Thin-film Calorimeter Research Articles

Resolving the Heat of Trimethylaluminum and Water Atomic Layer Deposition Half-Reactions.

Atomic layer deposition (ALD) is a surface synthesis technique that is characterized by self-limiting reactions between gas-phase precursors and a solid substrate. Although ALD processes have been demonstrated that span the periodic table, a greater understanding of the surface chemistry that affords ALD is necessary to enable greater precision, including area- and site-selective growth. We offer new insight into the thermodynamics and kinetics of the trimethylaluminum (TMA) and H2O ALD half-reactions with calibrated and time-resolved in situ pyroelectric calorimetry. The half-reactions produce 3.46 and 2.76 eV/Al heat, respectively, which is greater than the heat predicted by computational models based on crystalline Al2O3 substrates and closely aligned with the heat predicted by standard heats of formation. The pyroelectric thin-film calorimeter offers submillisecond temporal resolution that uniquely and clearly resolves precursor delivery and reaction kinetics. Both half-reactions are observed to exhibit multiple kinetic rates, with average TMA half-reaction rates at least 2 orders of magnitude faster than the H2O half-reaction kinetics. Comparing the experimental heat with published computational literature and additional first-principles modeling highlights the need to refine our models and mechanistic understanding of even the most ubiquitous ALD reactions.

Effects of wire materials on radiative heat flux and spectral characteristics of a capillary discharge plasma jet

Capillary discharge plasmas have been widely used in many fields. This paper investigates the influence of different explosion wire materials on the radiative heat flux and spectral characteristics. Three kinds of wire (aluminum, copper and nickel) with 0.3 mm diameter were used as initiation wires to produce an electrothermal plasma, with a capacitor charging voltage of 3 kV. A thin-film calorimeter with high response speed was used for the measurement of the radiative heat flux. At the same time, spectral analysis was performed using a spatially resolved spectrometer with an ICCD. Based on the results, the electrical energy transported to the plasma that is calculated from the voltage and current traces follows the order Cu > Al > Ni, while the radiative heat flux follows the order Al > Cu > Ni. The trends can be explained by the secondary combustion (chemical reaction with the air) of Al and the fact that the dissipated energy is smaller than the vaporization energy for Ni but larger for Al and Cu, resulting in a bad performance in jet heat flux due to the incomplete vaporization of Ni wires. For the Cu and Al cases, a four-stage evolution process of the emitted radiation, which describes the emission, absorption and decay processes, can be found from the spectral analysis.

Thin-Film Calorimetry – Novel Analytical Tool for In Situ Characterization of Lithium-Ion Batteries

Precise knowledge of the thermodynamic and calorimetric properties of the active materials used in lithium ion batteries is a key issue for the improvement of the entire system, considering especially lifetime and safety aspects. In current research the role of thin-film batteries becomes more and more important. Here, it is crucial that thin films behave, in general, differently compared to bulk electrodes. Therefore, conventional calorimetric techniques are not suitable for these requirements.The presented measurement technique Thin-Film Calorimetry (TFC) is based on high-temperature stable piezoelectric langasite (La3Ga5SiO14) resonators, so that the measurement range extends to about 1000 °C. Their resonance frequency fR exhibits a strong temperature dependence. By measuring fR , the device is working as a precise temperature sensor. Thin films with a thickness in the range from 100 nm to several micrometres of the material of interest are applied to the resonators. The production or consumption of latent heat during phase transformation of these layer(s) results in temperature fluctuations with respect to the furnace where the sensor is placed. They are reflected in frequency deviations fTr . As the dependence of the resonance frequency on temperature as well as mass and specific heat capacity of the resonator are known, fTr is used to calculate the amount of heat QTr generated (exothermic) or consumed (endothermic) by the phase transformation. For the present system, the uncertainty of the measured quantity of heat ΔQTr is estimated to be 0.6 mJ. Dividing QTr by the film’s mass gives the enthalpy ΔH. Thereby, this mass is accessible by the Thin-Film Calorimeter itself when operating the device prior and after film deposition in the microbalance mode. Figure 1 shows the TFC setup.Metallic layers of tin and aluminium are used to test and establish this technique. The enthalpies of solid-liquid as well as of solid-solid phase transformations are observed in the correct manner.To establish this technique in the field of battery materials several electrode materials as well as solid electrolytes are investigated. For example, the anode material molybdenum disulphide is characterized. A phase transformation indicated by a frequency shift is observed by annealing the thin film up to a temperature of 478 °C (see Figure 2). Combining these thermodynamic data with structural investigations, this transformation is identified as crystallization with an associated enthalpy of -183.2 J/g. Another example is the cathode material lithium manganese oxide (LMO) which is investigated in the temperature range from room temperature up to 775 °C under varying atmospheres. In ambient air, three phase transformations at 330 °C, 410 °C, and 600 °C are measured while at measurements performed under 0.5% H2/Ar four phase transformations at 390 °C, 470 °C, 730 °C, and 758 °C occur. The associated thermodynamic parameters as well as the structural changes are evaluated. By this, a thermodynamic and structural mapping of the materials can be given as shown in Figure 3.Current research focuses on the investigation of all-solid-state thin-film electrochemical cells which are monitored at constant temperature during dis-/charging. Thereby, the heat generation with respect to the state of charge is investigated.The newly developed measurement technique Thin-Film Calorimetry (TFC) enables to investigate thin films including battery layer sequences. Thereby, even in-situ measurements during (de-)lithiation of thin-film cells are performable. The temperature range extends far beyond the common operation range of lithium ion batteries. Even if this is not necessarily apparent at first glance, this feature enables to determine calorimetric data up to the synthesis temperatures. By this, information is gathered about the applicable thermodynamic process conditions a material can stand during battery production.

Dynamics of the temperature distribution in ultra-fast thin-film calorimeter sensors

To study phase-transition kinetics on submillisecond time scales a set of thin-film silicon–nitride gauges for ultra-fast scanning nanocalorimetry was constructed and utilized for controlled ultra-fast cooling, as well as heating, up to 106K/s. In this paper an analytical solution for the model describing the heat transfer in a membrane gauge at ultra-fast heating and cooling is presented. The dynamics of the temperature distribution in the membrane gauge is calculated and compared with experimental data. The origin of limitations imposed on the maximum attainable controlled cooling rate is investigated.

High-temperature thin-film calorimetry: a newly developed method applied to lithium ion battery materials

A thin-film calorimeter has been developed to investigate the thermodynamic properties of thin films including battery layer sequences. A new approach, i.e., the application of high-temperature stable piezoelectric resonators as highly sensitive planar temperature sensor, is chosen. Thin films with a thickness of several micrometers of the material of interest are deposited on the resonators. The production or consumption of latent heat by the active layer(s) results in temperature fluctuations with respect to surroundings, in our case the furnace in which the sensor is placed. The temperature fluctuations can be easily monitored in situ via changes of the resonance frequency of the resonator. This enables us to extract the temperature and time dependence of phase transformations as well as the associated enthalpies. To cover a temperature range from −20 to 1000 °C, high-temperature stable piezoelectric langasite (La3Ga5SiO14) resonators are applied. Initially, aluminum and tin layers are used to test the calorimeter. The temperature and enthalpy of the solid–liquid phase transformation agree well with the literature data. Further, the thermodynamic data of the battery materials to be used as cathode, solid electrolyte, and anode in lithium ion batteries are investigated by the newly developed method. The cathode materials Li(Ni0.8Co0.15Al0.05)O2-δ (NCA) and LiMn2O4-δ (LMO) are amorphous after deposition and crystallize during heating. NCA shows this transformation at 455 °C with an enthalpy of −4.8 J/g. LMO undergoes three phase transformations at 330, 410 and 600 °C. They require initially an activation which is followed by an exothermic enthalpy. The associated energies (activation; enthalpy) are (+67.2; −50.2) J/g, (+29.3; −29.3) J/g, and (+20.4; −26.2) J/g, respectively. The solid electrolyte Li3.4V0.6Si0.4O4-δ (LVSO) shows no phase transformation up to its decomposition at about 220 °C. The anode material molybdenum disulfide (MoS2) exhibits a phase transformation at 480 °C with an enthalpy of −183.2 J/g.

Fabrication, characterization and modeling of single-crystal thin film calorimeter sensors

Thin film based nanocalorimetry is a powerful tool to investigate nanosystems from a thermal point of view. However, nanocalorimetry is usually limited to amorphous or polycrystalline samples. Here we present a device that allows carrying out experiments on monocrystalline silicon. The monocrystalline silicon layer consists of the device layer from a silicon-on-insulator wafer and lies on a low-stress free-standing silicon nitride membrane. We applied a number of characterization techniques to determine the purity and quality of the silicon layer. All these techniques showed that the silicon surface is as pure as a standard silicon wafer and that it is susceptible to standard surface cleaning procedures. Additionally, we present a numerical model of the nanocalorimeter, which highlights that the silicon layer acts as a thermal plate thereby significantly improving thermal uniformity. This nanocalorimeter constitutes a promising device for the study of single-crystal Si surface processes and opens up an exciting new field of research in surface science.

Heat transfer in symmetric U-shaped microreactors for thin film calorimetry

We describe the results of two-dimensional finite difference analysis of the thermal profile, in both transient and steady state, of a symmetric U-shape designed high-sensitive nanocalorimeter. The thin film calorimeter, with a heat capacity of 100 nJ K−1 at room temperature, consists of a 180 nm thick freestanding silicon-rich nitride membrane on which thin film heaters and sensors are deposited. Simulated temperature profiles are in good agreement with in situ experimental data obtained at the heater and sensor locations. The first-order solid-to-liquid transition of indium films, from a few Å to hundreds of nm thick, was used as an experimental reference of the thermal profiles obtained from the 2D modeling. Temperature differences inside the sample region induced by the symmetric U-shape design of the Pt heaters limit the use of the nanocalorimeter to two different heating rate regimes. At low heating rates, β < 10 K s−1, especially with a thermal layer, the temperature profile is reasonably flat so that small samples can be characterized in power compensation mode. At heating rates faster than 4 × 104 K s−1 the nanocalorimeter works in adiabatic mode and measures transitions occurring in the sample directly loaded underneath the heater.

In Situ Characterization of Photopolymerizable Systems Using a Thin-Film Calorimeter

A thin-film calorimeter (TFC) was used to provide quantitative characterization of photopolymerizable systems. Photopolymerization exotherms measured using the TFC are compared with real-time infrared (RTIR) spectroscopic results. The TFC successfully monitored the in-situ polymerization kinetics of thin/thick films because of its increased sensitivity/resolution over traditional calorimetric instrumentation. Reproducible polymerization exotherms of trimethylolpropane triacrylate were measured on films as thin as 163 nm. Flexibility in the sample cell construction enables the characterization of both volatile reactants (closed cell) and the effect of oxygen inhibition (open cell). Several molecular systems were evaluated. First, exotherms for the reaction of a series of volatile hexene monomers with monofunctional thiol illustrate the effect of ene location on alkene reactivity. Second, vinyl acrylate, a volatile difunctional monomer with a unique polymerization mechanism, was shown to homopolymerize sign...

Design and performance of a thin-film calorimeter for quantitative characterization of photopolymerizable systems

A thin-film calorimeter (TFC) was designed for the quantitative characterization of photopolymerizable systems. A detailed description of its construction indicates the ease with which a TFC can be assembled and the flexibility inherent in its design. The mechanics of operation were optimized to yield a significantly faster instrument response time than other calorimetric methods such as photodifferential scanning calorimetry (photo-DSC). The TFC has enhanced sensitivity, more than an order of magnitude greater linear response range to changes in light intensity than that of the photo-DSC, resulting in the ability to measure both smaller and larger signals more accurately. The photopolymerization exotherm curves are reproducible and can be collected over a broad range of film thicknesses.

Ultra-fast isothermal calorimetry using thin film sensors

A new application of an ultra-fast thin film calorimeter is described. A commercially available sensor, thermal conductivity gauge TCG-3880 (Xensor Integrations, NL) was utilized as a measuring cell for isothermal calorimetric measurements. The sensor consists of a thin membrane (500 nm) with a semi conducting heater and a semi conducting thermopile, which are produced using integrated-circuit (IC) technology. To allow fast scanning sample size should be small too. Samples with masses of about 100 ng were measured. Different modes of operation were realized: (i) temperature scans over hundreds of Kelvin with rates up to 10,000 K/s; (ii) isothermal heat flow measurements with a time resolution of a few milliseconds; (iii) ac-mode for heat capacity measurements in the frequency range from 1 to 140 Hz at scanning or isothermally. In contrary to classical ac calorimeters beside heat capacity enthalpy changes can be monitored simultaneous. All these modes can be combined to realize experimental conditions needed to follow isothermal processes in time. As an example, we present isothermal crystallization experiments for fast crystallizing polymers. Heat flow as well as heat capacity was measured after quenching the sample from above the melting temperature to the crystallization temperature. Crystallization kinetics for polycaprolactone (PCL) and polyethylene (PE) are presented in a wide temperature range. We were able to follow isothermal crystallization of PCL in the whole temperature range between melting and glass transition.

Time resolved calorimetry of te films during pulsed laser annealing

Using a pyroelectric thin-film calorimeter the temperature of 30 nm thick Te films during pulsed laser annealing was studied in real-time. A XeCl excimer laser pumped dye laser with Raman shifter was utilized to study the wavelength and energy dependence. No significant wavelength dependence was noticed. Depending on the pulse energy, however, melting, boiling and crystallization of the Te films was observed. These findings support a strictly thermal model for laser annealing and optical recording with Te based media. In addition boiling was identified as the prevalent mechanism for the loss of material.