts is investigated by combining levitation with elastic neutron scattering and X-ray scattering using synchrotron radiation. Muon Spin Rotation (µSR) experiments show magnetic ordering in deeply undercooled Co80Pd20 alloys. The onset of magnetic ordering stimulates nucleation. Results on nucleation undercooling of zirconium are presented showing the limit of maximum undercoolability set by the onset of homogeneous nucleation. Metastable phase diagrams are determined by applying energy-dispersive X-ray diffraction of Ni-V alloys with varying concentration. Nucleation is followed by crystal growth. Rapid dendrite growth velocity is measured on levitation-processed samples as a function of undercooling ∆T by using high-speed video camera technique. Solute trapping in dilute solid solutions and disorder trapping in intermetallic compounds are experimentally verified. Measurements of glass-forming Cu-Zr alloy show a maximum in the V(∆T) relation that is indicative for diffusion-controlled growth. The influence of convection on dendrite growth of Al50Ni50 is shown by comparative measurements of dendrite growth velocity on Earth and in reduced gravity. Eventually, faceting of a rough interface by convection is presented as observed on Ni2B alloys.

Containerless Processing in the Study of Metastable Solids from Undercooled Melts

An undercooled melt possesses an enhanced free enthalpy that enables to crystallize metastable solids in competition with their stable counterparts. Crystal nucleation selects the crystallographic phase whereas the growth dynamics controls microstructure evolution. We apply containerless processing such as electromagnetic and electrostatic levitation to containerlesss undercool and solidify metallic melts. Heterogeneous nucleation on container-walls is completely avoided leading to large undercooling with the extra benefit that the freely suspended drop is direct accessible for in situ observation of crystallization far away from equilibrium. Results of investigations of maximum undercoolability on pure zirconium are presented showing the limit of maximum undercoolability set by the onset of homogeneous nucleation. Rapid dendrite growth is measured as a function of undercooling by a high-speed camera and analysed within extended theories of non-equilibrium solidification. In such both supersaturated solid solutions and disordered superlattice structure of intermetallics are formed at high growth velocities. A sharp interface theory of dendrite growth is capable to describe the non-equilibrium solidification phenomena during rapid crystallization of deeply undercooled melts.

Heterogeneous Nucleation with Finite Activation Energy and Perfect Wetting: Capillary Theory Versus Experiments with Nanometer Particles, and Extrapolations on the Smallest Detectable Nucleus

Analytical expressions are provided for charged and uncharged particles in the perfect-wetting limit of Fletcher's classical heterogeneous nucleation theory with finite activation energy G*. A dimensionless logarithm of the critical supersaturation can be represented as a single universal curve versus a dimensionless nucleus radius for neutral particles, while a uniparametric family of curves is required for charged particles. Comparison with experimental critical supersaturations for nanoparticles and molecular ions shows qualitative agreement, with substantial quantitative disagreement. At increasing sizes the predicted critical diameter augmented by a fixed length ∼1 nm approaches the Kelvin limit. Predicted activation probability curves versus size (supersaturation) are steep, whence stringent experimental definitions of the thermodynamic state and the seed particle are required for meaningful comparison with theory. Activation curves for mobility-selected WOx particles are broader than predicted, even with a well-defined supersaturation, while those of mobility-purified molecular ions agree qualitatively with theory, even in a turbulent mixing nucleation chamber. Recent work by CitationWinkler (2009) based on the Vienna expansion chamber and mobility-purified molecular ions has anticipated these conclusions. The theory is combined with similar considerations on the onset of homogeneous nucleation, to draw conclusions on the smallest nucleus size that may be activated by heterogeneous nucleation. Water is predicted to be able to detect uncharged nuclei with diameters as small as 0.6 nm. The associated possibility to develop detectors for relatively small single neutral molecules is enticing.

Crystal nucleation and dendrite growth of metastable phases in undercooled melts

An undercooled melt possesses an enhanced free enthalpy that opens up the possibility to crystallize metastable crystalline solids in competition with their stable counterparts. Crystal nucleation selects the crystallographic phase whereas the growth dynamics controls microstructure evolution. We apply containerless processing techniques such as electromagnetic and electrostatic levitation to containerlesss undercool and solidify metallic melts. Owing to the complete avoidance of heterogeneous nucleation on container-walls a large undercooling range becomes accessible with the extra benefit that the freely suspended drop is direct accessible for in situ observation of crystallization far away from equilibrium. Results of investigations of maximum undercoolability on pure zirconium are presented showing the limit of maximum undercoolability set by the onset of homogeneous nucleation. Rapid dendrite growth is measured as a function of undercooling by a high-speed camera and analysed within extended theories of non-equilibrium solidification. In such both supersaturated solid solutions and disordered superlattice structure of intermetallics are formed at high growth velocities. A sharp interface theory of dendrite growth is capable to describe the non-equilibrium solidification phenomena during rapid crystallization of deeply undercooled melts. Eventually, anomalous growth behaviour of Al-rich Al–Ni alloys is presented, which may be caused by forced convection.

Heterogeneous Nucleation Experiments Bridging the Scale from Molecular Ion Clusters to Nanoparticles

Generation, investigation, and manipulation of nanostructured materials are of fundamental and practical importance for several disciplines, including materials science and medicine. Recently, atmospheric new particle formation in the nanometer-size range has been found to be a global phenomenon. Still, its detailed mechanisms are mostly unknown, largely depending on the incapability to generate and measure nanoparticles in a controlled way. In our experiments, an organic vapor (n-propanol) condenses on molecular ions, as well as on charged and uncharged inorganic nanoparticles, via initial activation by heterogeneous nucleation. We found a smooth transition in activation behavior as a function of size and activation to occur well before the onset of homogeneous nucleation. Furthermore, nucleation enhancement for charged particles and a substantial negative sign preference were quantitatively detected.

Condensation particle counting below 2 nm seed particle diameter and the transition from heterogeneous to homogeneous nucleation

Particle detection by condensation particle counters (CPCs) is ultimately limited by the onset of homogeneous nucleation. At vapour supersaturations around the homogeneous nucleation limit the diameter of critical clusters is typically about 2 nm. It is widely assumed that only particles larger than critical clusters can be activated by vapour condensation and the general detection limit of CPCs is therefore currently accepted to be around 2 nm particle diameter. Using an expansion type CPC with n-propanol as working fluid we investigated the transition from heterogeneous to homogeneous nucleation, clearly showing that particles are activated much before the onset of homogeneous nucleation, even at particle diameters as small as 1.4 nm. For particle diameters below 2 nm we have usually found condensation particle counting to be influenced by the simultaneous presence of ions as generated in a bipolar diffusion charger. In this paper we illustrate how the presence of ions influences particle number concentration measurement and how ions can be removed in order to obtain accurate seed particle number concentrations for particle diameters down to 1 nm.

Cavitation dynamics on the nanoscale

The ultrafast excitation of gold nanoparticle sols causes a strong nonequilibrium heating of the particle lattice and subsequently of the water shell close to the particle surface. Above a threshold in laser fluence, which is defined by the onset of homogeneous nucleation, nanoscale vapor bubbles develop around the particles, expand and collapse again within the first nanosecond after excitation. We show the existence of cavitation on the nanometer and subnanosecond time scale, described within the framework of continuum thermodynamics.

Evaluation of homogeneous droplet formation inside UCPC (TSI model 3025)

The droplet formation characteristics inside an Ultrafine Condensation Particle Counter (UCPC, TSI model 3025) were tested changing working temperatures of the counter as well as flow rates and carrier gases. The onset of homogeneous nucleation was determined as a function of the temperature of the saturator and condenser. Preliminary tests were carried out using the UCPC for measurements of homogeneous nucleation rates of n-butanol vapour. The observed nucleation rates were compared with those predicted by classical nucleation theory and by self-consistent correction to the classical theory. Homogeneous droplet formation at nearly standard operation conditions of the UCPC were observed for carrier gases such as air and nitrogen. Moreover, in helium particle formation was observed already at standard operation conditions of the UCPC.

Homogeneous gas-phase nucleation in silane pyrolysis

Dilute and particle free mixtures of silane in the range of 100 ppm to 10% by volume in different carrier gases were decomposed thermally in a tube reactor. The onset of homogeneous nucleation was determined as a function of temperature and silane concentration for each carrier gas using a CNC with a detection limit of ca 0.01 μm. The gaseous decomposition by-products, disilane, trisilane and hydrogen were measured simultaneously using gas-phase chromatography. The onset of gas-phase nucleation was found to be inversely proportional to the temperature and influenced by the nature of the carrier gas. In inert gases, no chemical reaction took place between the decomposition products of silane and the carrier gas. In hydrogen, equilibrium displacement with the primary decomposition product (SiH 2) lead to a retardation of particle formation. Thus, the temperatures of onset of gas-phase nucleation was higher for mixtures in hydrogen than for mixtures in inert gases.

Information about the Growth Process from Aluminium Concentration Profiles in Ga1−xAlxAs Liquid Phase Epitaxial Layers

AbstractThe variation in the composition of Ga1−xAlxAs LPE layers with x < 0.37 as a function of layer thickness is examined by electron beam microprobe analysis. From the combination of these results with the experimentally established relation between layer thickness and cooling interval information is obtained about the onset of homogeneous nucleation in the LPE growth.