In his comedy routine "Let's Get Small" Steve Martin discusses getting small while driving. He pretends he is driving a car while being so short as to barely reach the bottom of the steering wheel. Martin: "So I'm driving like.. [extends arms high in the air like he's reaching up to a giant steering wheel] and, uh.. a cop pulls me over. And he makes me get out, he looks at me and he says, "Heyyy.. are you small"? I said, "No-o-o! I'm not!" He says, "Well, I'm gonna have to measure you." They have this little test they give you - they give you a balloon.. and if you can get inside of it, they know you're small….”
Martin’s comedy bit came to mind as I was reading about interesting recent developments in nanotechnology as related to electronics. Nanotechnology is the science of getting really small: it involves the manipulation of matter at dimensions between approximately 1 and 100 nanometers (or 100x10−9 meters), where unique phenomena enable novel applications. To put things in perspective a hydrogen atom is about 0.1 nanometer across. In electronic components we have seen application of nanomaterials in supercapacitors by using carbon nanotubes (CNTs) and graphene, whose extremely high specific surface area of up to 2,600 m2/g and high electrical conductivity make it well-suited for use as an electrode material, since increasing electrode surface area boosts the amount of energy that a supercap can hold.
Researchers have been hard at work studying the use of nanotechnology as a means of improving many of the shortcomings of today’s rechargeable battery technology, such as too-long recharging time and too-little capacity. In passive components, nanotechnology has played a leading role in development of oxide layers in tantalum capacitors and barium titanates in MLCCs (more on nanotechnology and passives shortly).
But let’s first spend a few minutes on nanostructures and attempts to develop batteries with higher energy storage density than existing lithium ion batteries. One can’t underestimate the importance of energy storage in future generations of products ranging from portable electronic devices (cellphones, tablets etc.) to hybrid electric vehicles. Consider, for example, the Chevrolet Volt hybrid, where currently energy is stored onboard in a 16.5-kWh, T-shaped lithium-ion battery pack that weighs 435 lb. and consists of 288 individual cells arranged into nine modules. Running on electric power only with a fully charged battery you can drive up to 38 miles (as they say in the TV commercials actual range varies with conditions) in a Volt, which is pretty good given previous efforts but nowhere near what is needed if you are going to create a vehicle where the driver does not have to worry constantly about how far he or she can go without re-charging (to alleviate these fears and extend the Volt's range the Chevy has a small, naturally aspirated 1.4 liter 4-cylinder gasoline-fueled internal combustion engine with approximately 80 hp (60 kW) powering a 55 kW generator).
The vast majority of all today’s rechargeable batteries are so-called ‘Lithium Ion’ batteries, which are comprised of an electrolyte providing positively charged lithium ions, an anode (usually made of graphite or silicon), which discharges electrons into a device giving it power and a cathode where electrons re-enter the battery after they have traversed the circuit.
Recent efforts have focused on high-capacity electrode materials such as lithium metal, silicon and tin as anodes; an anode of lithium metal would enhance battery efficiency because lithium has the highest specific capacity (3,860 mAh g–1) and the lowest negative electrochemical potential (−3.040 V vs. the standard hydrogen electrode). However, there are overriding issues in making a pure lithium anode. For one thing lithium ions expand as they gather on the anode during charging, forming dendrites − hair-like structures. These dendrites can short circuit the battery. While all anode materials, including graphite and silicon, expand somewhat during charging, lithium’s expansion can be so strong and uneven that it can warp and crack the battery casing, creating safety concerns as the anode and electrolyte produce heat when they come into contact. We’ve seen from real life automotive and aviation examples that Lithium batteries can overheat to the point of fire, or even explosion. If the lithium anode can be contained you still have to consider the fact that a lithium metal anode is so chemically reactive that it will consume the electrolyte and the battery’s ability to recharge declines rapidly, as measured by its Coulombic efficiency (the ratio expressed as a percentage between the energy removed from a battery during discharge compared with the energy used during charging to restore the original capacity).
Here’s the good news: researchers at Stanford University, reporting in a paper published in the journal Nature Nanotechnology, said they have made significant progress in the effort to design a pure lithium anode. The Stanford researchers built a protective layer of connected carbon domes on top of their lithium anode. The Stanford team calls this layer “nanospheres”. The 20-nanometer thick amorphous carbon nanosphere protective layer resembles a honeycomb and is chemically stable, strong and flexible enough to move freely with the lithium as it expands and contracts. The layer also protects against chemical reactions with the electrolyte.
The researchers claim that a commercialized, rechargeable battery with a lithium metal anode would triple the battery life of a cell phone and provide an electric vehicle with a 300 mile range. But we are not quite there yet. Previous anodes of unprotected lithium metal achieved approximately 96 percent efficiency, which dropped to less than 50 percent in just 100 cycles. Generally, to be commercially viable, a battery must have a Coulombic efficiency of 99.9 percent or more, ideally over as many cycles as possible. The Stanford team’s new lithium metal anode achieves 99 percent efficiency at 150 cycles. That’s close, but some more work remains to be done before we pop the champagne corks.
Nanotechnology and Passive Components
As we have noted sub-100nm particles have been used to develop thinner dielectric layers for capacitors for a long time. High-k dielectric thin films of (doped) barium titanate (BaTiO3) are used in multi-layer ceramic capacitors (MLCCs), whose capacity is inversely proportional to the layer thickness − ultrathin layers yield the highest capacitances per unit volume. To further increase the capacity of MLCC’s per unit volume, thinner layers and finer starting powders are required.
But working in the nanometer range presents a number of challenges, including materials preparation, materials application and powder handling. Hurdles exist with regard to dispersion, viscosity and the reduction of dielectric constant with grain size (the dielectric constant of BaTiO3, for example, is known to decrease substantially when the particle dimensions move into the nanometer range). The goal is to use nanotechnology to create materials compatible with existing production technology. In a paper presented at CARTS International 2014 (Santa Clara, CA) in April entitled “Novel Ceramic Capacitor Electrode Structures,” Alan Rae and Liang Chai of the NanoMaterials Innovation Center (Alfred, NY) noted that “An ideal situation would be if we could squeeze more capacitance out of existing MLCC production equipment and materials. A lot of emphasis in MLCCs has been on the dielectric and the emphasis on the electrode has been on the switch to processing base metals – which was a major technological step. What if we took these electrodes and transformed them in ways which increased capacitance without compromising other properties?”
Rae and Chao suggested that one approach was to use a coated layer under the electrode print to self-assemble a dispersed nano-electrode from a non-nano precursor, effectively increasing electrode area while using existing dielectrics and electrode materials. A novel way to enhance the effective electrode area, the authors wrote, would be by creating 3D structures that would increase the effective electrode area and thereby enhance the performance of existing dielectric materials. Initially using a precious metal electrode (PME) system, they found that certain structures could yield “a 500% increase in apparent dielectric constant.” They further cited a patent held by Apricot Materials Technology which describes a conventional tape casting process “where an intermediate layer between the electrode and the dielectric is printed using a metal-coated ceramic or other substrate material.” They said that during firing, this layer “self-assembles into a layer of finely-dispersed metal particles in grain boundary triple points which act as the electrode enhancing structure in the capacitor.”
Currently a major program is underway at the authors’ lab (for more information on the Nano Materials innovations Center see http://www.nanomic.org/) to explore the effects of processing parameters and define the process envelope. The authors concluded in their paper that they believe the electrode enhancement measures discussed can be extended across the range of PME and Ni base metal electrode (BME) systems.