Search

Application of carbon nanotubes (CNT) is hampered by the lack of practical methods for separating the raw production mixtures into species with a given structure and properties. In this work, we propose a new, cost effective separation method, suitable for the industrial scale processing of SWCNTs. The separation is afforded by passing a surfactant solution of unsorted CNTs through a column, filled with chemically modified cotton wool. This yields two fractions, heavily enriched with semiconducting and metallic CNTs, respectively. The as-separated fractions are fully characterized by UV–Vis spectroscopy, PL spectroscopy, Raman spectroscopy, and HRTEM.

Li-ion batteries have polymeric binder in their electrodes resulting in high internal resistance and hence long charge times and compromised product lifetime. We have developed Li-ion batteries utilizing binder-free NMC cathodes containing carbon nanotube (CNT). Our proprietary processing of CNT into viscous organic or water-based slurry without surfactant or dispersant reduces or eliminates the need for organic binder and carbon black additives in these cathodes. The benefits of incorporating our processed CNT slurry into NMC cathode electrodes are realized by significant gains in specific energy and power. When the organic binder in the cathode is completely removed, our Li-ion batteries retain superior performance metrics over conventional binder-containing cells, including significantly reduced impedance (-31%) and increased capacity (5%). Li-ion pouch cells containing these binder-free NMC/CNT cathodes show 2.5% higher specific energy. Our cells with binder-free cathode complete 500 cycles with 50% less capacity fade than cells with binder, and display more than 2x improvement in capacity at 10C rate discharge. A processing advantage of eliminating the polymeric binder from the formulation mix was demonstrated by the use of IPA as dispersing solvent in place of the costlier and toxic NMP solvent. Li-ion cells containing binder-free NMC/CNT cathodes from IPA mix preserved their performance advantages over conventional NMP-based binder-containing cells. Figure 1

We have developed supercapacitors utilizing electrodes made of binder-free carbon nanotube (CNT) / activated carbon (AC) composites. Due to the absence of organic binder our supercapacitors exhibit a significantly higher volumetric capacitance of ~20 F/cc compared to conventional supercapacitors while maintaining low resistance. This results in higher energy in the same package or smaller, more compact package size (lower volume) with same energy. Our cells pass 1,000 hour float tests at 2.7V between -35°C and 70°C, as well as high voltage operation at 3V and 70°C. Operation for 100,000 continuous cycles at 85°C was also achieved. Flat format supercapacitor pouch cells > 100 F have been demonstrated that can be integrated into automotive panels for autonomous power. Notable features of our CNT/AC composite supercapacitor technology include: Scalable R2R coating of CNT/AC electrodes without additives. Option of water-based process with no solvent recovery Binder-free CNT/AC electrodes provide 20% higher energy density than conventional AC-based electrodes (50% higher energy density at 3V) CNT/AC supercapacitor technology is 100% packaging agnostic Fabrication of 18650 EDLC cells rated at ~ 100 F demonstrated, and pouch cells rated at ~ 350 F demonstrated The flat pouch-cell format allows installation in automotive panels as power sources for emergency door lock/unlock applications. Figure 1

High-voltage (≥4.0 V) operation of supercapacitor devices was demonstrated using carbon nanotubes as active electrode materials combined with room temperature ionic liquids as electrolyte. Pouch cells were assembled with four different ionic liquids, 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM-BF4), diethyl-N-methyl-N-(2-methoxyethyl) ammonium bis(trifluoromethanesulfonyl)imide (DEME-TFSI), diethyl-N-methyl-N-(2-methoxyethyl)ammonium tetrafluoroborate (DEME-BF4), and 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr14-TFSI). Cyclic voltammetry showed the maximum operational voltage to be 4.5 V for DEME-TFSI and 4.7 V for DEME-BF4. Compared to electric double layer capacitor (EDLC) cells using propylene carbonate electrolyte at 2.7 V, capacitance increased by 20% using BMIM-BF4 at 4.0 V, DEME-TFSI at 4.5 V, DEME-BF4 at 4.7 V, and Pyr14-TFSI at 4.3 V, with tripling of energy density and comparable power density using Pyr14-TFSI-based EDLCs. Long-term cyclability using BMIM-BF4 ionic liquid electrolyte operating at 4.0 V showed retention of >80% of initial capacitance after 65,000 continuous cycles without doubling of initial cell equivalent series resistance.

This work reports a low-cost custom electrochemical instrument capable of performing rapid and accurate electrochemical impedance spectroscopy (EIS) for supercapacitors over a broad frequency band (10 mHz to 2 kHz). Conventionally, EIS is measured via sinusoidal perturbations; however, such an approach suffers from lengthy measurement time. Chirp signals have been shown previously to reduce EIS measurement time for supercapacitors for relative narrow frequency bands (1 Hz to 2 kHz). However, to characterize supercapacitors comprehensively, much broader frequency bands are required. Here, we present a custom instrument with an adaptive measurement algorithm for performing EIS measurements in a wide frequency range of 10 mHz to 2 kHz with low measurement uncertainties. The results obtained using this new technique has been validated here with a commercial instrument on several types of supercapacitors. Furthermore, measurement time on average decreases from 1500 s to less than 400 s. The overall cost of the custom instrument is 90% lower as compared to the commercial instrument. The custom instrument’s accuracy, time efficiency and low cost are expected to benefit electrochemical researchers.

Electric double layer capacitors (EDLCs) are a category of supercapacitors that generally compliment the performance of batteries through delivery of high power with significantly longer cycle life. The conventional active electrode material utilized in these EDLCs is activated carbon (AC) possessing high surface area through templated porosity. However, one of the primary limitations with AC-based supercapacitors using organic electrolytes is the operational voltage maximum around 2.75 V. Operation at higher voltages that push the 2.75 V barrier is the next essential developmental breakthrough to enable significant boosts to both energy and power densities of supercapacitors. Research into alternative active electrode materials is equally critical to enable and support high voltage operation of supercapacitors. This work demonstrates the fabrication of carbon nanotube (CNT) electrodes from organic-based dispersions without the inclusion of surfactant or binder, and subsequent assembly into supercapacitor devices. The performance of pouch cells assembled from the CNT electrodes and using conventional organic electrolytes at operating voltages ≥ 2.7 V is discussed. A significant and linear voltage dependence of capacitance (~ 3 times higher than AC) was observed for cells with CNT electrodes up to operational voltages ~ 3.5 V, which are currently not achievable using AC-based cells. Repeatable charge-discharge operation of the CNT-based EDLCs was demonstrated at 3.5 V and 70°C, with consequential increase to capacitance by ~ 25 - 30 % compared to 2.7 V operation. Power and energy densities were increased by 1.5 and 2 times respectively at 3.5 V compared to operation at 2.7 V. The performance characteristics from such CNT-based pouch cells were benchmarked against commercial-grade AC supercapacitor devices using organic electrolytes and rated at 2.75 V (single cell). Long-term performance advantages were observed for the CNT-based monocells at 3.5 V and 70°C, which passed 100k cycling tests at full discharge. Under the same test conditions, commercial AC-based cells failed early during the 100k cycling. Response times approaching 2 ms (at -45° phase angle) were also demonstrated with EDLC devices using thin CNT electrodes with 1 mF capacitance. Such EDLC devices may potentially rival aluminum electrolytic capacitors (AECs) for alternating current line-filtering applications, offering significant space savings compared to the bulky AEC components. Finally, the scalable fabrication of binder-free CNT electrodes from surfactant-free dispersion of the material was demonstrated by roll-to-roll, slot-die coating onto an Al foil current collector. The processing of these conformal CNT coatings, assembly into 18650 cylindrical cells, and the resulting EDLC performance characteristics are discussed. The CNT-based 18650 cells had an average capacitance of 7 F at 2.7 V, with low average ESR of 7 mΩ at 1 kHz, and average response time of 120 ms at -45° phase angle. Figure 1

Doping ferromagnetic nickel–iron alloys with chromium causes the Curie temperature to be reduced. We have demonstrated that solid state solutions of Ni–Fe–Cr can be formed by implanting Cr ions into ferromagnetic NiFe alloy films, thus creating paramagnetic films. We find that the magnetic moment and coercivity decrease steadily with Cr+ dose, reaching zero at room temperature with the onset of paramagnetism. Using Cr+ implantation in conjunction with a lithographic mask we have patterned continuous Ni80Fe20 films into separate regions that are ferromagnetic and paramagnetic at room temperature. This magnetic patterning process may have applications for the manufacture of magnetic write heads, such as for the notching process used to constrain the stray field from the write gap.

All-metallic giant magnetoresistive (GMR) sensors in a current-perpendicular-to-plane (CPP) geometry are attractive candidates for future high-density recording sensors due to their low intrinsic resistance. With increasing recording density the cross-sectional area of CPP sensors (such as TMR or GMR sensors) must also decrease, resulting in a rapid increase in sensor impedance unless the RA product of the sensor stack also decreases. Thus, as recording densities increase above 300 Gb/in2, and sensor sizes decrease below 50nm, low-resistance CPP-GMR sensors become of greater interest to minimize impedance, reduce noise and improve the bandwidth characteristics of the read head. Here we present results on the magnetotransport properties, fabrication, and performance of all-metal CPP-GMR shielded read heads with ultra-narrow physical widths between 40 and 60 nm.

The rapidly increasing technological advances in society place more demands on efficient energy storage and the correspondingly important fast release of the stored energy. Supercapacitors are utilized where energy needs to be stored and released quickly, however they exhibit limitations to these demands that arise from various sources within the device such as the necessary incorporation of resistive additives to the electrode formulation. In the case of activated carbon (AC) electrodes, this constitutes the insulating binder, while for carbon nanotube (CNT) electrodes, the inclusion of appropriate surfactant is almost always necessary to promote stable dispersion for processing. This work discusses the use of surfactant-free and binder-free CNT electrodes for the fabrication of supercapacitor devices. The performance characteristics from pouch cells containing such CNT electrodes in organic electrolytes were benchmarked against commercial-grade AC supercapacitor devices on both cell volume and cell weight bases. EDLC performance advantages realized from eliminating the additives of surfactant and binder were significantly lower ac impedance at low temperature operation (80% reduction @ -30°C), higher capacitance retention after 100,000 cycles, and superior retention of capacitance at high temperature (95% of initial capacitance @ 70°C). The CNT-based supercapacitors of this work exhibited these performance advantages while still maintaining very competitive energy and power densities.

The temperature-dependent magnetic response of exchange-coupled FePt/FeRh thin films is described. The FePt forms a high magnetocrystalline anisotropy, high-coercivity ferromagnetic layer. The FeRh layer is antiferromagnetic at room temperature but, upon heating above a transition temperature, becomes ferromagnetic with a large magnetic moment and low magnetocrystalline anisotropy, forming an exchange–spring system and significantly lowering the coercive field of the composite system. This feature opens intriguing possibilities for media applications for thermally assisted magnetic recording where the ferromagnetic phase of FeRh is exploited to help write the media while the antiferromagnetic phase supports the long-time stability.

It is experimentally shown that the critical current for onset of spin-torque instability in current-perpendicular-to-plane spin-valves can be strongly enhanced using "synthetic ferrimagnet" free-layers of form FM1/Ru/FM2 (FM=ferrromagnet). However, this enhancement occurs for only one polarity of bias current. A two-macrospin model is shown to reproduce the observations. The model suggests that this phenomenon is related to a polarity-dependent, spin-torque induced co-resonance between the two natural dynamic modes of the FM1/FM2 couple. The resonance condition facilitates energy transfer out of the spin-torque destabilized mode into the other stable mode whose effective damping is actually enhanced by spin-torques, thereby delaying the onset of instability of this coupled system to larger critical currents., Comment: 10 pages (manuscript form), with 4 embedded figures. Submitted for publication. Version #2 contains one additional equation (Eq. 4)

The temperature and field hysteresis of the magnetization in the first order antiferromagnetic to ferromagnetic phase transition in FeRh films grown onto $c$-axis sapphire and MgO (001) are investigated. The transition to the ferromagnetic state upon heating and antiferromagnetic state upon cooling is generally broad indicating a heterogeneous transition due to defects so that antiferromagnetic and ferromagnetic domains coexist during the transition. However, the nucleation of antiferromagnetic domains upon cooling is abrupt for FeRh on $c$-axis sapphire which is indicative of homogeneous nucleation and growth of antiferromagnetic domains. The transition is further broadened when measuring with fields applied out of plane of the sample due to internal demagnetization fields. Temperature dependent remanent magnetization measurements reveal that field induced magnetization changes are irreversible during heating, but reversible during cooling. The field dependence of the shift in transition temperature is qualitatively modeled with an Ising spin type model utilizing a mean field approach. From this calculation a shift of $\ensuremath{-}10\phantom{\rule{0.3em}{0ex}}\mathrm{K}∕\mathrm{T}$ in transition temperature is determined in good agreement with the experimentally observed shift of $\ensuremath{-}8$ and $\ensuremath{-}9\phantom{\rule{0.3em}{0ex}}\mathrm{K}∕\mathrm{T}$ for FeRh films grown onto MgO (001) and c-axis sapphire, respectively.

Using x-ray magnetic circular dichroism, we have detected the very interfacial spins that are responsible for the horizontal loop shift in three different exchange bias sandwiches, chosen because of their potential for device applications. The "pinned" uncompensated interfacial spins constitute only a fraction of a monolayer and do not rotate in an external magnetic field since they are tightly locked to the antiferromagnetic lattice. A simple extension of the Meiklejohn and Bean model is proposed to account quantitatively for the exchange bias fields in the three studied systems from the experimentally determined number of pinned moments and their sizes.

Interest in the magnetic properties of the ordered equiatomic alloy FeRh has been revived in recent years due to potential applications in heat assisted magnetic recording. This is based on the existence of a first order hysteretic phase transition from strongly anisotropic antiferromagnet (AF) to relatively isotropic ferromagnet (F) upon warming to ∼370 K. Here we investigate this transition, which shows significant coupling between structural, magnetic, and electronic degrees of freedom, via wide temperature range magnetotransport and magnetometry measurements on ordered epitaxial MgO(001)/FeRh(1000 A) films. Consistent with bulk measurements, a large decrease in resistivity (∼30%) occurs on warming through the transition. The transition temperature shifts with applied magnetic field at −0.75 K/kOe due to the field-induced stabilization of the F phase, leading to large negative magnetoresistance (∼40% in 90 kOe) in the transition region (340–400 K). Isothermal field cycles reveal consistent behavior whe...

A spin-wave Doppler technique is used to measure the drift velocity of the magnetization in current-carrying (CoFe)1−xGex alloys. For a current density of 1011 A/m2, we obtain a large enhancement of drift velocity with increased Ge concentration from 3.1±0.2 m/s for CoFe to 8.2±0.6 m/s for (CoFe)0.7Ge0.3. Interpretation of these values yields current polarization increasing from 0.84±0.04 to 0.95±0.05 as the Ge doping increases. While both spin-dependent conductivities decrease with increasing Ge concentration, the maximum in polarization is associated with a stronger relative change in the minority spin conductivity.

We determined the exchange anisotropy and rotational anisotropy of IrMn(7 nm)/Fe(t=3–20 nm)/IrMn(7 nm) exchange-biased structures using conventional ferromagnetic resonance (FMR) and network analyzer FMR (NA-FMR). Compared to single Fe layer films of identical thickness, we observe an isotropic downward shift and an angular variation of the FMR resonance field in the multilayer structures. The isotropic shift originates from the rotational anisotropy, while the angular variation originates from the exchange anisotropy. Both exchange anisotropy and rotational anisotropy increase with decreasing Fe thickness in the exchange-biased structures. The isotropic downward shift of the resonance field translates to an upward shift of the resonance frequency, and can be used to boost the operational frequency of microwave devices (bandpass/stop filters) by several gigahertz.

The magnetics and magnetotransport of current perpendicular to the plane (CPP) giant magnetoresistive (GMR) spin valves utilizing thin CoPt hard magnet pinning layers on Cr seed layers are investigated. 50-A-thick CoPt18 films grown on >20A Cr seed layers exhibit a coercivity of ∼1.5kOe and a resistivity of 31μΩcm. The low critical thickness of the CoPt hard magnet compared to IrMn or PtMn antiferromagnets makes it an attractive pinning material for small gap CPP sensors required for high magnetic recording densities. Furthermore, CoPt∕Ru∕CoFe trilayers exhibit strong antiparallel coupling and the low CoPt resistivity minimizes parasitic resistance yielding a higher magnetoresistance. Moreover, free layers with coercivities and coupling fields in a range of 5–10 Oe have been measured, demonstrating that free layer softness does not suffer from the proximity to the CoPt hard magnet due to the relatively thick spacer layers compatible with CPP GMR sensors. In particular we deposited CoPt18 pinned antiferrom...