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The two-step strategy screening based high throughput calculation is an effective method to search for highly active single atom catalysts (SACs) for efficient nitrogen reduction reaction (NRR), sharing a common feature of active metal centers MN4. However, this method faces the lack of descriptor for designing SACs due to limited data. Herein we proposed a robust empirical rule to estimate 312 credible limiting potentials for NRR over 104 C3N-supported SACs. It is found that the limiting potential scales clearly with the adsorption free energies of two key reaction intermediates (∗N2H and ∗NH2), which are significantly dependent on the structure relating descriptor φ, and the activity of SACs with and without N coordination exhibits similar catalytic efficiency. We identified four high-performance SACs having moieties of CrC2N2, CrC4, MnC4 and OsC4 with high selectivity towards NRR and competitive limiting potentials of −0.40, −0.45, −0.53 and −0.54 V, respectively. The φ values for these four SACs are around 19–25 which fall within its optimal ranges of 20–25 validating its prediction fidelity. This work broadens the discovery of two-dimensional carbon-nitrides supported SACs free of N coordination towards efficient N2 reduction and opens a new route of descriptor-guided design of efficient SACs for electrocatalysis process.

As metal-free carbon based catalysts, boron (B)-doped carbonaceous materials have proved to exhibit superior catalytic performance toward nitrogen reduction reaction. However, this strategy of heteroatom doping encounters the synthesis challenges of precise control of the doping level and homogeneous distribution of the dopants, and in particular, these materials cannot be utilized in electrochemical N2 reduction because of poor electrical conductivity. Accordingly, via first-principles calculations, we here predicted two stable two-dimensional crystalline compounds: BC6N2 and BC4N, which have small band gaps and uniform distribution of NRR active sp2-B species and holey structures. Between them, the BC6N2 monolayer originally possesses nice NRR activity with limiting potentials of -0.47 V. In the proton-rich acid medium, the electronic properties of these two B-C-N monolayers could be further tailored to exhibit a metallic characteristic by H pre-adsorption. This drastically improves the conductivity and enhances their NRR performances as reflected by the limiting potentials of -0.15, -0.34, and -0.34 V for BC6N2 via enzymatic, distal, and alternating mechanisms, respectively. Besides, NRR on BC4N through enzymatic mechanism proceeds as the limiting potential moderated from -1.20 to -0.90 V. More than that, the competing hydrogen evolution reaction can be effectively suppressed. The current investigation opens an avenue of designing a 2D crystalline phase of MFC catalysts independent of heteroatom doping and gives insightful views of surface functionalization as an impactful strategy to improve the electrocatalytic activity of metal-free catalysts.

The 2015 Microwave Theory and Techniques Society (MTT-S) International Microwave Symposium in Phoenix, Arizona, played host to a total of 16 student design competitions. One of these, sponsored jointly by the MTT-12 Technical Committee on Microwave and Millimeter-Wave Packaging and Manufacturing and the MTT-8 Technical Committee on Filters and Passive Components, was based on the design of a substrate integrated waveguide (SIW) dual-band filter.

A novel receiver architecture based on frequency translation is proposed in this paper. The receiver treats a mono-lithically integrated time-varying transmission line (TVTL) as a capacitive mixer with minimum insertion loss and noise penalty, placing it in the first stage before a fixed, narrow bandpass filter. By varying the LO/carrier frequency of the TVTL mixer, the complete received frequency band is shifted so that the desired band can be selected with a fixed bandpass filter, without suffering signal-to-noise ratio (SNR) degradation that usually occurs in a conventional heterodyne receiver with a mixer at its first stage. Measured results of both the TVTL MMIC and the testbed of the frequency translational receiver validate the proposed concept and confirm its potential for applications in software defined radios.

The concept of distributedly modulated capacitors (DMC) is proposed as a form of time-varying transmission-line structure offering nonreciprocal propagation and coupling characteristics. The nonreciprocity is achieved by taking advantage of the additional dimension of time-variance in the transmission-line property. The complete theory, based on: 1) the distributed parametric effect on a time-varying transmission line and 2) the distributed capacitive mixers, is presented with emphasis on the theoretical bounds of the isolation and insertion performances of DMC. Simulations are carried out and a prototype consisting of double-balanced varactor diodes on microstrip lines is implemented on a Rogers board. The measured results agree well with the theoretical derivations and the simulation results. It is thus confirmed that the DMC has great potential of being a circulator device with broadband, minimum insertion loss, and synthesizable isolation characteristics. It can be integrated into an RF system front-end that allows transmission and reception of signals at the same time and the same frequency.

A methodology to control the frequency transmission zeroes (FTZs) of a dual-mode resonator based elliptic-response filter is presented. Varactor loaded feeding structures are introduced for exciting a stub-perturbed ring-resonator to control the external quality factors associated with even and odd modes of the resonator. The technique allows for control of FTZs on either sides of the passband without any significant change in the bandwidth or operating frequency of the filter. Because of the high-Q nature of ring resonator, assymetric frequency responses are achieved with an in-band insertion loss 55 db at 190 MHz from the center-frequency of 2.2 GHz. Additionally, the filter is provided with tunable source-load coupling to isolate the movement of two FTZs.

Distributedly modulated capacitors (DMC) is a discrete realization of time-varying transmission line (TVTL), which offers broadband nonreciprocity through time modulation of the transmission line property and compensating the circuit loss with the parametric gain [2–4]. The low-noise, broadband behavior of the DMC and its IC compatibility makes it ideal for Simultaneous Transmit and Receiving (STAR) applications. In this paper, a MMIC implementation of the DMC is presented with its performance experimentally verified. Furthermore, a circulator prototype consisting of a pair of DMC MMICs in a balanced architecture is assembled and tested. More than 25 dB TX-to-RX isolation and less than 4 dB RX loss is demonstrated in the experiment over the frequency of 0.7 to 2.5 GHz.

The tuning range of a high-Q resonator or a bandpass filter in conventional technology is very limited and often with high insertion loss in the passband. Recently, the directional parametric conversion effect in time-varying transmission lines (TVTLs) has been utilized to develop nonmagnetic nonreciprocal components operating like circulators [1, 2]. In a TVTL, the two frequencies, the original RF and the up-converted RF, are cross-coupled through a single-tone carrier. Such property can be used for a frequency-tunable high-gain narrowband amplifier through the creation of the sharp resonance at the up-converted frequency. In this work, the concept of the frequency-tunable notch amplifier based on the TVTL is validated and demonstrated with analysis and experiments. We propose to form a feedback loop of a TVTL in one of its practical implementations called Distributedly Modulated Capacitors (DMC). The narrowband gain of the notch amplifier can be tuned over a wide frequency range by just tuning the carrier frequency that modulates the TVTL. An experiment prototype on a Rogers board demonstrated larger than 17 dB gain with a less than 10 MHz bandwidth at a center frequency tunable from 1.48 to 1.65 GHz.

Non-reciprocity in passive components is traditionally achieved with magnetic material such as ferrite. Such components are often bulky and narrowband. Another drawback of magnetic components is that they are not compatible with the current integrated circuit (IC) technology. On the other hand, active nonreciprocal components using transistors suffer from poor noise performance and power handling capability.

An indented antenna array is proposed for its low return loss and mutual coupling which may help to reduce the feedback in two-way full-duplex repeater. The essential idea is to take advantage of the different phase delays between the transmitting return and receiving paths created by the indention. An indention with linear progressive phase delays can make transmitting returns destructively combined while the receiving paths constructively combined. For the same reason, the antenna array front to back ratio can be improved as the aperture is no longer symmetrical toward both front and back sides. The reduction of transmitting to receiving (T/R) leakage due to the improved return loss and the increase of isolation due to a lower mutual coupling make these arrays ideal for full-duplex repeater applications. An eight-element indented dipole array is designed and simulated in ADS. The results demonstrate 18dB T/R isolation and at least 7dB front to back ratio improvement, which implies 14dB improvement in mutual coupling.

An indented quasi-Yagi antenna array is proposed for the purpose of achieving better impedance match by creating and combining out-of-phase return losses from each individual antenna element. The array architecture, jointly with non-reciprocal components such as circulators, can provide higher isolation between the transmitting and receiving paths in a full-duplex radio, without limiting the choices of antenna polarizations and radiation patterns. An antenna array of four meandered quasi-Yagi dipole elements with feeding network is designed and fabricated on Rogers 4350 substrate. The fabricated indented array has demonstrated 16dB average isolation improvement and 13dB return loss improvement over the frequency from 3.3GHz to 3.4GHz.

Transmission lines with time-varying reactance exhibit interesting behaviors such as parametric amplification and mixing [1]. In this paper, such a transmission line realized with distributedly modulated capacitors (DMC) is demonstrated with its active conversion gain, low noise figure as well as non-reciprocity. DMC is offered a great potential to operate in an RF front-end either as a broadband low-noise amplifier (LNA) or an integrated circulator. A practical implementation is presented with varactors and microstrip lines connected in double-balanced fashion. DMC shows in simulations that 9 dB gain with below 0.5 dB noise figure over the input signal frequency of 1 to 6 GHz is achievable when it is optimized for gain and noise performance. Alternatively when the circuit is optimized for isolation, greater than 13 dB isolation can be achieved from 500 MHz to 1.8 GHz, which has been verified with the experiment.