Porous Silicon Membranes for Removing Carbon Dioxide From Natural Gas

Tu Xiang Zheng

30 years ago I was assigned to prepare porous silicon membranes for study of removing carbon dioxide from natural gas based on Knudsen diffusion. Natural gas mainly consists of 79-84 mol % methane and 5-8% mol % Carbon dioxide. To meet specifications carbon dioxide must be removed before a natural gas can be delivered to the pipeline.

Knudsen’s diffusion occurs in a porous membrane, whose pore sizes are smaller than the mean free path of the gas molecules. The mass flux of a gas through the porous membrane can be expressed as:

J_k = D_k (∂ρ/∂L)              (1)

Where J_k is mass flux of the gas through the porous membrane, D_k is the Knudsen coefficient, ρ is the density of the gas, and L is the thickness of the membrane.

The Knudsen coefficient is defined as

D_k = d_p /3 (8R_gT/πM_g)^1/2         (2)

Where d_g is the diameter of the pores, R is the gas constant (8.3144J/mol k in SI units), M_g is the molecular weight of the gas (in units of kg/mol) and T has units of k.

Hence, for Knudsen diffusion, the square root of the inverse ration of the molecular weights of the gases will determine the mass flux of the gases through the porous membrane. As shown in the following table, the square root of the ratio of the molecular weights between the methane and the carbon dioxide is 1.66 that represents the mass flux ration between the carbon dioxide and the methane through the porous silicon membrane.

The porous silicon was prepared by anodization of silicon wafers in concentrated HF solutions. The used silicon wafers were p-type silicon wafer (0.01-0.001 Ω-cm), polished on one side and oriented along the crystalline direction. The used HF solutions were composed of HF wt 49% and ethanol and the anodization took place in a double tank cell. The anodization was carried out in the dark at a constant current density from 18 to 36 mA/cm². The obtained porous silicon membrane had a pore size from 6 to 10 nm and a porosity of about 50 %.

In order to form a porous silicon membrane first, applied current density was 36mA/cm² and anodization time was 30 minutes. As a result, a porous silicon layer with a thickness of about 30 microns was created. Then, the applied current density was abruptly increased in order to enable the porous silicon layer detached from the silicon wafer. The abrupt increase applied current density led to a high porosity layer and high released gas pressure. So the porous silicon layer was easy being detached from the silicon wafer.

As an alternative, the porous silicon membranes can be obtained by combination of forming and etching of the porous silicon. First, a thick porous silicon layer was formed in a silicon wafer and then etched in a diluted KOH solution. Secondly, a thin porous silicon layer was formed by etching the leaved silicon layer in the silicon wafer.

After fabrication of the porous silicon membrane, the sample was rinsed gently with ethanol. Then a final rinse was carried out with hexane in order to minimize the possibility of shattering of the membranes due to strong capillary forces and thermal stresses exerted when ethanol evaporates from the pores. Finally, the membranes were dried in a nitrogen flow.

Low Power and Fast Response Carbon Dioxide Gas Sensors

Tu Xiang Zheng

Carbon dioxide can cause negative health affects to humans including drowsiness and at high enough concentrations suffocation. It has been recommended that the maximum time averaged exposure to the atmosphere containing 5,000 ppm carbon dioxide is not over eight hours. As such it is highly desirable to be able to measure carbon dioxide in order to control indoor air quality and in environmental monitoring.

The present author provides a MEMS carbon dioxide gas sensor as shown in the above figure. The sensor is based on a silicon wafer and fabricated utilizing CMOS technologies. Since the sensor is required to be operated at an elevated temperature a thermal insulating pad is formed in the silicon wafer which is used to support the sensor body. Both a resistor for heating and a thermopile for temperature sensing are formed on the thermal insulating pad. Then depositing an electrical insulating layer and laying a tin dioxide layer is formed thereon. By employing such device structure with good thermal insulation to the silicon wafer, the sensor presents a series of advantages such as miniaturized size, low power consumption, and fast response.

Reducing the size of the sensor is the most effective way to reduce overall power consumption. The size of the heater of the sensor can be reduced as small as 60 µm x 800 µm. This affords field operation on a single 9V battery for an acceptable time. In addition, shortening the thermal response time enables the sensor to be operated for a very brief period during measurement. This pulse heating provides a further reduction in the power consumption. All these capabilities make the sensors ideal for the applications ranging from low power wireless to cell phones and wearable and conformal sensing systems.

The thermopile of the sensor is not able to measure the absolute temperature, but generate an output voltage proportional to the temperature difference between the heater and the silicon wafer. This is suitable for the sensor to adapt a pulse width modulation (PWM) based temperature controlled circuit. It is a key to stabilize the temperature of the sensor since the response of the sensor increases and reaches the maximum at a certain temperature, and then decreases rapidly with increasing the temperature. In the modulation circuit a microcontroller is programmed to generate different modulating 8 bit digital signal which is converted to analog signal using digital to analog converter (DAC). The analog signal is then used to control a PWM based driver circuit which drives the heater of the metal oxide gas sensor.

The pulse width modulation (PWM) based temperature controlled circuit has been reported to have three advantages:

• A cyclic temperature variation can give a unique response for each gas as rate of reaction of the different analytic gases are different at different temperature;
• Low temperature may lead to the accumulation of incompletely oxidized contaminant, which may get removed during cyclic oscillating voltage; and
• Thermal cycling can lead to improvements in sensitivity because for each gas there is a heater voltage for which it shows maximum conductance-temperature characteristics.

Micromachined Vertical Vibrating Gyroscopes

Tu Xiang Zheng

As well known, micromachined vibrating gyroscopes have gained popularity in recent years. A main application of these sensors is to determine the yaw (vehicle rotation about its vertical axis) or roll (vehicle rotation about its lengthwise horizontal axis) angle of the vehicle. These sensors also play a key role in consumer electronics for platform stabilization in camcorders and video-game headsets.

The above mentioned US patent describes a micromachined vertical vibrating gyroscope that can be used to counteract the rolling effect on a vehicle, and thus, are a preferred stabilization tool for vehicles such as airplanes, ships, and cars. The emergence of micromachining technology has generated the possibility to produce gyroscopes that present many advantages over their counterparts, such as lower cost, small size, higher performance, and lower power consumption. These advantages explain why the gyroscopes have been widely used in smart phones.

The present gyroscope is implemented by vibrating the proof mass in a direction parallel to the substrate, either lateral or rotational, and sensing vertical displacement or torsional motion due to Coriolis effect.

The gyroscope consists of three single crystal silicon assemblies: an outer single crystal silicon assembly, an intermediate single crystal silicon assembly, and an inner single crystal silicon assembly. The outer assembly includes a plurality of arc-shaped anchors arranged in a circle and extending from a single crystal silicon substrate coated with an insulating annulus thereon. The intermediate assembly is a suspended wheel concentric with the arc-shaped anchors. The inner assembly is a suspended hub concentric with the circle formed by the anchors and having no axle at its center. The three assemblies are connected to each other through several flexures. The intermediate suspended wheel is driven into rotational vibration by lateral comb capacitors. Input angular rates are measured by two vertical capacitors. The gyroscope is fabricated utilizing a bipolar-compatible process comprising steps of buried layer diffusion, selective epitaxial growth and lateral overgrowth, deep reactive ion etching, and porous silicon processing.

In operation of the gyroscope a voltage is applied to the lateral driving capacitors. The intermediate wheel is then stimulated into rotational vibration about the coordinate z-axis that is set to be vertical to the substrate plane. For the rotational vibration of the wheel, the flexures provide flexible mechanical support. As the rotational angular becomes too large the stops begin to abate the vibration so as to prevent the flexures from damaging. The lateral monitoring capacitors is used to measure the frequency and amplitude of the rotational vibration of the wheel. When the substrate experiences an angular rate about the coordinate x-axis that is set to be perpendicular to the flexures a Coriolis force is induced. The Coriolis force exerts on the inner vibrating hub and causes the hub to be rotationally vibrated about the coordinate y-axis.

In the balance state the two vertical capacitors and are designed to be completely equal. When the hub rotates about the coordinate y-axis, the two vertical capacitors are no longer equal. If the hub rotates counterclockwise, the capacitance of the vertical capacitor will increase and the capacitance of the vertical capacitor will decrease. If the rotation direction reverses, the difference of the capacitance also reverses. Since the difference of the capacitance of the two vertical capacitors depends upon the input angular rate, the input angular rate can be determined by measuring the difference of the capacitance of the two vertical capacitors.

The measurement circuit of the gyroscope can be adopted in open loop or in close loop. In open loop the amplitude of a carrier signal can be modulated by the difference of the capacitance of the two vertical capacitors. After demodulation with the carrier frequency and the driving signal frequency a DC voltage proportional to the input angular rate can be yielded as the output of the measurement circuit. In close loop the yielded signal is first fed to a rebalance circuit. The rebalance circuit then provides a rebalance voltage applying to the vertical capacitors to null the rotation of the inner vibrating hub about the coordinate y-axis. The rebalance voltage is proportional to the input angular rate. So the measurement circuit can be implemented as a Σ∆ interface circuit. 

1986 Year’s Transparent Silicon Membranes Used for Ion Implantation Self Annealing

Tu Xiang Zheng

 

The above picture shows a thin silicon membrane supported by a thick silicon frame. In order to indicate the very small thickness of the thin silicon membrane a business card is placed under the silicon membrane. A letter “POSIFA” of the business card can be seen clearly through the thin silicon membrane, which means that the membrane is very thin so as to be transparent. Actually the thin silicon membrane is n-type single crystal silicon with a 3 Ω-cm resistivity and has 1 micron in thickness and 1cm in diameter. In general when the thickness of a silicon membrane is less than 5 micron the membrane can transparent part of the visible light.

The thin silicon membrane was fabricated by the present author in early 1986 year. At that time the present author was asked to fabricate a thin single crystal silicon membrane for the studies of single crystal silicon ion implantation self annealing. Thermal annealing in a furnace is the technique normally used to remove lattice damage and restore the electrical properties of ion implanted single crystal silicon. As an alternative, the annealing can be done utilizing the heating effect produced by the same ion beam during the implantation process. The heating temperature should be very high so that the ion implanted silicon layer can enter epitaxial regrowth phase. On the other hand the cooling of the silicon layer is slow enough allowing epitaxial regrowth to be carried out. This is why the thin single crystal silicon membrane is required, which can provide an excellent thermal insulation.

Etching stop technology enables the formation of thin single crystal silicon membranes. An early etch stop technology is based on P⁺ etch stop layers. This is because the rate of silicon etching depends on the boron concentration and decreases so dramatically that anisotropic etchants, especially KOH, barely attack boron doped (P⁺) silicon layers with boron concentrations around 10^-19cm^-3. Unfortunately, heavy boron doped silicon membrane can not be used for the studies of single crystal silicon ion implantation self annealing. The base resistivity of the silicon to be implanted should be higher than 1 Ω-cm so as to be able to fabricate semiconductor devices.

A lightly doped p-n junction can be used as an etch stop by applying a bias between the wafer and the etchants. But the etchants are limited to be alkali such as KOH. It has long been known that metal impurities including alkali metals (Na, K, and Li) can affect a variety of silicon device characteristics, including junction leakage, surface and bulk recombination, emitter to collector shorting, and gate oxide integrity. In addition, in order to apply a positive voltage to the silicon wafer a metal contact needs to be made thereon, which is not accepted by the silicon device fabrication due to the same reason.

The present author did a lot of literature survey and did not find any useful reference materials. The present author knew that he faced to a tough target and a difficult challenge, but he did not give up. He tried many new ways and finally found the best one is selective formation and selective etching of porous silicon.

Porous silicon is a material which is formed by anodization that is electrochemical oxidation of single crystal silicon in concentrated hydrofluoric acid (HF) solutions. The formation reaction is highly dependent on the type and level of silicon doping, and the material can be selectively formed on particular regions of a wafer that present appropriate doping characteristics obtained by diffusion or ion implantation.

The process started with an n/n ⁺ epitaxial silicon wafers with heavily boron doped substrates. The silicon wafer is patterned on the frond side and inserted into an etching cell for electrical contacting. The silicon wafer serves as anode. Two types of etching cells are used: double-cells and single-cells. By the use of a double cell the wafer is contacted with the aid of electrolyte on both sides. That means the electrodes of HF-resistant material (platinum or silicon) are placed into electrolyte and the induced current /applied voltage goes through the electrolyte to contact the wafer. The positive potential is applied on the back side and the negative potential on the front side of the silicon wafer, where the porous layer is generated. Then the porous silicon is etched selectively so as to leave a thin silicon membrane suspending by a silicon frame of the wafer. The etchants such as a mixture of HF and H₂O₂  may be used for selective etching of porous silicon.

Image Sensing Using Micromachined Ultrasonic Sensor Arrays

Tu Xiang Zheng 

Ultrasonic sensors convert ultrasound waves to electrical signals or vice versa. They detect a wide range of materials, are not influenced by problematic surfaces, and are largely immune to environmental influences. They have many uses in medicine as well as in other various advanced technologies including electronics, chemicals and construction. As well known, an ultrasonic sensor applied to the abdomen of a pregnant woman sends ultrasonic waves into the body and receives the echoes back from the inside, which are used for making visual images. These real time images showing the appearance and movement of the fetus allow observation of the development of the fetus.

Silicon based capacitive ultrasonic sensor arrays bring revolutionary improvement in performance and represent a major advance in ultrasonic sensor technology.

These sensors benefit from the economies of scale found in semiconductor manufacturing and are well suited for high-volume applications that demand high-performance sensors at low costs. A similar sensor array can be found in the US Patent 6,359,276 B1, which is used for sensing infrared image.

As shown in the above figure, each sensor of the array comprises two electrodes facing each other, one of which is fixed and the other is movable. The two electrodes are separated by an insulating layer and an air gap. It can operate on transmit and receive mode, by converting electrical energy into acoustic energy or vice versa through the displacement of the movable electrode.

When a voltage is applied between both electrodes and the membrane is pulled down to the bottom electrode by electrostatic forces. The membrane moves until the electrostatic force has equilibrium with internal force of the membrane. AC signals cause vibrations of the thin diaphragm and generate ultrasonic waves. Furthermore, the receiver can detect an ultrasonic wave using the change of capacitance when displacement of the membrane is caused by the pressure of an arriving ultrasonic wave.

In according to this patent, the sensor array is disposed in a silicon substrate in which there already exists a CMOS circuit with readout electronics. Each sensor includes a silicon nitride membrane bridging a cavity recessed into the substrate. The membrane has four beams. The distal ends of the beams are anchored to the substrate, so that the membrane is supported by the substrate and the surface of the beams is aligned with the plane surface of the substrate. The surface of the membrane and beams is coated with silicon dioxide film. A metal layer is disposed on the surface of the silicon dioxide film. The end portions of the metal layer are disposed on the beams and keep in contact with the proximal end portions of electrical conductors which already exist on the surface of the substrate. The cavity is a narrow gap, so that the membrane can touch the bottom of the cavity without damage as it is forced to bend downward. The trenches between the membrane and the beams and between the beams and the edges of the substrate are also narrow, so that the membrane and beams can touch the edge of the substrate, as they are forced to bend in the lateral directions. 

It was reported by Andrew et al. that MEMS technology makes it possible to produce air and gas ultrasonic sensors that can operate at higher frequencies (200 kHz to 5 MHz). The ability to make microscopic structures with MEMS technology permits the fabrication of very small sensors that emit high-frequency ultrasound. The smaller the sensor implies the higher the frequency of the ultrasonic signal. This was realized by Lemmerhirt et al. They developed a CMOS based ultrasonic 32 X 32 sensor array. The array was built with CMOS process for 3D image acquisition. Each sensor has 100 µm diameter membrane with 60 µm diameter top electrode and 0.6 µm gap. The center frequency of each element is 1.8 MHz.

Wireless Meter of Methane Number and Mass Flow of Natural Gas

Tu Xiang Zheng

  

Natural gas is used in an amazing number of ways. Although it is widely seen as a cooking and heating fuel in most households, natural gas has many other energy and raw material uses that are a surprise to most people who learn about them. In 2012 about 30% of the energy consumed across the United States was obtained from natural gas. It was used to generate electricity, heat buildings, fuel vehicles, heat water, bake foods, power industrial furnaces, and even run air conditioners.

Natural gas is a gaseous mixture chemically composed by methane, smaller fractions of higher molecular weight hydrocarbons and inert gases (mainly N₂ and CO₂). The different components ratio in the gas mixture determines its physical and chemical properties and consequently, its quality. Concretely, composition fluctuations affect to properties such as methane number.

The methane number is the parameter used to quantify the quality of the natural gas.  A 100 methane composition is given 100 methane number and as the higher hydrocarbons and inert gases percentage increases the methane number decreases. It is assigned that a 100 methane composition is used as the knock resistant reference fuel. Every natural gas engine has a higher than a 65 methane number to prevent engine knocking.

For methane number measurement many different sensor techniques are available in a variety of classes. Sensor principles include electrical techniques, like electrochemical detection, or electrical detection of adsorption by induced capacitance changes, optical techniques; for instance infrared (IR) adsorption or Raman spectroscopy, chromatography, calorimetry and acoustic analyses. What most sensor techniques have in common is that their applicability into real time monitoring systems is limited, either because sensors are hard to integrate based on practical considerations like, size, cost or response time, or because the sensors rely on principles that generally do not apply to all gasses.

The present paper proposes a new method to determine on line the methane number of natural gas. The method is based on the measurement of the gas density and the correlation between the density and the methane number of natural gas. The gas density can be measured with a thermopile flow sensor combining a differential pressure sensor. These two sensors are installed in an orifice plate. When natural gas flows through the orifice plate the mass flow rate and the pressure drop of the gas flow can be measured simultaneously. Then the density of the natural gas can be calculated based on the Bernoulli equation which states that there is a relationship between the pressure drop and velocity of the natural gas flow.

The correlation between the specific gravity and the methane number of natural gases is shown as the following table. The table gives methane number: 48.1, 66.2, 76.4, 80.8, 91.4 and 100 and the volume percent of their corresponding compositions. Using the specific gravity of each composition the specific gravity of each methane number can be calculated which is also given in the table. The specific gravy of each composition of the natural gas is shown in another table. It can be seen that the methane number increases and the specific gravity of the different methane number gases decreases. It is not surprising because the specific gravity of methane is lower than all other composition of the natural gas. 

A proposed wireless natural gas meter is shown in the above figure. The meter can measure both the methane number and wirelessly send the data to a smart phone for the user to monitor the consumption and the quality of the natural gas precisely. In order to do so a key component of the meter is a thermopile flow sensor developed based on a mix of integrated circuit manufacturing and micro-machining process. Some of the advantages of the thermopile flow sensors can be listed as

• Direct mass flow sensing;
• Large dynamic range;
• Fast response;
• Excellent low flow sensitivity;
• Low power consumption;
• Small size, mass, volume;
• low cost; and
• Easy to integrate in gas or fluid transport networks.

Thermopile Flow Sensors with Differential pressure sensors for Measurements of Mass Flow Rate, Density and Void Fraction of Gas-Liquid Two-Phase Flow Fluids

Tu Xiang Zheng

Gas-liquid two-phase flow exists broadly in chemical, petroleum and metallurgical industries. The measurements of gas-liquid two-phase flow parameters in real time without separating the phases is desirable in order to reduce costs, increase production and reach excellence in oil and gas transport. Although many measurement techniques have been developed, it is yet difficult to measure some flow parameters because of the complexity of the two-phase flow. It is necessary to explore new measurement techniques.

This paper proposes a measurement technique of gas-liquid two-phase flow parameters which has the advantages of low cost, simple structure and non-intrusiveness. This technique is based on the combined use of a thermopile flow sensor and a differential pressure sensor. The setup is shown in the above figure which consists of a venture, a bypass tube, a thermopile flow sensor, and a differential pressure sensor. The thermopile flow sensor is installed at the central point of the bypass flow tube and the differential pressure sensor is used to measure the pressure difference between the inlet and the outlet pressure of the bypass tube. The mass flow rate measured by the thermopile flow sensor can be used to derive the main mass flow rate passing through the venture according to a ratio dependent on the setup structure. Knowing the mass flow rate and pressure difference the density of the gas-liquid two-phase fluid can be obtained. The void fraction of the gas-liquid two-phase fluid can be further calculated using know pure liquid density and pure gas density.  

As can be seen from the above figure, there are two flow loops: the venture loop and the bypass loop. In accordance with the nature of these two parallel loops, it is reasonable to have their pressure drop to be equal.

According to the homogeneous model of two-phase flow two phases travel at equal velocities and mix well; therefore, they can be treated as if there is only one phase.

Using Bernoulli's equation in the special case of incompressible flows (such as the flow of water or other liquid, or low speed flow of gas), the theoretical mass flow rate through the venture can be given by:

m_v = C_vA₂{2(p_up-p_down)/ρ[1-(A₂/A₁)²]}^1/2   (1)

where m_v is the mass flow rate of the fluid, C_v is the discharge coefficient = (actual flow rate) / (theoretical flow rate), A₁ and A₂ is the cross-sectional area of the venture in the indicated section of the venture, p_up, p_down are the fluid's static pressure in the indicated section of the venture, and ρ₁ is the fluid's density passing through the venture.

Similarly, the theoretical mass flow rate through the bypass tube can be given by:

m_b = C_bA₃[2(p_up-p_down)/ρ₂]^1/2                       (2)

where m_b is the mass flow rate of the fluid, C_b is the discharge, A₃ is the cross-sectional area of the bypass tube, p_up, p_down are the fluid's static pressure in the indicated section of the bypass tube, and ρ₂ is the fluid's density passing through the bypass tube.

In arriving at the homogeneous model for two-phase flow, area averaging is performed for both phases and the density ρ₁ and ρ₂ must be equal a similar ρ. Forever the void fraction α of the two-phase fluid must satisfied the relation:

  ρ = (1-α)ρ_l + αρ_v                                           (3)

where ρ_l and ρ_v are the densities of the pure liquid and the pure gas, respectively.

It is clear that a thermopile flow sensor with a differential pressure sensor can be used to measure the mass flow rate, density and void fraction of a gas-liquid two-phase flow. A proposed setup comprises a thermopile, a differential pressure sensor, a venture tube and a bypass tube. Using the venture equation and the homogeneous model form the mass flow rate measured by the thermopile flow sensor and the pressure difference measured by the differential pressure sensor the density and the void fraction of the two-phase fluid can be derived respectively. This technique requires the gas and the liquid mixes so well that the mixture can be seen as one single phase approximately. The mixing degree of the gas and the liquid affects the measuring accuracy. For un-complete mixing two-phase fluid correction is needed to improve the accuracy.