Effect of milling method and time on the properties and electrochemical performance of LiFePO4/C composites prepared by ball milling and thermal treatment

Effects of ball milling way and time on the phase formation, particulate morphology, carbon content, and consequent electrode performance of LiFePO₄/C composite, prepared by high-energy ball milling of Li₂CO₃, NH₄H₂PO₄, FeC₂O₄ raw materials with citric acid as organic carbon source followed by thermal treatment, were investigated. Three ball milling ways and five different milling durations varied from 0 to 8 h were compared. LiFePO₄/C composites could be obtained from all synthesis processes. TEM examinations demonstrated LiFePO₄/C from ball milling in acetone resulted in sphere shape grains with a size of ∼60 nm, similar size was observed for LiFePO₄/C from dry ball milling but in a more irregular shape. The ball milling in benzene resulted in a much larger size of ∼250 nm. The LiFePO₄/C composites prepared from dry ball milling and ball milling in acetone showed much better electrochemical performance than that from ball milling in benzene. SEM examinations and BET measurements demonstrated that the high-energy ball milling effectively reduced the grain size. A ball milling for 4 h resulted in the best electrochemical performance, likely due to the proper amount of carbon and proper carbon structure were created.     

An investigation of fractal characteristics of mesoporous carbon electrodes with various pore structures

Fractal characteristics of mesoporous carbon electrodes were investigated with various pore structures using the N₂ gas adsorption method and the transmission electron microscopy (TEM) image analysis method. The mesoporous carbons with various pore structures were prepared by imprinting mesophase pitch used as a carbonaceous precursor with different colloidal silica particles. All imprinted mesoporous carbons were composed of two groups of pores produced from the carbonisation of mesophase pitch and from the silica imprinting. The overall surface fractal dimensions of the carbon specimens were determined from the analyses of the N₂ gas adsorption isotherms. In order to distinguish the surface fractal dimension of the carbonisation-induced pore surface from that fractal dimension of the silica-imprinted pore surface, the individual surface fractal dimensions were determined from the image analyses of the TEM images. From the comparison of the overall surface fractal dimension with the individual surface fractal dimensions, it was recognised that the overall surface fractal dimension is crucially influenced by the individual surface fractal dimension of the silica-imprinted pore surface. Moreover, from the fact that the silica-imprinted pore surface with broad relative pore size distribution (PSD) gave lower value of the individual surface fractal dimension than that pore surface with narrow relative PSD, it is concluded that as the silica-imprinted pores comprising the carbon specimen agglomerate, the individual surface fractal dimension of that pore surface decreases.     

Characterization of single wall carbon nanotubes by nonane preadsorption

The preferential blocking of the interior adsorption sites of single walled carbon nanotubes (SWNTs) by n-nonane is demonstrated. Following adsorption of n-nonane and evacuation for 24 h at 323 K, it was found that interior sites with diameters less than ∼14 Å remained filled with n-nonane, blocking the physical adsorption of N₂ on these sites at 77.3 K. We demonstrate that “nonane blocking” is a very useful technique for nanotube porosity characterization.     

Preparation of short carbon nanotubes by mechanical ball milling and their hydrogen adsorption behavior

Short multi-wall carbon nanotubes (MWNTs) with open tips were obtained by mechanical ball milling. The microstructure characteristics of MWNTs before and after ball milling were checked by transmission electron microscopy (TEM). The effect of ball milling on the hydrogen adsorption behavior of the MWNTs was studied. The hydrogen adsorption experiments were carried out at room temperature under a pressure of 8-9 MPa. The hydrogen adsorption capacity of carbon nanotubes milled for 10 h was 0.66 wt%, which was about six times that of MWNTs without milling. For the carbon nanotubes milled with MgO for 1 h, a hydrogen adsorption capacity of 0.69 wt% was obtained. The enhancement of hydrogen adsorption might result from the increase of defects and surface area of the MWNTs caused by ball milling.     

Electrode properties of a double layer capacitor of nano-structured graphite produced by ball milling under a hydrogen atmosphere

Nano-structural graphite prepared by ball milling under H₂ or Ar atmosphere was studied as an electrode for electric double layer capacitors (EDLCs) by means of a conventional 2-electrode galvanostatic method. Especially, the product prepared under H₂ atmosphere using zirconia balls revealed 500 m² g⁻¹ surface area and showed 12 F g⁻¹ specific capacitance, which was comparable to that of an activated carbon with large specific surface area of 3000 m² g⁻¹ examined as a reference. A proper condition of the milling time is rather a shorter time than ∼8 h, where the graphitic feature is remained in the ball milled product. On the other hand, for the sample prepared by using steel balls, the specific capacitance per surface area was several hundreds times smaller than the others, indicating that the small amount of Fe contamination during milling played a negative role for the EDLC properties.     

多壁碳纳米管的球磨处理对其吸附储氢性能的影响

研究了球磨改性对多壁碳纳米管储氢性能的影响,球磨处理前后的碳纳米管微观结构采用TEM和XRD进行表征. 结果发现,球磨处理能使碳纳米管长度变短,管端口打开,缺陷增多,表面积增大,球磨处理12 h的碳纳米管的吸附量从未球磨的1.60%(w)提高到2.55%(w),表明球磨改性能明显提高碳纳米管的吸附量.     

Structure and gas permeability of multi-wall carbon nanotube buckypapers

We report on the filtration behavior, scanning electron microscopy (SEM) and gas permeability of multi-wall carbon mats (buckypapers). The SEM-apparent macropore diameter, image fractal dimension and lacunarity (a measure of translational invariance) of the samples averaged at 38 nm, 1.82 and 0.55, respectively. Their N₂ adsorption analysis revealed an average BET specific surface area of 197 m²/g, BJH pore diameter of 2.67 nm and FHH fractal dimension of 2.492. These parameters were rather insensitive to the preparation conditions. The effective diffusivity of six common laboratory gases (O₂, N₂, H₂, He, CO₂, CH₄) through buckypapers of different thicknesses was also measured. Results fell into the 3-12 × 10⁻⁹ m² s⁻¹ range and correlated with the kinetic diameter of the gases.     

Anomalous reduction in surface area during mechanical activation of boehmite synthesized by thermal decomposition of gibbsite

Mechanical activation of boehmite (γ-AlOOH), synthesized by thermal decomposition of gibbsite, has been carried out in a planetary mill up to 240 min. After an initial decrease in particle size up to 15 min, the particle size shows an increase with further milling; the median size (d₅₀) has increased from 1.8 to 5 μm during 15 to 240 min of milling. Quite unexpectedly, the BET specific surface area of the sample (N₂ adsorption method) decreases continuously from 264 m²/g to 67 m²/g with milling. A detailed analysis of N₂ adsorption/desorption isotherms has indicated that the decrease in surface area is associated with: (a) change in narrow slit like pores with microporosity to slit shaped pores originating from loose aggregate of platelet type particles; and (b) shift of maxima in pore size distribution plot at ~ 2 nm and ~ 4 nm to dominantly ~ 23 nm size pores. Scanning electron microscopy (SEM) studies have revealed that during milling, initial breakage is followed by agglomeration/fusion of particles with consequent loss in porosity. Amorphisation, decrease in microcrystallite dimension (MCD) and increase in microstrain (ε) are indicated from a detailed analysis of X-ray powder diffraction patterns and Fourier Transform Infrared (FTIR) spectra. Reactivity of samples, expressed in terms of increase in dissolution in alkali (in 8 M NaOH at 90 °C) and decrease in boehmite to γ-Al₂O₃ transformation temperature, increases with milling time. The nature of correlations between reactivity and physico-chemical changes during milling has been analyzed and discussed.     

Hydrogen adsorption in different carbon nanostructures

Hydrogen adsorption in different carbonaceous materials with optimized structure was investigated at room temperature and 77 K. Activated carbon, amorphous carbon nanotubes, SWCNTs and porous carbon samples all show the same adsorption properties. The fast kinetics and complete reversibility of the process indicate that the interaction between hydrogen molecules and the carbon nanostructure is due to physisorption. At 77 K the adsorption isotherm of all samples can be explained with the Langmuir model, while at room temperature the storage capacity is a linear function of the pressure. The surface area and pore size of the carbon materials were characterized by N₂ adsorption at 77 K and correlated to their hydrogen storage capacity. A linear relation between hydrogen uptake and specific surface area (SSA) is obtained for all samples independent of the nature of the carbon material. The best material with a SSA of 2560 m²/g shows a storage capacity of 4.5 wt% at 77 K.     

Quenched solid density functional theory and pore size analysis of micro-mesoporous carbons

We present a new model of adsorption on micro-mesoporous carbons based on the quenched solid density functional theory (QSDFT). QSDFT quantitatively accounts for the surface geometrical inhomogeneity in terms of the roughness parameter. We developed the QSDFT models for pore size distribution calculations in the range of pore widths from 0.4 to 35 nm from nitrogen at 77.4 K and argon at 87.3 K adsorption isotherms. The QSDFT model improves significantly the method of adsorption porosimetry: the pore size distribution (PSD) functions do not possess gaps in the regions of ∼1 nm and ∼2 nm, which are typical artifacts of the standard non-local density functional theory (NLDFT) model that treats the pore walls as homogeneous graphite-like plane surfaces. The advantages of the QSDFT method are demonstrated on various carbons, including activated carbons fibers, coal based granular carbon, water purification adsorbents, and mirco-mesoporous carbon CMK-1 templated on MCM-48 silica. The results of PSD calculations from nitrogen and argon are consistent, however, argon adsorption provides a better resolution of micropore sizes at low vapor pressures than nitrogen adsorption.     

Phase transformation and gas–solid reaction of Al2O3 during high-energy ball milling in N2 atmosphere

Al₂O₃ powders were milled in N₂ atmosphere using an attritor ball mill. The phase transformation and gas–solid reaction of Al₂O₃ during high-energy ball milling in N₂ atmosphere were investigated. It is found that the γ-Al₂O₃ transform to α-Al₂O₃ as milling time increases. Milled for 20 h in N₂ atmosphere, Al₂O₃ becomes partially amorphous and the gas–solid reaction between Al₂O₃ and N₂ takes place. A new phase with cubic aluminum nitride (AlN) structure forms during milling which is different from normal hexagonal AlN produced by carbon thermal reduction processing. As milling energy increases, the amount of this new phase with cubic AlN structure increases. The amount of this new phase is as high as 72% when Al₂O₃ is milled in N₂ atmosphere at 650 rpm for 40 h.     

Effect of ball milling on the corrosion resistance of magnesium in aqueous media

The influence of the high-energy ball milling on the corrosion behavior of magnesium in aqueous media has been investigated through electrochemical experiments complemented by morphological, structural, chemical and surface analyses. The milling duration was varied from 0 to 40 h. Polarization curves show that the milling procedure improves the magnesium corrosion resistance in passive conditions (KOH solution) and in more active corrosion conditions (borate solution). This is illustrated by the corrosion potential which becomes nobler with milling. The variation of the polarization resistance and related corrosion current with milling time is also an indication of the improvement of the Mg corrosion resistance due to the milling. Moreover, the passive current is significantly lower for milled Mg. The Mg crystallite size is reduced from >100 to 34 nm after 10 h of milling and does not decrease significantly with further milling. The iron contamination of the Mg powder due to the erosion of the milling tools is very low (0.09 wt.% after 40 h of milling). In contrast, a significant oxygen contamination occurs during milling (2.6 wt.% after 40 h of milling). XPS and AES data indicate MgO enrichment in the bulk of the milled Mg without significant MgO increase at the powder surface. The corrosion improvement was attributed to the increase through the milling process of the density of surface defects and grain boundaries susceptible to increase the number of nucleation sites for Mg hydroxylation in aqueous media, leading to the rapid formation of a dense and protective Mg(OH)₂ layer.     

Synthesis and characterisation of nanoporous carbide-derived carbon by chlorination of vanadium carbide

Nanoporous carbide-derived carbon (CDC) was synthesised from vanadium carbide (VC) powder via gas phase chlorination in the temperature range from 500 to 1100 °C. The XRD analysis of nanoporous carbon powder samples was carried out to investigate the structural changes (graphitisation) of nanoporous carbons synthesised. The first-order Raman spectra showed the absorption peak at ∼1582 cm⁻¹ and the disorder-induced (D) peak at ∼1345 cm⁻¹. The low-temperature N₂ adsorption experiments were performed and a specific surface area up to 1305 m² g⁻¹ and total pore volume up to 0.66 cm³ g⁻¹ were obtained.     

Process investigation, electrochemical characterization and optimization of LiFePO4/C composite from mechanical activation using sucrose as carbon source

LiFePO₄/C composite was synthesized by mechanical activation using sucrose as carbon source. High-energy ball milling facilitated phase formation during thermal treatment. TG-DSC and TPR experiments demonstrated sucrose was converted to CH_x intermediate before completely decomposed to carbon. Ball milling time, calcination temperature and dwelling time all had significant impact on the discharge capacity and rate performance of the resulted power. The optimal process parameters are high-energy ball milling for 2-4 h followed by thermal treatment at 700 °C for 20 h. The product showed a capacity of 174 mAh/g at 0.1C rate and around 117 mAh/g at 20C rate with the capacity fade less than 10% after 50 cycles. Too low calcination temperature or insufficient calcination time, however, could result in the residual of CH_x in the electrode and led to a decrease of electrode performance.     

Injection molding of surface modified powders with high solid loadings: A case for fabrication of translucent alumina ceramics

Solid loading is a critical key to the fabrication of ceramic compacts with high densities via ceramic injection molding. As reported in most previous work, solid loading of ultra-fine alumina feedstock system could be achieved only up to ∼58 vol% with stearic acid (SA) as the surface modification agent. In present work, different from the traditional work in which SA has been introduced just in the powder blending process, we have successfully prepared the feedstock with a much higher solid loading up to ∼64 vol% by a prior ball milling treatment of ceramic powders with a small amount of SA before the traditional blending process. It can be attributed to that SA can be coated homogeneously around the powder surfaces by a chemical reaction induced by ball milling treatment. Highly translucent Al₂O₃ ceramics have been fabricated, which suggests an alternative route for fabrication of translucent ceramics with high quality.     

Influence of Pd on the structure and electrochemical hydrogen storage properties of Mg50Ti50 alloy prepared by ball milling

High-energy ball milling was used to modify the physico-chemical and the electrochemical hydrogenation properties of Mg₅₀Ti₅₀ alloy via the addition of Pd. This was done by first ball milling Mg and Ti together for (20 − x) hours. 3.3 at.% Pd was then added and ball milling was resumed for x hours. X-ray diffraction and X-ray photoelectron spectroscopy analyses revealed that the alloying of Pd with pre-milled Mg₅₀Ti₅₀ was initiated after only a few minutes and was completed after 5 h of milling. The maximum discharge capacity of the Mg₅₀Ti₅₀-3.3 at.% Pd electrode increased significantly with the milling time (from 35 mAh g⁻¹ for 5 min to 480 mAh g⁻¹ for 20 h of milling). The exchange current density increased with the milling time and was directly related to the Pd surface concentration, suggesting that Pd plays a key role in facilitating the charge-transfer reaction. In contrast, the incorporation of Pd had a minor effect on the hydrogen diffusion coefficient. The electrochemical pressure-composition isotherms revealed a significant destabilization of the hydride as the milling time with Pd increased. No significant improvement in the hydrogen storage properties of Mg₅₀Ti₅₀-Pd electrodes was observed for Pd concentrations higher than 3.3 at.%.     

Influence of carbon sources on LiFePO4/C composites synthesized by the high-temperature high-energy ball milling method

Carbon-coated LiFePO₄ composites were synthesized by a new method of high-temperature high-energy ball milling (HTHEBM). Fe₂O₃ and LiH₂PO₄ were used as raw materials. Glucose, sucrose, citric acid and active carbon were used as reducing agents and carbon sources, respectively. In this method, high-energy ball milling and carbon coating worked together and, therefore, fine and homogeneous LiFePO₄/C particles with excellent properties were obtained in a relatively short synthesis time of 9 h. Moreover, the synthesis process could be completely finished at a relatively lower temperature of 600 °C for high-energy ball milling transforming mechanical energy into thermal energy. The results of X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electrochemical performance tests indicated that carbon source had an important influence on the properties of LiFePO₄/C composites synthesized by the HTHEBM method. It was proved that the LiFePO₄ composites coated with glucose had the best properties with 1 μm geometric mean diameter and 150.3 mA h g⁻¹ initial discharge capacity at a current rate of 0.1 C. After the 20th cycle test, the reversible capacity was 148 mA h g⁻¹ at 0.1 C, showing a retention ratio to the initial capacity of 98.5%.     

Fractal characteristic of three Chinese coals

Experimental and theoretical investigation about coal/char structure is presented. Surface structures of parent coal and char with different burn-off ratios were analyzed. We introduced the fractal theory into Scanning Electron Microscopy image analysis and utilize the particle surface fractal dimension (D_ps) to quantitatively describe the surface character of coal/char particles. D_ps of three Chinese coals reach their maximum in the 35-45 wt% char burn-off interval and then decrease with increasing carbon burn-off ratio. The inner-pore information of coal/char particles was determined by N₂ isotherm adsorption/desorption. Using fractal BET model, internal surface fractal dimension (D_s) of coal/char particles was calculated. The D_s change trend of three Chinese coals is similar to their S_BET development. It means the D_s can quantitatively describe the inner pore structure character of coal/char particles.     

Particle sizes effects on electrical properties and densification of laminated Ba1.002La0.003TiO3 ceramics

This paper investigated the influences of initial particle sizes on electrical properties and densification of laminated Ba_1.002La_0.003TiO₃ ceramics prepared by the reduction–reoxidation technique. Ba_1.002La_0.003TiO₃ powders with average size of∼820 nm and∼260 nm were prepared by the planet ball milling and the sand milling, respectively. For ceramic samples from ∼260 nm particles, the inflection point where the densification rate begins to decrease occurs at a higher sintering temperature than that of ceramic samples from∼820 nm particles. An abnormal growth of grains is observed in ceramic samples from∼820 nm particles, which resists reoxidation. Samples from∼260 nm particles are prone to be globally reoxidized and exhibit a much greater change in grain boundary resistance and RT resistivity after reoxidation. A possible mechanism of the oxygen diffusion in the reoxidation process is proposed, which verifies that samples with smaller grains as well as lower density are easily oxidized to a deeper degree.     

Degradation of high temperature MEA with PBI-H3PO4 membrane in a life test

The article reports on the results of a 780 h life test of high temperature MEA with PBI-H₃PO₄ membrane. The MEA was loaded by current density 0.2 A cm⁻² for 763 h at 160 °C in hydrogen-air feed. The load was discontinued 14 times during the life test including three complete shut downs. In the course of the life test MEA characteristics were studied by electrochemical methods. Pt particle size growth was evaluated by ex situ measurements of electrochemical hydrogen adsorption/desorption with the cathode catalyst sampled after the life test and with pristine catalyst. Possible changes of electrochemically active surface area (ESA) of carbon support were monitored by electrochemical impedance studies (EIS) performed in the course of the MEA life test. Average Pt particle diameter was found 3.8 and 7.8 nm for pristine catalyst and for catalyst sampled after the life test, respectively. ESA of carbon support remained unchanged, membrane resistance decreased by ∼20%, hydrogen crossover increased by a factor of 14, although remained insignificant. Voltage loss rate in the life test was ∼25 μV h⁻¹. The major cause of the MEA degradation was identified as a loss of Pt ESA by particle size growth.