Technology

 

Overview

Technology Available to Update Your Product Line

Question: Do you have a product that can measure all the thermal properties of today's nano-scale samples?

Question: Can your products simultaneously measure mass, heat flow and viscoelasticity of thin films, coatings, powders and membranes?

Question: Do your customers want to make easy and accurate nanogram measurements without the limitations on the sample uniformity or placement of traditional QCM techniques?

Answer: If these capabilities would help fill out your instrument offering, this technology is your answer. Contact us for details.

THE TECHNOLOGY - WHY IS IT NEEDED

The world is full of surfaces – just look around. Some are natural but most have a manmade coating.

Scientists and engineers have made thin films, coatings, and membranes with useful properties such as protecting the underlying substrate or allowing certain molecules to pass through. Many important chemical and biological processes occur in these films either in their production or use. When a gas interacts with a solid film it may adsorb or dissolve in the film or react catalytically at the surface. For example:

  • A polymer-based finish, while stable at room temperature, will oxidize at higher temperatures.
  • A pharmaceutical tablet coating, while stable at low humidity, will absorb moisture and soften when exposed to higher humidity.
  • When a spray paint is applied, the volatiles evaporate, the coating cures, and a viscous film becomes a glassy solid.
  • Many coatings (e.g. supported catalysts) are designed to be reactive.
  • Coatings can be made deliberately active (e.g. nutrient agar plates), or deliberately lethal (bactericidal coatings) to microorganisms.

In all these processes, heat is generated or absorbed, the thin film gains or loses mass, the viscoelastic properties of the solid film change, and the desired properties may be enhanced or destroyed.

WHAT THE TECHNOLOGY DOES

The quartz crystal microbalance/heat conduction calorimeter (QCM/HCC), was developed at Drexel University to study chemical and biological processes in thin films1,2,3. Heat conduction calorimetry has been previously used to measure adsorption energetics in solids. Gas sorption instruments or thermal gravimetric analyzers have measured mass release in solids when heated. Measurements of shear and loss modulus are usually done at low frequency on large samples with dynamic mechanical analyzers. Until the QCM/HCC, however, there had been no instrument that simultaneously measured heat generation, mass uptake or release, and viscoelastic property changes in the same, sub-milligram solid film sample.

The first commercially available nanobalance-calorimeter based on the QCM/HCC principle, the Masscal™G1, is now available from Masscal Corporation. It employs a patented mass/heat flow sensor4 with a sensitivity sufficient to detect molecular monolayer formation in all signal channels. The mass sensor used in the G1 is a piezoelectric shear mode resonator made of quartz, termed a quartz crystal microbalance (QCM). When the resonator is electrically driven at its natural acoustical frequency, the decrease in resonant frequency is proportional to the increase in mass per unit area of a thin film deposited at its surface. The thin film may be a polymer6, protein, paint or coating, chemical sample, catalyst, or metal, but it must adhere to the QCM surface. Subsequent mass changes are followed as the film is exposed to atmospheric pressure gas mixtures with varying partial pressures of adsorbing or reacting gases5.

Mass/heat flow sensor

Figure 1. QCM

An uncoated QCM has a sharply defined resonance frequency ~5 MHz. The mechanical damping of the quartz that gives rise to this broadening can be determined by measuring an electrical quantity called the Ωmotional resistance ΩR of the QCM (for typical bare QCM's, R ~ 10 ohms). When thin, stiff films are deposited on the QCM surface the increase in R is small, but softer, thicker films (i.e., rubbery polymers 5-10 microns thick) can increase R by hundreds of ohms. The difference in motional resistance of the coated and uncoated QCM is proportional to the shear loss compliance J" of the film (3). For organic films gaining or losing volatile components, the changes in J" indicate the extent to which the film is being plasticized7.

QCM/HCC Transducers

Figure 2.

In Masscal's mass/heat flow sensor, the QCM is thermally coupled to a heat sink through a Peltier thermocouple plate. Any heat flow generated by processes in the thin film on the QCM's upper surface is detected as a voltage change by the thermocouple plate - the heat conduction calorimetry (HCC) principle. In the Masscal G1 (Figure 2), the mass/heat flow sensor is placed in a quasi-adiabatic thermal environment and is exposed to a slow flow of gas mixture at ambient pressure. Three quantities are measured simultaneously: the thermal power P(t), the mass change m(t), and the change in motional resistance R(t) of the damped oscillator when the sample film reacts with the probe gas. Any solid which can be prepared as a thin film on a gold-coated quartz substrate is amenable to study. Films of 1- 2 cm2 area and 0.1-10 µm thickness are optimal. Methods of film preparation so far have included spin-coating, spray-coating, and electrochemical deposition. Solids studied have included many polymers, Pd and Pt metal films, the proteins lysozyme and myoglobin, the molecular solids C60 and C60/piperazine, pharmaceutical film-coat materials, and nutrient-containing agar films.

The Masscal G1 Nanobalance-Calorimeter

Figure 3. The Masscal G1 Nanobalance-Calorimeter

RELATION TO NANOTECHNOLOGY

According to the National Science Foundation, the term nanotechnology refers to a wide range of technologies that measure, manipulate, or incorporate materials and/or features with at least one dimension between approximately 1 and 100 nanometers (nm). Such applications exploit the properties, distinct from bulk/macroscopic systems, of nanoscale components. The thermodynamic and kinetic properties of nanomaterials must be measured with methods more sensitive than the normally employed calorimetric or gravimetric techniques. Knowledge of these properties is essential in assessing their performance. The Masscal G1 is ideally suited for such studies.

A question of central importance to nanotechnology is the long-term stability of materials with nanostructures in challenging environments, such as high temperature and high humidity, or in the presence of oxidizing agents or solvent vapors. With atomic compositions varying systematically at the nanometer scale, nanomaterials contain many more contacts between different functional groups and molecular subunits than do typical materials. Nanoparticles have much higher ratios of surface to volume than do larger particles, and are often much more reactive. How sensitive are nanomaterials to moisture or to oxidative degradation? Answers to these questions will determine the ultimate usefulness of the many extraordinary new nanomaterials being synthesized today.

References

  1. AL Smith, H Shirazi, I Wadso, The OCM/HCC: simultaneous, isothermal, high sensitivity measurements of mass change and heat flow in polymer and fullerene films, Recent Advances in the Chemistry and Physics of Fullerenes and Related Materials. Electrochem. Soc., San Diego, CA, 1998, p. 576-85.
  2. AL Smith, HM Shirazi: Quartz microbalance microcalorimetry: a new method for studying polymer-solvent thermodynamics. J. of Thermal Analysis and Calorimetry 59 (2000) 171-86.
  3. AL Smith, HM Shirazi: Principles of Quartz Crystal Microbalance/Heat Conduction Calorimetry: Measurement of the Sorption Enthalpy of Hydrogen in Palladium. Thermochim. Acta. 432 (2005) 202-11.
  4. AL Smith, Mass and Heat Flow Measurement Sensor, U. S. Patent Office 6,106,149. Allan L. Smith, U. S. A., 2000.
  5. AL Smith, SR Mulligan, HM Shirazi: Determining the Effects of Vapor Sorption in Polymers Using the Quartz Crystal Microbalance/Heat Conduction Calorimeter. J. Polymer Sci. Part B Polymer Physics 42 (2004) 3893-906.
  6. RA Cairncross, JG Becker, S Ramaswamy, R O'Connor: Moisture Sorption, Transport, and Hydrolytic Degradation in Polylactide. Appl. Biochem and Biotechnology 131 (2006) 774-85.
  7. AL Smith, in P. Zarras, B. Richey, T. Wood, B. Benicewicz (Eds.), New Developments in Coating Technology. ACS Symposium Series, Washington DC, 2006.