Li-Min Wang

Effect of aspect ratio and surface defects on the photocatalytic activity of ZnO nanorods

ZnO, aside from TiO2, has been considered as a promising material for purification and disinfection of water and air, and remediation of hazardous waste, owing to its high activity, environment-friendly feature and lower cost. However, their poor visible light utilization greatly limited their practical applications. Herein, we demonstrate the fabrication of different aspect ratios of the ZnO nanorods with surface defects by mechanical-assisted thermal decomposition method. The experiments revealed that ZnO nanorods with higher aspect ratio and surface defects show significantly higher photocatalytic performances.

Ultrahard nanotwinned cubic boron nitride

Cubic boron nitride (cBN) is a well known superhard material that has a wide range of industrial applications. Nanostructuring of cBN is an effective way to improve its hardness by virtue of the Hall–Petch effect—the tendency for hardness to increase with decreasing grain size. Polycrystalline cBN materials are often synthesized by using the martensitic transformation of a graphite-like BN precursor, in which high pressures and temperatures lead to puckering of the BN layers. Such approaches have led to synthetic polycrystalline cBN having grain sizes as small as ∼14 nm (refs 1, 2, 4, 5). Here we report the formation of cBN with a nanostructure dominated by fine twin domains of average thickness ∼3.8 nm. This nanotwinned cBN was synthesized from specially prepared BN precursor nanoparticles possessing onion-like nested structures with intrinsically puckered BN layers and numerous stacking faults. The resulting nanotwinned cBN bulk samples are optically transparent with a striking combination of physical properties: an extremely high Vickers hardness (exceeding 100 GPa, the optimal hardness of synthetic diamond), a high oxidization temperature (∼1,294 °C) and a large fracture toughness (>12 MPa m1/2, well beyond the toughness of commercial cemented tungsten carbide, ∼10 MPa m1/2). We show that hardening of cBN is continuous with decreasing twin thickness down to the smallest sizes investigated, contrasting with the expected reverse Hall–Petch effect below a critical grain size or the twin thickness of ∼10–15 nm found in metals and alloys.

Direct determination of kinetic fragility indices of glassforming liquids by differential scanning calorimetry: Kinetic versus thermodynamic fragilities

A calorimetric method of obtaining directly the fragility of liquids from the fictive temperatures of variably quenched glasses, is outlined. “Steepness indexes” m, have been determined for a group of molecular liquids of diverse character, and vary in the range 50–150. The values obtained mostly agree well with those from earlier studies using dielectric relaxation, heat capacity spectroscopy, and viscosity data. In our method there is the advantage that the fragility is determined from the relaxation process that is basic to the calorimetric glass transition temperature measurement, namely, that of the enthalpy. The calorimetric measurements also yield the liquid and glass heat capacities, and entropies of fusion, permitting relationships between thermodynamic and kinetic responses to be examined simultaneously. We study glycerol, dibutylphthallate, 9-bromophenanthrene, salol, orthoterphenyl, propylene carbonate, decalin and its nitrogen derivative decahydroisoquinoline, and find the latter two to be th...

Fragility and thermodynamics in nonpolymeric glass-forming liquids

For nonpolymeric supercooled liquids, the empirical correlation m = 56Tg DeltaCp(Tg)/DeltaHm provides a reliable means of correlating dynamic and thermodynamic variables. The dynamics are characterized by the fragility or steepness index m and the glass transition temperature Tg, while thermodynamics enter in terms of the heat capacity step DeltaCp at Tg and the melting enthalpy DeltaHm. The combination of the above correlation with the 23 rule for the Tg/Tm ratio yields an expression, m = 40DeltaCp(Tg)/DeltaSm, which was rationalized as the correlation of the thermodynamic and kinetic fragilities. Defining a thermodynamic fragility via DeltaCp(Tg)/DeltaSm also reveals that the slopes in Kauzmanns original DeltaS(T)/DeltaSm versus T/Tm plot reflect the fragility concept [Chem. Rev. 43, 219 (1948)], so long as Tm/Tg = 1.5. For the many liquids whose excess heat capacity is a hyperbolic function of temperature, we deduce that the fragility cannot exceed m = 170, unless the Tg/Tm = 2/3 rule breaks down.

Many-Body Nature of Relaxation Processes in Glass-Forming Systems

Most glass-forming systems are composed of basic units interacting with each other with a nontrivial anharmonic potential. Naturally, relaxation and diffusion in glass formers is a many-body problem. Results from recent experimental studies are presented to show the effects of many-body relaxation and diffusion manifested on the dynamic properties of glass formers. Considering that the effects are general and critical, the problem of glass transition will not be solved until the many-body nature of the relaxation process has been incorporated fundamentally into any theory.

Potential energy, relaxation, vibrational dynamics and the boson peak, of hyperquenched glasses

We describe a combination of laboratory and simulation studies that give quantitative information on the energy landscape for glass-forming liquids. Both types of study focus on the idea of suddenly extracting the thermal energy, so that the system obtained for subsequent study has the structure, and hence potential energy, of a liquid at a much higher temperature than the normal glass temperature Tg .O ne t ype of study gives information on the energy that can be trapped in experimental glasses by hyperquenching, relative to the normal glass, and on the magnitude of barriers separating basins of attraction on the landscape. Stepwise annealing studies also give information on the matter of energy heterogeneity and the question of ‘nanogranularity’ in liquids near Tg .T heother type of study gives information on the vibrational properties of as ystem c onfined to a given basin, and particularly on how that vibrational structure changes with the state of configurational excitation of the liquid. A feature in the low frequency (‘boson peak’) region of the density of vibrational states of the normal glass becomes much stronger in the hyperquenched glass. Qualitatively similar observations are made on heating fragile glass-formers into th es upercooled and stable liquid states. The vibrational dynamics findings are supported and elucidated by constant pressure molecular dynamics/normal mode MD/NM simulations/analysis of the densities of states of different inherent structures of a model fragile liquid (orthoterphenyl (OTP) in the Lewis– Wahnstrom approximation). These show that, when the temperature is raised at constant pressure, the total density of states changes in a manner that can be well represented by a two-Gaussian ‘excitation acros st hecentroid’, leaving a thir da nd major Gaussian component unchanging. The low frequency Gaussian component, which grows with increasing temperature, has a constant peak

Dynamics of glass-forming liquids. IX. Structural versus dielectric relaxation in monohydroxy alcohols

The prominent Debye-type but non-Arrhenius dielectric relaxation is a feature common to many monohydroxy alcohols in their liquid state. Although this exponential process is often considered to reflect the primary structural relaxation, only a faster, smaller, and nonexponential relaxation peak correlates with viscous flow and mechanical relaxation. We provide dielectric relaxation data for 2-methyl-1-butanol, 2-ethyl-1-hexanol, and 3,7-dimethyl-1-octanol across ten decades in time. Based on these and previous results, we show that there exists a variety of dielectric to mechanical relaxation time ratios in the viscous regime, but a universal value of 100 for that ratio appears to evolve in the high temperature limit. The temperature dependence for both the relaxation time and strength of the Debye peak differs from the typical behavior of structural dynamics in terms of the alpha process. The implications of these findings for rationalizing the Debye-type dielectric process of hydrogen-bonded liquids are discussed.

Three Dimensional Carbon-Nanotube Polymers

Eight fascinating sp(2)- and sp(3)-hybridized carbon allotropes have been uncovered using a newly developed ab initio particle-swarm optimization methodology for crystal structure prediction. These crystalline allotropes can be viewed respectively as three-dimensional (3D) polymers of (4,0), (5,0), (7,0), (8,0), (9,0), (3,3), (4,4), and (6,6) carbon nanotubes, termed 3D-(n, 0) or 3D-(n, n) carbons. The ground-state energy calculations show that the carbons all have lower energies than C(60) fullerene, and some are energetically more stable than the van der Waals packing configurations of their nanotube parents. Owing to their unique configurations, they have distinctive electronic properties, high Youngs moduli, high tensile strength, ultrahigh hardness, good ductility, and low density, and may be potentially applied to a variety of needs.

Calorimetric versus kinetic glass transitions in viscous monohydroxy alcohols

An extensive comparison of calorimetric and dielectric measurements is carried out for generic molecular liquids and monohydroxy alcohols with focus on the identification of the dielectric modes which are associated with the glass transition. For generic liquids, the calorimetric glass transition temperatures (T g-cal) are always greater than their kinetic counterparts (T g-kin), but the difference remains below 3 K. Also, the nonexponentiality parameters of the Tool-Narayanaswamy-Moynihan-Hodge model applied to the calorimetric data and the stretching exponents of the dielectric measurements show remarkable agreement. The same behavior is found for glass-forming monohydroxy alcohols, provided that the faster and smaller non-Debye relaxation rather than the large dielectric Debye process is assigned to the structural relaxation. The study emphasizes that the dielectric signature of the glass transition in monohydroxy alcohols is a dispersive loss peak that is faster and significantly smaller than the prominent Debye feature.

Exponential probe rotation in glass-forming liquids

Using time resolved optical depolarization, we have studied the rotational behavior of molecular probes in supercooled liquids near the glass transition temperature T(g). Simultaneously, the dynamics of the liquid immediately surrounding these rigid probes is measured by triplet state solvation experiments. This direct comparison of solute and solvent dynamics is particularly suited for assessing the origin of exponential orientational correlation functions of probe molecules embedded in liquids which exhibit highly nonexponential structural relaxation. Polarization angle dependent Stokes shift correlation functions demonstrate that probe rotation time and solvent response time are locally correlated quantities in the case of smaller probe molecules. Varying the size of both guest and host molecules shows that the size ratio determines the rotational behavior of the probes. The results are indicative of time averaging being at the origin of exponential rotation of probes whose rotational time constant is slower than solvent relaxation by a factor of 20 or more.