Nathan D. Orloff

In alcohols, the hydrogen bonded network impacts important thermophysical properties, such as boiling point and viscosity. Hydrogen bonding in alcohols is influenced by solvent accessibility (steric hindrance) of the hydroxyl group. Quantifying hydrogen bonding of alcohols is critical to understanding their dissolution and phase separation in water and other solvents. The relaxation dynamics of this network occur on multiple timescales and length scales, and dielectric spectroscopy offers a direct path to accessing the collective dynamics of the hydrogen bonded network. Here, we used microwave microfluidic spectroscopy to quantify the effects of steric hindrance of propanol and butanol isomers at 25 and 30 °C from 100 MHz to 40 GHz. The dielectric spectra were fitted with a model that includes two relaxations: a Debye relaxation (Δɛ1) and a non-Debye (Δɛ2) relaxation. Our results suggest that the relative static dielectric constant (ɛs), the Debye peak amplitude (Δɛ1), and the relaxation time constant (τ1) are all influenced by both steric hindrance and temperature. We develop a simple metric for steric hindrance in alcohols to explain the trends in the Debye relaxation time constant (τ1). Our results can be used to develop dielectric models and inform molecular simulation studies.

Dielectric spectroscopy can provide information about the relaxation dynamics and intermolecular interactions in fluids. The past century of research has resulted in many hypotheses about intermolecular interactions in alcohols, including hydrogen bonding dynamics. In spite of extensive efforts to measure the dielectric properties of alcohols, there is a range of specificity of results in the literature. The range of reported values in existing data makes it challenging to validate models of relaxation dynamics and examine trends. Here, we report the dielectric spectra of eight primary linear monohydroxy alcohols, with accompanying uncertainties, measured at 25 °C from 100 MHz to 110 GHz using microwave microfluidic spectroscopy. The spectra of alcohols were fitted with a model containing two relaxations: a dominant Debye relaxation and a higher frequency non-Debye relaxation. We validated our fitting approach by comparing the residuals to the uncertainties. We found that the Debye relaxation changes systematically as a function of the number of carbons in the alkyl chain, decreasing in magnitude and increasing to higher time constants. The non-Debye relaxation also shifts downward in frequency as the chain length increases. Overall, these results are a comprehensive set of broadband dielectric spectra that can be used to test and validate models for intermolecular interactions in alcohols.

Despite the industrial relevance of sapphire and gallium arsenide (GaAs) for electronic applications, there are few reports on the value of complex permittivity and dielectric loss at frequencies above 110 GHz using integrated circuit topologies such as coplanar waveguides. Available materials characterization reports at microwave frequencies use methods that quantify bulk material properties and may miss the effective material properties present in the substrate structure and most relevant for integrated circuits at the frequencies of interest. Using a wafer-level method to extract the complex permittivity of sapphire and GaAs to 220 GHz, we found that the complex permittivity of intrinsic GaAs (εon−wafer≈13) and the convoluted in-plane and out-of-plane complex permittivity of c-plane sapphire (εon−wafer≈10.5) did not show dispersion up to 100 GHz. This result shows the importance of wafer-level tests for sapphire, where on-wafer permittivity diverges from nominal single-axis values. Above about 100 GHz, different data analysis approaches yielded increasingly different permittivities. This result provides useful data to electronics designers, materials scientists, and physicists while also providing insight into mechanisms that could produce unexpected frequency dependence.

Multiline thru-reflect-line (mTRL) is a calibration algorithm for vector network analyzers (VNAs) that uses S-parameter measurements of transmission lines with different lengths. While the conventional thru-reflect-line algorithm chooses the line lengths based on phase relationships, choosing the line lengths in mTRL is not as straightforward because the thru is not always the common line. This work tested a simple algorithm to iteratively compute all line lengths with a recursive fraction, only assuming that the longest and the shortest line lengths are known. A cursory theoretical analysis showed that certain recursive fractions should outperform others in terms of calibration error, S-parameter uncertainty, kit compactness, and resilience to the removal of any two lines from the kit. To test this analysis, we fabricated and measured 51 different mTRL calibration kits and benchmarked them against a kit whose normalized uncertainty was optimized by brute force Monte Carlo simulations with a supercomputer. After analyzing the kits, we performed a comparative analysis to determine which recursive fraction kits most closely matched the behavior of the optimized kit. We then developed a figure of merit to capture the strengths and weaknesses of each kit, revealing that the user should choose a recursive fraction within the range of 53/128 ( $\approx 0.41$ )–37/64 ( $\approx 0.58$ ) to design line lengths for mTRL. The proposed algorithm offers a simple and practical solution for designing high-performance mTRL line kits, enhancing measurement accuracy and robustness for high-frequency measurement applications.

Electromagnetic simulations are a ubiquitous tool for microwave engineers. We benchmark simulations by their agreement with experimental results. Transmission lines are an excellent test case because they have well-established measurement methodologies and are straightforward to simulate. However, when disagreements between simulation and experiment arise, finding the root cause can be quite difficult due to the number of design parameters. Here, we investigated a fabrication-motivated model that reduces this complexity by leveraging correlations between transmission line design parameters. We validated our model by comparing its prediction of cross-sectional geometry to scanning electron microscopy (SEM) measurements. Our procedure provides a simple path for microwave engineers to improve simulations on a broad range of on-wafer devices.