Hafnium Dichloride Research Articles

Facile Preparation of a Novel HfC Aerogel with Low Thermal Conductivity and Excellent Mechanical Properties.

Aerogels emerge as captivating contenders within the realm of high-temperature thermal resistance and thermal insulation. Nevertheless, their practical applications are usually constrained by their inherent brittleness when subjected to rigorous conditions. Herein, employing hafnium dichloride oxide octahydrate (HfOCl2·8H2O) as the hafnium source and resorcinol-formaldehyde (RF) as the carbon precursor, hafnium carbide (HfC) aerogels are fabricated via the sol-gel method complemented with carbothermal reduction reaction. Investigations are conducted into the effects of various molar ratios, duration, and temperatures of calcination on the microstructural features and physico-chemical characteristics of the as-prepared HfC aerogel. The aerogel shows a high BET-specific surface area (601.02 m2/g), which is much larger than those of previously reported aerogels. Furthermore, the HfC aerogel exhibits a low thermal conductivity of 0.053 W/(m·K) and a compressive strength of up to 6.12 MPa after carbothermal reduction at 1500 °C. These excellent thermal insulation and mechanical properties ensure it is ideal for the utilization of high-temperature thermal resistance and thermal insulation in the fields of aerospace.

Effects of hafnium sources and hafnium content on the structures and properties of SiBNC–Hf ceramic precursors

AbstractThe synthesis of SiBNC–M (M is metal element) multinary ceramic precursors has mainly been through chemical modification of the SiBNC ceramic precursors with the introduction of the metal element/compound. This misses the simplicity, efficiency, and combined benefits of single reactor synthesis routes. We herein adopt a one‐pot method to synthesize SiBNC–Hf ceramic precursors (PBSHZ) from different Hf sources, and with varying Hf content. The starting materials include hafnium tetrachloride, hafnium dichloride, trichlorosilane, hexamethyldisilazane, and boron trichloride. Fourier transform infrared spectroscopy (FT‐IR), nuclear magnetic resonance (NMR), X‐ray photoelectron spectroscopy (XPS), and thermogravimetric analysis are employed to characterize the structures and properties of the PBSHZ. The results reveal the successful incorporation of Hf into the precursors. Compared to the hafnium tetrachloride source, the precursor synthesized from hafnium dichloride yields more ceramics and shows better solubility, due to fewer Hf–Cl bonds available for reaction. Meanwhile, there is a positive correlation between the rise in the Hf content of PBSHZ and the ceramic yield. However, the solubility and processability of the precursors decline, due to the multiplication of the cross‐linking degree in the molecular structure. Further FT‐IR, NMR, XPS, and thermogravimetry–mass spectrometry (TG–MS) analysis indicate a full polymer‐to‐ceramic transformation at 1000°C pyrolysis temperature. At this point, the SiBNC–Hf ceramics mainly consist of an Si–B–N skeleton, HfN, and free carbon.

Unbridged 1- and 2-substituted bis(silylindenyl) zirconium(IV) and hafnium(IV) dichloride complexes as catalyst precursors for ethylene polymerization

Twelve unbridged metallocene dichloride complexes of the types [1-(4-XC6H4SiMe2)-η5-Ind]2MCl2 and [2-(4-XC6H4SiMe2)-η5-Ind]2MCl2 (X=Me, MeO, F; M=Zr, Hf) with differently 1- and 2-substituted indenyl ligands have been synthesized, characterized and applied for catalytic ethylene polymerization. After activation with methylaluminoxane (MAO), all complexes are catalysts for ethylene polymerization. However, in nearly all cases, the species with the silyl substituent in the 1-position show much higher activities than those with the same substituent in 2-position of the indenyl moiety. For instance, the MAO activated complex bis(η5-1-(dimethyl-4-tolylsilyl)idenyl) zirconium(IV) dichloride (3), displayed an almost five times higher activity (3980kg PE/mol cat.h) than the isomeric bis(η5-2-(dimethyl-4-tolylsilyl)indenyl) zirconium(IV) dichloride (19)/MAO catalyst (870kg PE/mol cat.h). The same trend was observed for the para-fluorophenyl silyl indenyl complexes 23 and 17. This behavior may be explained in a way that the same silyl substituent in position 2 can exert more steric hindrance around the metal center compared to the 1-substituted analogues. The GPC characterization of the produced polyethylenes showed that some of the resins have a bimodal molecular weight distribution indicating at least two different active sites that are involved in the polymerization process.

Preparation and high‐temperature behavior of HfC–SiC nanocomposites derived from a non‐oxygen single‐source‐precursor

AbstractA single‐source precursor for the preparation of HfC‐SiC ceramics was synthesized via a Grignard reaction using bis(cyclopentadienyl)hafnium(IV) dichloride, trans‐1,4‐dibromo‐2‐butene, and (chloromethyl)trimethylsilane as raw materials. The composition, structure, pyrolysis process and high‐temperature behavior of the precursor were investigated. The results show that the precursor with a backbone comprising Hf–C, Si–C and CH=CH groups exhibits good solubility in common solvents, such as tetrahydrofuran, dimethylbenzene, and chloroform. Pyrolysis of the precursor at 1000°C yielded a microcrystalline HfC phase with a ceramic yield of 63.86 wt%. The pyrolytic products at 1600°C were HfC–SiC nanocomposite ceramics, which exhibited good thermal stability up to 2400°C. The formation of a (Hf,Si)C solid‐solution would be beneficial for densification during the sintering process. The non‐oxygen structure, high ceramic yield, homogeneous composition and excellent high‐temperature behavior of the pyrolytic products make the as‐prepared precursor a promising material for the preparation of high‐performance ultra‐high‐temperature ceramics.

Hydrogenation of polystyrene-b -polybutadiene-b -polystyrene mediated by group (IV) metal metallocene complexes

Various group (IV) metal complexes, namely bis(cyclopentadienyl) titanium dichloride, bis(pentamethylcyclopentadienyl) titanium dichloride, cyclopentadienyl titanium trichloride, pentamethylcyclopentadienyl titanium trichloride, bis(cyclopentadienyl) zirconium dichloride, and bis(cyclopentadienyl) hafnium dichloride, were used as the catalysts for mediating styrene–butadiene–styrene hydrogenation. The catalytic efficiency of these catalysts was examined. The results show that catalyst activity strongly depends on the chemical structure of the metallocene complex. We also found that trialkylaluminum has a significant influence on the hydrogenation activity and thermal stability of metallocene catalysts. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 2141–2149

Ethylene Polymerization with a Hafnocene Dichloride Catalyst Using Trioctyl Aluminum and Borate: Polymerization Kinetics and Polymer Characterization

The polymerization of ethylene is investigated in a semibatch solution reactor using bis(n‐propylcyclopentadienyl)hafnium dichloride catalyst and tetrakis(pentafluorophenyl) borate dimethylanilinium salt ([B(C6F5)4]−[Me2NHPh]+) as the catalytic system. Trioctylaluminum (TOA) is used as impurity scavenger and alkylating agent. Ethylene pressure, polymerization temperature, TOA, borate, and catalyst concentrations are changed to investigate ethylene polymerization kinetics with this catalyst system. A 23 central composite design, augmented with extra runs to further explore the effect of some factors, is used as the statistical basis for the polymerization study. Ethylene propagation follows first‐order kinetics. Chain transfer to monomer, β‐hydride elimination, and transfer to TOA are the main chain transfer reactions. In addition to alkylating the catalyst precursor, TOA also deactivates the catalyst. The mode of reactor addition for catalyst, borate, and TOA has also been studied. When TOA and borate are added sequentially to the reactor, followed by the catalyst, the polymerization activity is lower than when the catalyst and borate are added simultaneously, suggesting that complexation with borate avoids deactivation reactions with TOA. image

Alkoxido-, amido-, and chlorido derivatives of zirconium- and hafnium bis(amido)cyclodiphosph(V)azanes: Ligand ambidenticity and catalytic productivity

Bis(tert-butylamido)cyclodiphosph(III)azanezirconium- and hafnium dichloride are good polyethylene pre-catalysts, but due to the basicity of their phosphorus(III) atoms they suffer from ligand degradation. To lessen the Lewis-acid attacks on these complexes, the phosphorus-nitrogen ligands were oxidized with sulfur to afford the corresponding bis(tert-butylamido)cyclodiphosph(V)azanes, which are devoid of such very Lewis-basic sites. These modified ligands, however, no longer bind the metals in the desired κ2N,N′ fashion, but laterally in the catalytically inactive κN,κEl mode. Treatment of the Group 4 metal dichlorides with sodium tert-butoxide afforded the bis(tert-butylamido)cyclodiphosph(III)azanemetal di-tert-butoxides in high yields. These compounds were amenable to sulfur oxidation, yielding bis(tert-butylamido)cyclodiphosph(V)azanemetal di-tert-butoxides in equally high yields. The oxidized complexes retain the κ2N,N′ structure, prompting us to reinvestigate a similar oxidation of the bis(tert-butylamido)cyclodiphosph(III)azanemetal dichlorides. The dichloride complexes were more difficult to oxidize, but ultimately bis(tert-butylamido)cyclodiphosph(V)azanezirconium- and hafnium dichlorides were obtained and shown to be more productive catalysts than their phosphorus(III) counterparts.

Effects of Cl-Based Ligand Structures on Atomic Layer Deposited HfO2

Atomic layer deposition (ALD) of HfO2 is a key technology for the application of high dielectric constant gate dielectrics ranging from conventional Si devices to novel nanodevices. The effects of the precursor on the growth characteristics and film properties of ALD HfO2 were investigated by using hafnium tetrachloride (HfCl4) and bis(ethylcyclopentadienyl)hafnium dichloride (Hf(EtCp)2Cl2, Hf(C2H5C5H4)2Cl2) with O2 plasma reactant. The growth characteristics were significantly affected even by simply changing the precursor. Theoretical calculations utilizing geometrical information on the precursor and density functional theory revealed that the steric demands of the precursor ligands have a dominant effect on the different growth characteristics rather than the reaction probability of the precursor on the surface. The chemical compositional analysis results showed that the Cl residue in the HfO2 films was reduced by using Hf(EtCp)2Cl2 due to the lower number of Cl atoms in each Hf precursor molecule and...

Interpreting “Acidity” as a Global Property Controlling Comonomer Reactivity in Olefin Polymerization

A possible rationale for the different catalytic behaviors of systems based on rac-(ethylenebis(1-indenyl))zirconium dichloride (rac-EBIZrCl2), rac-(ethylenebis(1-indenyl))hafnium dichloride (rac-EBIHfCl2), and rac-(isopropylidenebis(1-indenyl))zirconium dichloride (rac-iPrBIZrCl2) toward ethene–styrene copolymerization has been sought by studying related active systems. For this purpose, the metallocene ion pairs (IPs) rac-EBIZrMe—MeB(C6F5)3, rac-EBIHfMe—MeB(C6F5)3, and rac-iPrBIZrMe—MeB(C6F5)3 have been synthesized and their structures in solution explored with ROESY and pulsed gradient NMR spectroscopy. The energetics of dynamical processes relevant for catalysis that can be used as indicators of the cation acidity have been studied with variable-temperature NMR experiments and density functional theory (DFT). NMR experiments successfully provided IP structural details in solution and also indicated the presence of an intricate dynamic behavior for all the IPs. DFT results, instead, indicated quantitatively how changing the metal and/or the ancillary ligand bridge influences the energetics of the active species and modifies the reaction energy profile. The theoretical results also drew attention to the fact that finding a rationale for the ligand influence on the catalytic behavior of rac-EBIZrCl2/MAO and rac-iPrBIZrCl2/MAO in ethene–styrene copolymerization requires not only considering the steric effects but also determining how subtle changes in the ligand sphere affect the capability of the metal center to accept electrons from the counteranion or the olefins.

Polymerization Behavior of C1-Symmetric Metallocenes (M = Zr, Hf): From Ultrahigh Molecular Weight Elastic Polypropylene to Useful Macromonomers

The C1-symmetric metallocenes rac-[1-(9-η5-fluorenyl)-2-(5,6-cyclopenta-2-methyl-1-η5-indenyl)ethane]zirconium dichloride (1) and rac-[1-(9-η5-fluorenyl)-2-(5,6-cyclopenta-2-methyl-1-η5-indenyl)ethane]hafnium dichloride (2) are known to produce elastic polypropylene. They were investigated concerning their temperature stability during the polymerization of propene. After activation of these complexes with triisobutylaluminum (TIBA)/[CPh3][B(C6F5)4], first polymerization experiments (80–100 °C) afforded moderate to high activities. However, at these polymerization temperatures, the molecular weights of the produced polymers are significantly decreased, resulting in a waxy appearance and, therefore, a loss of the elastic behavior. The main reason for this behavior, especially for the more significant decrease of the molecular weight in the case of hafnocene 2 compared with zirconocene 1, was revealed to occur due to a fast β-methyl transfer reaction. Hence, hafnocene 2 can form polymer chains with a high selectivity toward allylic chain ends. These macromonomers can be used in the catalytic insertion polymerization for the formation of new grafted copolymers. Initial copolymerization experiments with ethene were conducted.

Hafnium metallocene compounds used as cathode interfacial layers for enhanced electron transfer in organic solar cells

We have used hafnium metallocene compounds as cathode interfacial layers for organic solar cells [OSCs]. A metallocene compound consists of a transition metal and two cyclopentadienyl ligands coordinated in a sandwich structure. For the fabrication of the OSCs, poly[3,4-ethylenedioxythiophene]:poly(styrene sulfonate), poly(3-hexylthiophene-2,5-diyl) + [6,6]-phenyl C61 butyric acid methyl ester, bis-(ethylcyclopentadienyl)hafnium(IV) dichloride, and aluminum were deposited as a hole transport layer, an active layer, a cathode interfacial layer, and a cathode, respectively. The hafnium metallocene compound cathode interfacial layer improved the performance of OSCs compared to that of OSCs without the interfacial layer. The current density-voltage characteristics of OSCs with an interfacial layer thickness of 0.7 nm and of those without an interfacial layer showed power conversion efficiency [PCE] values of 2.96% and 2.34%, respectively, under an illumination condition of 100 mW/cm2 (AM 1.5). It is thought that a cathode interfacial layer of an appropriate thickness enhances the electron transfer between the active layer and the cathode, and thus increases the PCE of the OSCs.

Electronic structure of two catalytically important hafnocenes

The electronic structures of trichloro[(1,2,3,3a,7a-η)-1H-inden-1-yl]-hafnium and bis(cyclopentadienyl)hafnium dichloride have been investigated by HeI and HeII UV photoelectron spectroscopy and DFT/OVGF calculations. The chlorine lone pair ionization energies are reduced considerably on going from HfCl4 to metallocene hafnium chlorides. The electronic structure results have been related to the mechanism of olefin polymerization catalyzed by hafnocenes.

Synthesis, spectroscopic and antibacterial studies on bis(cyclopentadienyl)- hafnium(IV) derivatives with dithiocarbamates derived from α-amino acids

The reactions of bis(cyclopentadienyl)hafnium(IV) dichloride with novel dithiocarbamate ligands [Na 2 (S 2 C-NH-(R)-COO)], derived from α-amino acids (glycine, leucine, β-alanine and dl-alanine), have been studied in THF/DMF mixture and binuclear complexes of type [{(C 5 H 5 ) 2 HfCl} 2 (S 2 C-NH-(R)-COO)] have been isolated. Tentative structures of the complexes have been proposed on the basis of analyses, electrical conductance, magnetic moment and spectral (UV-Vis, FT-IR, 1 H NMR, 13 C NMR and FAB-mass) data. NMR spectra indicate that there is rapid rotation of the cyclopentadienyl ring around the metal-ring axis at 25 °C. Studies were conducted to assess the growth inhibiting potential of the ligands and complexes against various bacterial strains. The complexes are found to be potent bactericides than the ligands.

Activation of hafnocene catalyzed polymerization of 1-hexene with MAO and borate

A study of 1-hexene polymerization with ethylene- bis(9-fluorenyl) hafnium dichloride has been carried out using two different cocatalyst systems, methyl-aluminoxane/trimethylaluminum (MAO/TMA) and tris-isobutyl-aluminum/ N, N-dimethylanilinium tetrakis(pentafluorophenyl) borate (TIBA/borate). When MAO/TMA was used, 1-hexene polymerized into a low molar mass poly(1-hexene) with low catalytic activity. Activation with TIBA/borate increased polymerization activity drastically as well as the molar mass of the polymers. In order to analyze differences in the activity profiles, UV–Vis spectroscopy was employed to investigate ligand to metal charge transitions (LMCT) of the hafnocene dichloride during the activation process. The low catalytic activity and the fast chain transfer to the cocatalyst with MAO/TMA may originate from strong bonding between the metallocene cation and the MAO/TMA species thus obstructing monomer coordination and insertion.

Cubic phase stabilization in nanoparticles of hafnia-zirconia oxides: Particle-size and annealing environment effects

Amorphous hafnia (HfO2−y), zirconia (ZrO2−y), and hafnia-zirconia (xHfO2−y–(1−x)(ZrO2−y)) nanoparticles were prepared by combining aqueous solutions of hexamethylenetetramine (HMT) with hafnium dichloride oxide (HfOCl2⋅8H2O), zirconium dichloride oxide (ZrOCl2⋅8H2O), or a mixture of these two salts at room temperature. For pure hafnia, transmission electron microscopy showed that the lower cation concentration (0.01M) resulted in the precipitation of smaller amorphous nanoparticles relative to higher concentrations (0.015M–0.04M). Consequently, the lower concentration preparation route coupled with a reducing environment (H2:N2=9:91) during annealing at temperatures between 650 and 850°C allowed for nanoparticles with a cubic structure to be prepared as determined by x-ray diffraction. The structurally cubic hafnia nanoparticles were 6nm or less in diameter and equiaxed. Using the same method (0.01M total metal cation concentration and reducing environment during annealing), nanoparticles of cubic structure were prepared across the entire hafnia-zirconia compositional spectrum, with a critical particle size for the cubic structure of about 6nm. Nanoparticles of tetragonal and monoclinic structure were prepared by increasing the annealing temperature and/or using a less reducing environment. The unique role of HMT in sample preparation is discussed as well.

Synthesis and Characterization of (2,6-iPrNCN)HfCl2- and (3,5-MeNCN)2Hf2- (where NCN = 2,6-bis[phenylazanidyl]methylphenyl): New Trianionic Pincer Ligands

The synthesis, isolation, and metalation of the trianionic NCN-pincer ligand [2,6-iPrNCNLi3]2 (2) is described. The terdentate coordination ability of 2 is demonstrated in the synthesis of the hafnium dichloride anion [2,6-iPrNCNHfCl2][Li(DME)3] (3-DME). Complex 3-DME is distorted square pyramidal in the solid state, but NMR evidence indicates the complex is fluxional and trigonal bypyramidal in solution. Straightforward ligand modification provides the additional trianionic pincer ligand [3,5-MeNCNLi2]2[Li2(DME)6] (5). This sterically smaller ligand leads to the hafnium pincerate dianion complex [(3,5-MeNCN)2Hf][Li2(DME)2] (6), which contains two trianionic pincer ligands. The new compounds are characterized by elemental analysis, multinuclear NMR, and X-ray diffraction crystallography.

Violet–blue luminescence from hafnium oxide layers doped with CeCl3 prepared by the spray pyrolysis process

AbstractHfO2:CeCl3 coatings were deposited by the spray pyrolysis method employing hafnium dichloride oxide and CeCl3 dissolved in deionized water (18 MΩ/cm). The room temperature photoluminescence characteristics of the HfO2:CeCl3 films were studied as a function of the deposition parameters such as doping concentrations and substrate temperature. The presence of two different Ce3+ centres in HfO2 is detected from photoluminescence measurements. A reduction of the luminescence intensity is observed with an increase of both the CeCl3 concentration and the deposition temperature. X‐ray diffraction measurements of these films showed that the crystalline structure depends on the substrate temperature. For substrate temperatures less than 350 °C the deposited films are almost amorphous, while substrate temperatures higher than 400 °C produce diffraction peaks corresponding to the monoclinic phase of HfO2. The chemical composition of the films as determined by energy dispersive spectroscopy is also reported. Furthermore, the surface morphology characteristics of the coatings, as a function of the deposition temperature, are also presented. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Synthesis of propylene-ethylene copolymers in liquid propylene using ansa-metallocenes of the C 1 symmetry

The copolymerization of ethylene with propylene in the liquid propylene initiated by ansa-metallocenes of the C 1 symmetry, rac-[1-(9-η5-fluorenyl)-2-(5,6-cyclopenta-2-methyl-1-η5-indenyl)ethane]zirconium dichloride and rac-[1-(9-η5-fluorenyl)-2-(5,6-cyclopenta-2-methyl-1-η5-indenyl)ethane]hafnium dichloride, activated by methylaluminoxane has been studied. Triisobutylaluminum has been used as a cocatalyst. The propylene-ethylene copolymers thus prepared contain 5–60 mol % ethylene units. The reactivity ratios have been measured. In the case of the zirconocene-based catalyst, the molecular mass of the copolymers decreases with an increase in the content of ethylene units. The reverse situation is observed in the case of the hafnocene-based catalytic system. The copolymers are characterized by the low T g values (down to −45°C). Incorporation of a small amount of ethylene units (5 mol %) results in a rise in the elastomeric behavior of the polymers.

Mono- and bis(cyclopentadienyl)hafnium(IV) derivatives with alkyl xanthates

Hafnium(IV) alkyl xanthates of the types Cp 2 Hf(S 2 COR)Cl and CpHf(S 2 COR) 3 , where R= C 2 H 5 , n-C 3 H 7 n-C 4 H 9 and n-C 5 H 11 , have been prepared by the reactions of bis(cyclopentadienyl)hafnium(iv) dichloride with potassium alkyl xanthates in anhydrous tetrahydrofuran. Conductance and infrared studies suggest that these complexes are non-electrolytes in which all of the xanthate ligands are bidentate. 1 H and 13 C NMR spectra of these complexes indicate that there is rapid rotation of the cyclopentadienyl ring about the metal-ring axis and for the CpHf(S 2 COR) 3 complexes, non-equivalence of the alkyl xanthate ligands was observed.

Dichloro[(η5-cyclopentadienyl)dimethyl(η5-3-phenylindenyl)silane]hafnium(IV): a powder study

The title compound, [Hf(C22H20Si)Cl2], was solved by simulated annealing from laboratory X-ray powder diffraction data collected at room temperature. The mol­ecular structure comprises a hafnium dichloride centre, coordinated by a η5-cyclo­penta­dienyl and a η5-3-phenyl­indenyl unit, which are connected through a shared dimethyl­silicon linkage.