Critical Crystallite Research Articles

Direct Formation of Anatase (TiO2)/Silica (SiO2) Composite Nanoparticles with High Phase Stability of 1300 °C from Acidic Solution by Hydrolysis under Hydrothermal Condition

Anatase (TiO2)/silica (SiO2) composite nanoparticles that were directly synthesized from solutions of TiOSO4 and tetraethyl orthosilicate by hydrolysis under hydrothermal conditions at 200 °C showed far more enhanced phase stability than pure anatase-type TiO2. By the presence of 48.2 mol % SiO2 as mostly amorphous phase, the crystallite growth of anatase was completely suppressed to maintain the crystallite size (14−16 nm) of the as-prepared condition after heating at 1000 °C, and the anatase-to-rutile phase transformation was retarded to maintain the anatase-type structure up to 1300 °C with no trace of rutile phase. The critical crystallite size of anatase in the composite samples for the anatase-to-rutile phase transformation was suggested to be around 70−100 nm. The silica, which contained -OTi groups around tetrahedrally coordinated Si and was regarded to exist mostly as amorphous phase, was considered to play an important role in suppressing diffusion between anatase crystallites, which led to the retardation of the crystallite growth of anatase and consequently resulted in the suppression of the phase transformation.

Size Effect on Crystal Structures of Barium Titanate Nanoparticles Prepared by a Sol-Gel Method

Barium titanate nanoparticles were synthesized by the hydrolysis of complex alkoxide precursor that was prepared in a reflux of metallic barium and tetraethylorthotitanate in solvent. The hydrolysis was performed by the addition of water/ethanol solution to the precursor solution. As reflux time increased, particle sizes, which were measured with transmittance electromicroscopy, became smaller followed by sharpening of size distribution. As water concentration and benzene content in the hydrolysis increased, the particle size increased with crystallite size that was determined with X-ray diffractometry. No significant difference was observed between the particle and crystallite sizes up to 30 nm. Over the size of 30 nm, the particle size was larger than the crystallite size because of generation of polycrystallites. Annealing treatments in air at 400 and 1000°C also increased both the particle and crystallite sizes. The crystallite sizes estimated from the (111) peaks were smaller than those from (110) peaks in a range of sizes larger than 40 nm, which indicated that the critical crystallite size of transformation from cubic to tetragonal structures was approximately 40 nm.

Unusual calcination temperature dependent tetragonal–monoclinic transitions in rare earth-doped zirconia nanocrystals

The dependence of tetragonal-monoclinic (t → m) transitions in the hydrothermally prepared (ZrO2)0.98(RE2O3)0.02 (RE = Sc, Y) nanocrystals upon the calcination between 400–1400 °C were explored by means of X-ray diffraction (XRD), high temperature X-ray diffraction (HTXRD), near infrared Fourier transform (FT) Raman and ultraviolet (UV) Raman spectroscopies. XRD and FT Raman scattering analysis indicated that the monoclinic fraction varied discontinuously with increasing the temperature from 400 to 1400 °C, which is unusual and is mainly ascribed to the microstrain effect. HTXRD analyses (from room temperature to 1400 °C) revealed that the as-prepared samples underwent tetragonal → monoclinic (t → m) transformation at 500 °C for (ZrO2)0.98(Sc2O3)0.02 and at 200 °C for (ZrO2)0.98(Y2O3)0.02 during the cooling process. UV Raman spectra suggested that the t → m transformation initially occurred at the surface region of the calcined samples. Hence, the monoclinic phase was enriched on the surface of the zirconia grains. Finally, the size effect of the stabilizers (Y3+ and Sc3+) on the critical crystallite size and the metastability of the tetragonal phase were also discussed.

Crystallite size growth and the derived dilatometric effect during θ- to α-phase transformation of nano-sized alumina powders

The existence of critical and primary crystallite sizes ( d cα=17 nm and d p=51 nm), in which the primary crystallites are mono-crystallite particles formed during θ- to α-phase transformation of the ultrafine alumina powders are examined. θ-Al 2O 3 powders were CIPed into compacts, thermally treated at 2°C, 5°C, and 10°C/min heating rates and coordinated with an analysis of their thermal expansion characteristics. Both the direct measurement and the thermal expansion/shrinkage characteristic of the compacts reflect that the primary crystallite is a mono-crystallite particle. The thermal expansion profile can be divided into three steps. A thermodynamic model relating the free energy change and the crystallite size is formulated. The need for a critical size can be attributed to the surface tension and the free energy change per unit volume. The existence of the primary size can then result from the occurrence of strain energy induced by the coalescence of the critical crystallites.

Examinations on the Critical and Primary Crystallite Sizes during θ- to α-Phase Transformation of Ultrafine Alumina Powders

Examination on the existence of the critical and primary crystallite size (dcα ∼ 17 nm and dp = 50−55 nm) phenomena during θ- to α-phase transformation of the nanosized Al2O3 powder was designed and conducted using two θ-Al2O3 powders of different crystallite sizes. Due to the critical size, the size dependence of the transformation temperature, the rate of transformation, and the inevitable presence of residual θ-phase in the thermal-treated and re-treated “100%-transformed” sample can be expected. Due to the two characteristic sizes and the formation of primary crystallites by the coalescence growth of the critical crystallites, the variation in mean α-size during the transformation can be predicted by means of the difference in the growth rate of the two θ-powder (different crystallite sizes) systems associated with proper thermal treatment techniques. Further, it is important to note that the presence of primary crystallites at various thermal-treated samples becomes possible.

Thermal stability of nanocrystalline fcc and hcp Ni(Si) synthesized by mechanical alloying of Ni90Si10

The thermal stability of nanocrystalline fcc and hcp Ni(Si), obtained by mechanical alloying of Ni90Si10, has been studied. The allotropic transformation from fcc to hcp Ni(Si) is accompanied by a volume expansion of 8.6% and is observed when fcc Ni(Si) reaches a critical crystallite size of 10nm. The hcp phase transforms to stable fcc Ni(Si) at 573K. It has been identified that the lattice distortion in nanometre-sized crystallites from the equilibrium configuration and the decrease in the interfacial energy with grain refinement act as self obstacles in controlling the grain growth of nanocrystalline materials.

Electrostrictive properties of nanosize PbTiO/sub 3/

Properties of nanosize PbTiO(3) are discussed from the standpoint of the effects of crystallite size on electrostrictive behavior. Experimental results based on crystallite size-dependent primary and secondary order parameters (spontaneous polarization and strain) show that the critical crystallite size (CCS) for the stabilization of the cubic paraelectric phase at 298 K is around 15 nm. The CCS inversely scales with the cubic-tetragonal transition temperature (T(tr)). Electrostrictive coefficients exhibit an order of magnitude increase at 28 nm, which is attributed to the anharmonicity of the perovskite lattice. First order calculations indicate that the increase in the electrostrictive coefficients offsets the decrease in spontaneous polarization and results in an increase in piezoelectric moduli suggesting that piezoelectric activity could, in principle, be observed in the nanosize regime.

Face-centered-cubic to Hexagonal-close-packed Transformation in Nanocrystalline Ni(Si) by Mechanical Alloying

An allotropic transition from face-centered-cubic (fcc) to hexagonal-close-packed (hcp) Ni(Si) solid solution in Ni95Si5 and Ni90Si10 during nanocrystallization by mechanical alloying is reported. The transformation was identified as a defect-induced melting accompanied by a volume expansion of 8.6% and was observed when fcc Ni(Si) reached a critical crystallite size of 10 nm. Calculation based on equation of state showed that a 37% reduction in tetragonal shear modulus and a negative pressure of about 8.7 GPa were generated at the onset of transformation.

Thermal stability of nanocrystalline Ni silicides synthesized by mechanical alloying

Solid state reactions induced by mechanical alloying followed by isothermal annealing in elemental blends of Ni and Si have been studied over the entire composition range of the Ni–Si system. Above a critical crystallite size, the congruent melting phases, γ-Ni 31Si 12 and NiSi, become unstable and react with second phase available to form equilibrium non-congruent, β 1-Ni 3Si and α-NiSi 2, after isothermal annealing at 723 and 1023 K, respectively. Formation of equilibrium ε-Ni 3Si 2 has not been observed in the composition range 33–50at.% Si even after annealing at 1073 K. The results also indicate that the crystallite size of non-congruent melting phases is well above 100 nm even at the stage of their evolution, suggesting that they are probably stable only in the bulk state. The grain growth is faster for the non-congruent phases when compared to that of congruent melting compounds. The interfacial energy of the compounds plays an important role in controlling their stability and grain growth in the nanocrystalline state.

Mechanical-milling-induced amorphization of Se: A crystallite destabilization model

Complete solid-state amorphization has been realized in elemental Se by means of mechanical milling of crystalline Se powder. Quantitative X-ray diffraction and thermal analyses were employed to characterize the amorphization process and indicated that the amorphization onset corresponds to a critical crystallite size and a drop in microstrain. During the major amorphization process, the remaining crystallite size remains unchanged with a constant lattice expansion. A new kinetics model of crystallite destabilization is proposed for the solid-state amorphization which satisfactorily explains the experimental observations.

Cubic to Tetragonal Phase Transformation of Ultrafine BaTiO3 Crystallites at Room Temperature

The critical crystallite size for the cubic (c-) to tetragonal (t-) phase transformation of ultrafine BaTiO3 crystallites was examined. Crystallites were prepared by calcination of barium titanyl oxalate tetrahydrate (BTOT) at various temperatures as to obtain crystallites with different sizes. The c- BaTiO3 crystallite which is thermodynamically considered as a high-temperature form can exist at room temperature if the crystallite is smaller than certain size. The critical size for the c→t phase transformation is ∼30 nm. The size may become larger if the crystallite suffers higher lattice strain which can be induced by different powder preparation methods.

Size driven phase transition in nanocrystalline BaTiO 3

Nanocrystalline powders of BaTiO 3 with a narrow size distribution were produced by a modified sol-gel method. The structure of the powders covering crystallite-sizes from 10 nm to 1 μm was determined by x-ray diffractometry (XRD). Differential scanning calorimetry (DSC) was used to examine the temperature dependent ferroelectric-paraelectric phase transition. This structural phase transition was found to disappear below a critical crystallite size of 49 nm. In conformity with this observation a structural transition towards a cubic symmetry became apparent in the x-ray diffractograms and also the disappearance of the second harmonic generation (SHG) was found at the same crystallite size. Raman scattering experiments proved the existence of polar TiO 6-building blocks. All experimental findings are in excellent agreement with theoretical predictions.

Effect of Composition and Size of Crystallite on Crystal Phase in Lead Barium Titanate

Lead titanate, barium titanate, and lead barium titanate powders (>99.9% pure), the particle size of which varied from 0.03 to 0.15 μm depending on the calcination temperature and the composition, was prepared from barium lead titanyl oxalate, which was previously prepared by reacting high‐purity ammonium titanyl oxalate with barium and lead acetate. The critical crystallite size of BaTiO3 powder from the cubic to the tetragonal phase is around 1 μm. Pb0.3Ba0.7TiO3 powder with an average size of 0.057 μm showed the tetragonal phase.

Preparation of platinum on activated carbon

In a study of the preparation of Pt/C catalysts, a set of modified activated carbon carriers has been prepared by nitric acid or air oxidation, or thermal decarboxylation. The carbons obtained by these methods had largely identical pore structures, but differed considerably in surface chemistry. A series of activated carbons with different surface chemistry was impregnated with aqueous hydrochloroplatinic acid solutions. Upon impregnation, the carbon reduced a certain amount of the Pt IV complex to Pt II, which was strongly chemisorbed at the walls of the carrier's mesopores. Weak physisorption was also observed. The capacity of the carrier for the reductive chemisorption was governed by its surface chemistry, and thus was strongly dependent on the pretreatment of the activated carbon. Aqueous alkaline formaldehyde solutions were then applied as reductor. After a period of slow growth, during which small zero-valent platinum particles were formed, a critical crystallite size was reached. Subsequently, a fast autocatalytic growth was observed until the supply of unreduced platinum became rate limiting. During the reduction process the carrier surface chemistry was (slightly) modified. After the reduction was completed, a catalytic autoredox reaction of excess formaldehyde to methanol and formate was observed. By properly selecting the preparation conditions, platinum-on-carbon catalysts combining a high metal load (5%) and a high metal dispersion (60%) could be obtained.

Stability of Metastable Tetragonal Zirconia

This research was undertaken to determine the reason for the occurrence of metastable tetragonal zirconia (ZrO2). Metastable tetragnal ZrO2 was prepared by low temperature calcunation of zirconium oxychloride octahydrate ZrOCl2⋅8H2O. The tetragonal-to-monoclinic (t-m) transformation in pure metastable tetragonal ZrO2 powder was investigated with special reference to the isothermal annealing temperature, the variation of crystallite size and the annealing environment. The results indicate that the metastable tetragonal ZrO2 is stabilized thermodynamically by surface free energy and there is a so-called “critical diameter (crystallite size)”. Moreover, thermodynamic calculation showed that the t-m transformation temperature depends on the critical crystallite size.

Sol-Gel Synthesis and Characterization of Zirconia and Yttria Stabilized Zirconia Powders

Powders of pure zirconia and zirconia containing 3 mol% yttria were prepared by sol-gel transformation followed by calcination techniques. The powders were characterized for their purity, chemical homogeneity, thermal behaviour, particle size, surface area, crystallite size and content of the tetragonal and monoclinic phases. The results show that as-prepared powders were of high purity, chemically homogeneous, X-ray amorphous and contained sub-micron particles. Addition of yttria increased the temperature stability of the tetragonal phase, while the critical crystallite size for the transformation remained unchanged. Yttria stabilized zirconia powders could be hot- pressed to 97% theoretical density at 1450°C and the sintered body consisted of uniform 0.70 μm grains having about 45% tetragonal phase.

Some studies of mixed oxide thin film transistors

The behaviour of mixed oxide TFTs, with Pr6O11 and Yb2O3 in the ratio of 1 : 1 by weight, both in dry air and in the ambient atmosphere is reported. CdS-(Pr6O3+Yb2O3) and CdSe-(Pr6O11 + Yb2O3) were used as the semiconductor-insulator (S-I) combinations with Al electrodes. The tranconductance, output resistance, amplification factor and gain-bandwidth product of the fabricated TFTs were calculated from the various characteristic curves. In addition, the trap density, critical donor density, mobility and crystallite size were also evaluated

Thin film transistors with Er 2O 3 gate insulators

Er 2O 3 was used to fabricate thin film transistors (TFTs) using the multiple pumpdown (MPD) method of device fabrication. CdS-Er 2O 3, CdSe-Er 2O 3 and (CdS-CdSe)-Er 2O 3 were used as the semiconductor-insulator combinations. In the “mixed semiconductor” TFT a mixture of CdS and CdSe in the ratio of 1:1 by weight was used. The various transistor parameters are presented. Parameters such as critical donor density, trap density, mobility and crystallite size were calculated using a model proposed by Levinson et al.

Phase transformations of gel-derived magnesia partially stabilized zirconias

Both chloride-free (amorphous, group A) and chloride-containing (crystalline, group B) ZrO2-8.1 mol % MgO powders produced by means of gelatinized coprecipitation have been calcined at 450 to 1450° C and characterized by differential thermal analysis/thermal gravimetric analysis, X-ray diffraction, electron microscopy, and infrared spectroscopy. For both groups A and B, the powder density, chloride content, crystallite size, crystallization temperature, and the initial temperature for metastable tetragonal-to-monoclinic (Tm → M) transformation decrease with increasing concentration of NH4OH with which samples were prepared. High-temperature tetragonal-to-cubic transformation of group B also revealed the same relationship, but the inverse for group A. An explanation based on the nature of the Zr-O bond and internal strain have been proposed for both transitions. In addition, it is also found that silica can inhibit both transitions and raise the critical crystallite size for Tm → M transitions, but water vapour and chloride have the inverse effect on the Tm → M transition.

Thermodynamic analysis of the stability of supported metal catalysts

The stability of the bidimensional dispersed state of metal on a support is analyzed by comparing its free enthalpy with the free enthalpy of the state in which the metal atoms are agglomerated in the form of crystallites. The chief result of the analysis is the existence of a critical crystallite radius r c given by the equation r c ≡ 5δ 1 − cos θ , where δ represents the diameter of a platinum atom and θ the wetting angle of the metal on the support. For crystallite radii smaller than r c the bidimensional dispersed state is more stable, while for radii larger than r c the crystallite state is more stable.