Single-component Precursors Research Articles

Reactive laser synthesis of ultra-high-temperature ceramics HfC, ZrC, TiC, HfN, ZrN, and TiN for additive manufacturing

Ultra-high-temperature ceramics (UHTCs) are optimal structural materials for applications that require extreme high temperature resilience (Mp > 3000 °C), resistance to chemically aggressive environments, wear, and mechanical stress. Processing UHTCs with laser-based additive manufacturing (AM) has not been fully realized due to a variety of obstacles. In this work, selective laser reaction sintering techniques (SLRS) were investigated for the production near net-shape UHTC ceramics such as HfC, ZrC, TiC, HfN, ZrN, and TiN. Specifically, group IV transition metal and metal oxide precursor materials (<44 μm) were chemically converted and reaction-bonded into layers of UHTCs using single-step selective laser processing in 100 vol% CH4 or NH3 gas that might be compatible with prevailing powder bed fusion techniques. Conversion of either metals (Hf, Zr and Ti) or metal oxides (HfO2, ZrO2, and TiO2) particles was first investigated to examine reaction mechanisms and volume changes associated with SLRS of single-component precursor systems. SLRS processing of metal or metal oxide alone produced near stoichiometric UHTC phases and yields up to >99.9 wt% total for carbides and nitrides and during the rapid reactive in-situ processing scheme. However, for single-phase feedstocks, gas-solid reactivity induced volumetric changes (correlated with the stochiometry of the rocksalt-type UHTC carbide and nitride products) resulted in residual stresses and cracking in the model-AM product layer. To mitigate conversion-induced stresses of single-phase precursors, composite metal/metal oxide precursors were employed to compensate for the volume changes of either the metal (which expands during conversion) or the metal oxide precursor (which contracts). While conversion of the optimized composite materials produced HfC layers with as little as +0.9% volume change, results indicated interparticle adhesion must be optimized to obtain robust UHTC-AM layers. Computational models of carbon and nitrogen diffusion in host transition metal lattices corroborated experimental results where a progressive particle conversion might inhibit interparticle diffusion unless laser processing parameters are carefully optimized to favor reaction bonding over discrete particle conversion. While this method presents a host of processing considerations, we demonstrate how this reactive approach may be viable for the AM of numerous UHTC materials that are not readily produced using current methods.

High performance Li2ZnTi3O8@C anode material fabricated by a facile method without an additional carbon source

Li2ZnTi3O8@C anode material has been synthesized by a facile solid-state reaction method, in which C6H5Li3O7·4H2O is used as the lithium source, as well as the single-source carbon. With the formation of carbon-wrapped lithium oxide by the thermolysis of single-component precursor C6H5Li3O7·4H2O, Li2ZnTi3O8 particles are produced in a limited area, which could effectively hinder their growth/agglomeration. Moreover, the carbon layer coating on the outer surface of Li2ZnTi3O8 is also beneficial to the transfer of both Li+ and electrons, then an enhanced electrochemical performance is expected to achieve. From the electrochemical test results, a reversible capacity of 227.4 mAh g−1 could be delivered by Li2ZnTi3O8@C after 100 cycles at the current rate of 2 C.

Photothermal cancer therapy using graphitic carbon-coated magnetic particles prepared by one-pot synthesis.

We describe here a simple synthetic strategy for the fabrication of carbon-coated Fe3O4 (Fe3O4@C) particles using a single-component precursor, iron (III) diethylenetriaminepentaacetic acid complex. Physicochemical analyses revealed that the core of the synthesized particles consists of ferromagnetic Fe3O4 material ranging several hundred nanometers, embedded in nitrogen-doped graphitic carbon with a thickness of ~120 nm. Because of their photothermal activity (absorption of near-infrared [NIR] light), the Fe3O4@C particles have been investigated for photothermal therapeutic applications. An example of one such application would be the use of Fe3O4@C particles in human adenocarcinoma A549 cells by means of NIR-triggered cell death. In this system, the Fe3O4@C can rapidly generate heat, causing >98% cell death within 10 minutes under 808 nm NIR laser irradiation (2.3 W cm−2). These Fe3O4@C particles provided a superior photothermal therapeutic effect by intratumoral delivery and NIR irradiation of tumor xenografts. These results demonstrate that one-pot synthesis of carbon-coated magnetic particles could provide promising materials for future clinical applications and encourage further investigation of this simple method.

Facile single-component precursor for Cu2ZnSnS4 with enhanced phase and composition controllability

A simple and effective method has been demonstrated using soluble single-component precursors to achieve high quality Cu2ZnSnS4 (CZTS) thin films with enhanced phase and composition controllability. The soluble single-component precursors were composed of Cu2−xS colloidal nanocrystals (NC) and the homogeneously distributed Zn–Sn species on the surface. Zn species is presented as the surface ligand for the first time, in the form of Zn–Sn complex, providing both the colloidal stabilization and the essential components for CZTS. CZTS film composition is governed by the prescribed stoichiometry between Cu2−xS nanoparticles and the Zn–Sn complex in solution. This novel reaction process enables a favorable CZTS phase progression, with less than 175 °C formation temperature, and a minimized secondary phase growth mode, due to its local composition uniformity in both liquid and film state. Compared with the approaches based on molecular precursor or quarternary CZTS nanocrystals, the single-component precursor allows fine tuning of the kesterite materials for future advanced optoelectronics.

Modelling structure and properties of amorphous silicon boron nitride ceramics

Silicon boron nitride is the parent compound of a new class of high-temperature stable amorphous ceramics constituted of silicon, boron, nitrogen, and carbon, featuring a set of properties that is without precedent, and represents a prototypical random network based on chemical bonds of predominantly covalent character. In contrast to many other amorphous materials of technological interest, a-Si3B3N7 is not produced via glass formation, i.e. by quenching from a melt, the reason being that the binary components, BN and Si3N4, melt incongruently under standard conditions. Neither has it been possible to employ sintering of ?m-size powders consisting of binary nitrides BN and Si3N4. Instead, one employs the so-called sol-gel route starting from single component precursors such as TADB ((SiCl3)NH(BCl2)). In order to determine the atomic structure of this material, it has proven necessary to simulate the actual synthesis route. Many of the exciting properties of these ceramics are closely connected to the details of their amorphous structure. To clarify this structure, it is necessary to employ not only experimental probes on many length scales (X-ray, neutron- and electron scattering; complex NMR experiments; IR- and Raman scattering), but also theoretical approaches. These address the actual synthesis route to a-Si3B3N7, the structural properties, the elastic and vibrational properties, aging and coarsening behaviour, thermal conductivity and the metastable phase diagram both for a-Si3B3N7 and possible silicon boron nitride phases with compositions different from Si3N4 : BN = 1 : 3. Here, we present a short comprehensive overview over the insights gained using molecular dynamics and Monte Carlo simulations to explore the energy landscape of a-Si3B3N7, model the actual synthesis route and compute static and transport properties of a-Si3B3N7.

Mechanistic Insights on Acrylate Insertion Polymerization

Complexes [{(PwedgeO)PdMe}(n)] (1(n); PwedgeO = kappa(2)-P,O-Ar(2)PC(6)H(4)SO(2)O with Ar = 2-MeOC(6)H(4)) are a single-component precursor of the (PwedgeO)PdMe fragment devoid of additional coordinating ligands, which also promotes the catalytic oligomerization of acrylates. Exposure of 1(n) to methyl acrylate afforded the two diastereomeric chelate complexes [(PwedgeO)Pd{kappa(2)-C,O-CH(C(O)OMe)CH(2)CH(C(O)OMe)CH(2)CH(3)}] (3-meso and 3-rac) resulting from two consecutive 2,1-insertions of methyl acrylate into the Pd-Me bond with the same or opposite stereochemistry, respectively, in a 3:2 ratio as demonstrated by comprehensive NMR spectroscopic studies and single crystal X-ray diffraction. These six-membered chelate complexes are direct key models for intermediates of acrylate insertion polymerization, and also ethylene-acrylate copolymerization to high acrylate content copolymers. Studies of the binding of various substrates (pyridine, dmso, ethylene and methyl acrylate) to 3-meso and 3-rac show that hindered displacement of the chelating carbonyl moiety by pi-coordination of incoming monomer significantly retards, but does not prohibit, polymerization. For 3-meso,3-rac + C(2)H(4) right arrow over left arrow 3-meso-C(2)H(4,) 3-rac-C(2)H(4) an equilibrium constant K(353 K) approximately 2 x 10(-3) L mol(-1) was estimated. Reaction of 3-meso, 3-rac with methyl acrylate afforded higher insertion products [(PwedgeO)Pd(C(4)H(6)O(2))(n)Me] (n = 3, 4) as observed by electrospray ionization mass spectrometry (ESI-MS). Theoretical studies by DFT methods of consecutive acrylate insertion provide relative energies of intermediates and transition states, which are consistent with the aforementioned experimental observations, and give detailed insights to the pathways of multiple consecutive acrylate insertions. Acrylate insertion into 3-meso,3-rac is associated with an overall energy barrier of ca. 100 kJ mol(-1).

Hydrophobic monolayer preparation by Langmuir–Blodgett and chemical adsorption techniques

Alkylsiloxane and perfluoroalkylsiloxane monolayers are prepared on siliceous surfaces using the techniques of Langmuir–Blodgett deposition and solid–liquid chemical adsorption. Acid-catalyzed hydrolysis and polycondensation reactions provide two-dimensional siloxane networks at the liquid–vapor interface, which can be compressed to mean molecular areas of ∼22 and ∼32 Å 2 for pendent hydrocarbon and fluorocarbon chains, respectively. Subsequent Langmuir–Blodgett transfer onto glass substrates at moderate surface pressures leads to compact monolayers for single-component precursors, while mixed alkyl- and perfluoroalkylsilanes produce nonhomogeneous films characterized by transfer ratios greater than unity. As an alternate monolayer preparation technique, silane polymerization was performed directly on siliceous surfaces via a chemical adsorption mechanism. XPS analysis of a chemically adsorbed 1H,1H,2H,2H-perfluorodecylsiloxane film confirms a single adsorbed monolayer thickness in which the pendent fluoroalkyl chains align nonperpendicularly with respect to the surface. The surface free energy was determined to be 11.4 dyn cm −1 based on static contact angle measurements. AFM imaging shows the presence of surface defects due to oligomer deposition during the drying process. The use of solubilized trichloro-based silane coupling agents under anhydrous conditions is shown to produce surfaces with a minimal number of surface defects. The presence of undissolved silane material in the bulk solution significantly increases the number of surface defects.

The insertion reaction of acetonitrile on aryl nickel complexes stabilized by bidentate N,N′-chelating ligands

Organometallic complexes to be used as single component precursors in the catalytic dimerization/polymerization of olefins usually must contain a labile ligand that can easily be displaced by the olefin. This is the first step in the activation of the precursor. One commonly used labile ligand is a nitrile. Here we report an example of incompatibility between the nickel or palladium aryl bond and acetonitrile. Neutral [MBr(Mes)NN] complexes in which Mes=2,4,6-Me3C6H2, NN=diazabutadiene (DAD), pyridinylimine (PIM), 2,2'-bipyridine (bipy) or 1,10-phenanthroline (phen) gave the expected [M(Mes)(3,5-lut)(NN)][BF4] compounds and the unexpected [Ni(Mes){NH=C(Me)(2,4,6-Me3C6H2)}(NN)][BF4] complexes in the presence of TlBF4 and 3,5-lutidine or acetonitrile. The sequence of reactions that leads to the imine ligand must include an initial insertion of the nitrile on the sigma(Ni-Mes) bond. These ionic complexes remain stable under 20 bar of ethylene.

Synthetic Routes to Perovskite Phase Mixed Metal Oxides

AbstractWe are developing strategies to form single-component precursors to perovskite phase mixed metal oxides where the stoichiometry of the precursor is fixed at the molecular level to be that of the final desired phase. These strategies ean be applied to the formation of perovskite phase thin films. Two such methods are described herein, relying on ester elimination and alcohol elimination reactions. We have found the reaction between lead(II) acetate and titanium(IV) isopropoxide in ethanol results in ligand redistribution at room temperature. Reactions between (β-diketonate)2M(OR)2 where M = Ti, Zr and Sn and lead(II) acetate do not result in any detectable ester elimination under the reaction conditions employed, and this approach was not successful. Alcohol elimination experiments have been explored as an alternative strategy using a bifunctional glycolate ligand. Lead carbonate reacted with glycolic acid to form the corresponding lead glycolate complex Pb(O2CCH2OH)2. This compound reacted with (β-diketonate)2M(OR)2, M = Ti and Sn, to eliminate two equivalents of alcohol and form the corresponding single component (β-diketonate)2M(OCH2CO2)2Pb complexes. These complexes start to thermally decompose at 150°C and loss of weight is complete by 450°C to yield a product whose weight loss corresponds to the formation of PbTiO3 for M = Ti. Bulk thermolysis of a sample of (dpm)2Ti(OCH2CO2)2Pb at 400°C resulted in formation of crystalline perovskite phase lead titanate together with PbO, massicot and α-PbO phases.