Zhongliang Tian

Calculation of the surface free energy of fcc copper nanoparticles

Using molecular dynamics simulations with the modified analytic embedded-atom method we calculate the Gibbs free energy and surface free energy for fcc Cu bulk, and further obtain the Gibbs free energy of nanoparticles. Based on the Gibbs free energy of nanoparticles, we have investigated the heat capacity of copper nanoparticles. Calculation results indicate that the Gibbs free energy and the heat capacity of nanoparticles can be divided into two parts: bulk quantity and surface quantity. The molar heat capacity of the bulk sample is lower compared with the molar heat capacity of nanoparticles, and this difference increases with the decrease in the particle size. It is also observed that the size effect on the thermodynamic properties of Cu nanoparticles is not really significant until the particle is less than about 20 nm. It is the surface atoms that decide the size effect on the thermodynamic properties of nanoparticles.

Effect of Ni content on corrosion behavior of Ni/(10NiO-90NiFe2O4) cermet inert anode

Abstract Ni/(10NiO-90NiFe 2 O 4 ) cermet inert anodes with metal Ni content of 0, 5, 10, 15 and 20 (mass fraction, %) were prepared and their corrosion behavior in Na 3 AlF 6 -Al 2 O 3 melts was investigated in laboratory electrolysis tests. The results indicate that the content of metal Ni in anodes has little effect on the steady-state concentration of impurities Ni and Fe in electrolyte and the values range in (114–173)×10 −6 and (287–385)×10 −6 , but the content of impurities in the metal aluminum manifolds. There is preferential corrosion for metal Ni in NiO-NiFe 2 O 4 based cermet anodes. Considering the corrosion resistance and electrical conductivity, the cermet containing 5%Ni (mass fraction) behaves best among NiO-NiFe 2 O 4 based cermet anodes studied, and should be further studied.

Results from 100 h electrolysis testing of NiFe2O4 based cermet as inert anode in aluminum reduction

A 100 mm diameter cup-shaped inert anode for aluminum electrolysis consisting of cermet 17Ni/83(10NiO-90NiFe2O4) was prepared and the operating performance was evaluated in a laboratory cell with the electrolyte CR2.3 and Al2O3 concentration 7.43% (mass fraction). The results indicate that no major operational difficulties are encountered during the testing which lasts for 101.5 h and the inert anode exhibits good general performances. The steady-state average concentration of impurity Ni in the bath is close to the solubility, however, the Fe concentration is lower than its solubility. The contents of the main contaminants for aluminum produced are Ni 0.128 8%, Fe 1.007 4%. The corrosion rate of inert anode under electrolysis conditions based on the content of impurity Ni in metal aluminum is approximately 8.51 mm/a.

Preliminary testing of NiFe2O4-NiO-Ni cermet as inert anode in Na3AlF6-AlF3 melts

The electrical conductivity of cermet 83(90NiFe2O4-10NiO)-17Ni at different temperatures was measured in air, the operating performance of inert anode was evaluated in a laboratory electrolysis cell with various electrolyte compositions. The results indicate that the electrical resistivity of cermet studied has negative temperature coefficient, which is the characteristic of semi-conducting material. The proper addition of AlF3 in the bath can improve the corrosion resistance of cermet inert anode, but excess adding amount will cause the catastrophic corrosion. Post-examination of anodes shows that metal Ni leaches preferentially on the anode surface. Chemical dissolution, electrolyte penetration as well as electrochemical dissolution serve as major corrosion mechanisms.

Effect of sintering atmosphere on phase composition and mechanical properties of 5Ni/(10NiO-NiFe2O4)

Abstract 5Ni/(10NiO-NiFe2O4) cermet materials were prepared by cold pressing sintering method in different atmospheres. The phase compositions were studied by XRD, microstructure was studied by SEM and mechanical properties were studied by three-point-bending strength and heat cycle. The results show that Ni/(NiO-NiFe2O4) cermet can be obtained by sintering in vacuum or atmospheres with oxygen content of 2×10−5, 2×10−4 and 2×10−3, respectively, but the phase composition of the ceramics varies greatly with oxygen content of the sintering atmosphere. The cermet tends to have a high content of NiO when sintered in vacuum. The content of NiFe2O4 and Ni in the cermet materials respectively increases and decreases with increasing oxygen in the sintering atmosphere. In the vacuum, cermet has an average grain size of 3.90 μm and bending strength of 138.59 MPa, which is able to withstand thermal shock tests 6.67 times in average.

Effect of adding methods of metallic phase on microstructure and thermal shock resistance of Ni/(90NiFe2O4-10NiO)cermets

Abstract Ball mixing and electroless plating were respectively used as the adding methods of metallic phase to prepare Ni/(90NiFe2O4-10NiO) cermets for the inert anode in aluminum electrolysis. The microstructure and thermal shock resistance of cermet samples were studied. The results show that, for the samples prepared by ball mixing method, aggregation of metallic phase is found in either the green blocks or sintered samples and the extent of aggregation increases with the increase of metal content. For 6.5Ni/(90NiFe2O4-10NiO) cermets prepared with electroless plating method, the homogeneous and fine metallic particles are found in either the green compacts or sintered samples, but the relative density and thermal shock residual strength decrease by 3% and 28%–58% respectively, compared with samples prepared with ball mixing method.

Effect of Cu-Ni content on the corrosion resistance of (Cu-Ni)/(10NiO-90NiFe2O4) cermet inert anode for aluminum electrolysis

(Cu-Ni)/(10NiO-90NiFe2O4) cermet inert anodes containing metal Cu-Ni 0, 5, 10, 15 and 20 wt pct were prepared and their corrosion resistance to Na3AlF6-Al2O3 melts was investigated. The results indicate that the content of metal Cu-Ni has little effect on the steady-state concentration of Ni in the electrolyte and the values could not be used to effectively differentiate their corrosion resistance. The steady-state concentration of Fe decreases from 304×10−6 to 168×10−6 and that of Cu increases from 21×10−6 to 71×10−6 with the content of metal Cu-Ni increasing from 0 to 20 wt pct. Post-examination shows that metallic phase Cu-Ni is corroded preferentially during electrolysis and many pores are left at the anode surface. Considering the corrosion resistance and electrical conductivity, the cermet containing metal Cu-Ni 5 wt pct should be selected and studied further.

Effect of sintering atmosphere on corrosion resistance of Ni/(NiFe2O4-10NiO) cermet inert anode for aluminum electrolysis

Abstract A comparative study on the corrosion resistance of 17Ni/(NiFe2O4–10NiO) cermet inert anode prepared in different sintering atmospheres was conducted in Na3AlF6–Al2O3 melt. The results indicate that the corrosion rates of NiFe2O4-based cermet anodes prepared in the vacuum and the atmosphere with oxygen content of 2×10−3 (volume fraction) are 6.46 and 2.71 cm/a, respectively. Though there is a transition layer with lots of holes or pores, a densified layer is formed on the surface of anode due to some reactions producing aluminates. For the anode prepared in the atmosphere with oxygen content of 2×10−3, the thickness of the densification layer (about 50 μm) is thicker than that (about 30 μm) formed on the surface of anode prepared in the vacuum. The contents of NiO and Fe(II) in NiFe2xO4–y–z increase with the decrease of oxygen content in sintering atmosphere, which reduces the corrosion resistance of the material.

Preliminary Study on Preparation of Al-Sc Master Alloy in Na3AlF6-K3AlF6-AlF3 Melt

The Al-Sc Alloys show many advanced properties for a wide range of engineering applications. But the metal Sc must be added into the metal Al in the form of the Al-Sc master alloy, which is usually produced by the thermal reduction process. In this paper, the decomposition voltages of Sc2O3 and Al2O3 in Na3AlF6-K3AlF6-AlF3 melt were calculated based on the Nernst equation. And the Al-Sc master alloy was prepared in the melt at 950 °C by electrolysis. The results show that the Al-Sc master alloy can be prepared by the co-deposition of Sc3+ and Al3+ at the cathode if the concentration ratio of Sc2O3 to Al2O3 in the melt is more than 2:1 under the condition of the aluminum electrolysis. SEM and ICP analysis of the product indicates that the Al-Sc master alloy is made of the phase Al and the phase Al3Sc.

Studies on the Purification of Na 3 AlF 6− K 2 SiF 6− AlF 3 Melt

The purity of the electrolyte has an important influence on the content of the impurities in electrodeposited silicon. In this paper, effects of electrolysis temperature, current density and the time on the concentrations of two important impurities: phosphorus and iron in the electrolyte were investigated during the purification of Na3AlF6−10%K2SiF6−AlF3 melt. It was observed that the concentrations of P and Fe in the electrolyte decreased with the electrolysis temperature, current density and the electrolysis time increasing. X-ray fluorescence (XRF) of the electrolyte shows that the concentrations of P and Fe decrease from 4.2ppmw to 1.3ppmw and 204.0ppmw to 42.6ppmw, respectively. The removal rates of P and Fe are 69.08% and 79.12% when the electrolysis was performed for the duration of 3 hours and at a current density of 20mA•cm−2 while the operating temperature was 1225K.