Smart Air Supply Anti-Haze masks

Xiang Zheng Tu 

A smart air supply anti-haze mask, as shown in the above figure, comprises a sealed mask, a pollution filter, a micro-electric fan and a POSIFA thermal flow sensor based microcontroller. The mask is sealed to the face during inhalation and creates a breathing space by resting far away from the face. Two one - way valves are connected to the mask which are used to direct air flow in and out respectively. The pollution filter is made of multiple porous membranes and blocks against haze PM2.5 particles in the suctioned air.

The micro-electric fan moves enough filtered air to the mask through the in air flow valve.

The air is required to deliver to the mask according to an air flow waveform that is restored in the microcontroller. In order to do so the micro-electric fan is driven by a PWM signal that is send from the microcontroller. The PWM signal is generated by modulating an air flow rate signal measured by a thermal flow sensor. The thermal flow sensor can be installed in two ways. One is installed on the bask surface of the filter. In the first way the air flow rate is measured by the sensor immediately after passing the filter. In the secondly way the air flow rate is measured by the sensor immediately after the fan blowing.

In the second way the thermal flow sensor is installed in a laminar flow restrictor. Reference to the above figure, the restrictor is positioned in an air flow tube that is located between the micro-electric fan and the in one-way valve. The micro-electric fan produces a turbulent flow to the air flow tube. The restrictor consists of a plurality of collimated channels which are used in dividing the velocity components of the incoming flow stream into smaller components. Some of the velocity components cancel each other thereby presenting a more uniform velocity profile, reducing the turbulence of the flow, and allowing laminar flow passing through the channels.

As well known, laminar flow occurs at low Reynolds numbers, where viscous forces are dominant, and is characterized by smooth, constant fluid motion. For air flow in a channel, the Reynolds number is defined as

Re = (ρvD_H)/μ  = (QD_H) / (NυA)                        (1)

where:

Re is 2300 for air flow.

D_H is the hydraulic diameter of the channels (m).

Q is the volumetric air flow rate (m³/s).

A is the channel's cross-sectional area (m²).

N is the number of channels.

v is the mean velocity of air flow (m/s).

μ is the dynamic viscosity of air equaling to 1.983x10^-5 Pa·s.

ν is the kinematic viscosity of air equaling to 15.11x10^-6m²/s.

ρ is the density air equaling to 1.2754 kg/m³

The hydraulic diameter of the channels can be found by

D_H = 4A/P                                                             (2)

Where A is the cross-sectional area of the channel and P is the total perimeter of all channel walls that contact with the air flow. It should be noted that the length of the channel exposed to the flow is not included in the Reynolds number.

In the second way the air flow passes through a porous material such as a pollution filter. In this case Darcy‟s law is applicable which is stated as

Q = - (κAΔp) / (μL)                                         (3)

Where:

Q (m³/s) is the total discharge,

κ (m²) is the intrinsic permeability of the porous material,

A (m²) is the cross-sectional area to flow,

Δp (Pa) is the total pressure drop,

μ (Pa·s)  is the viscosity, and

L (m) is the length. 

Darcy‟s law is only applied for Re < 1, although it is sometimes not easy to define the pore diameter in a stringent way. Darcy’s law assumes laminar or viscous flow (creep velocity) and it does not involve the inertia term. Darcy’s law also assumes that in a porous material a large surface area is exposed to flow, hence the viscous resistance will greatly exceed acceleration forces in the flow.

So for a smart air supply anti-haze mask the thermal flow sensor is not necessary to install in a laminar flow restrictor. Instead it may directly install on the surface of the filter of the mask because the out flow of the filter is an air laminar flow. The laminar flow tends to flow without lateral mixing, and adjacent layers slide past one another. There are no cross-currents perpendicular to the direction of flow, nor eddies or swirls of flows.

As shown in the Darcy‟s law the total pressure drop (Δp) represents the viscous resistance to the flow. That is why the air flow is reduced while one wears a normal mask. There is no doubt that you want to protect you from air pollution and guard your health you will sacrifice some comfort or even though feel overly suffocating. But when wear a smart air supply anti-haze mask you will feel comfortable as usual and absolutely nothing will happen, because there is a micro electric-fan that can supply enough filtered air to you.

High Reliability of  POSIFA’s Thermal Water Flow Sensors

Xiang Zheeng Tu

 

A POSIFA thermal water flow sensor is fabricated in a silicon substrate. A combination of a heater and two thermopiles is used as sensing element of the sensor. A porous silicon layer is formed in the substrate for thermal insulation between the sensing element and the substrate, while the top layer is made of a SiO₂/Si₃N₄ stack thin film. The mechanism of water flow detection mainly depends on measuring the change in the electrical voltage of the thermopiles, associated with the heat convection transfer caused by the water flow.  In operation the sensor is heated by applying an electric voltage pulse to the heater. The pulse can be rectangular with pulse width 20ms, repeat frequency 1Hz resulting in 1.8mw power consumption. It has been measured that the Instantaneous peak temperature of the sensor is lower than 50⁰C.

It has been proved that Arrhenius' equation can be used for calculation of the failure rate of a semiconductor. The equation is expressed as

L = A exp (Ea / k T)                                                  (1)

Where

L is the lifetime of a semiconductor device

T is the absolute temperature (in kelvin)

A is the pre-exponential factor, a constant for each semiconductor device

Ea is the activation energy for each failure mechanism (in Joules mol-1)

R is the universal gas constant

Activation energy refers to the minimum amount of energy required to trigger a temperature-accelerated failure mechanism. The following table shows some activation energy values obtained for various failure mechanisms commonly encountered in the semiconductor devices.

Failure Mechanism	Accelerating Factors	Activation Energy
Oxide Film Defect	Electric Field, Temperature	0.3- 1.1 eV
Al Wire Corrosion	Humidity, Temperature, Voltage	0.7 - 0.9 eV
Al Wire Electromigration	Temperature, Current Density	0.5 – 0.7 eV

Since the sensors normally driven with electric voltage pulses the mean current density is very low. So Al wire electromigration for the sensor failure can be ignored and the main failure mechanism is Al wire correction. If voltage is applied, the leakage current between Al conductors will be added as a factor for Al corrosion. Al corrosion reaction proceeds as follows:

(a) Reaction on anode side

Under the normal ambient conditions, since the surface of “Al” is covered with oxide film, “Al” is in the passive state and exists stably. At the bias voltage application status, if the surface of the anode side adsorbs the Cl- ions diffused from the inside of the sealed resin, the Al wire protected by the passive state gibbsite may react and finally melt as:

At first, the hydroxide on the surface reacts with the Cl- ions to generate fusible salt.

Al(OH)3 + Cl- → Al(OH)2Cl + OH-                          (2)

The substrate Al exposed by this reaction reacts with the Cl- ions.

Al + 4Cl- → AlCl4 - + 3e-                                          (3)

In addition, when the sealed resin absorbs moisture, reaction with the moisture may start.

AlCl4 - + 3H2O → Al(OH)3 + 3H+ + 4Cl-              (4)

Finally Al(OH)₃ will be generated. Different from the protective oxide film, the generated Al(OH)₃ is not soluble, but has a high enough cubic expansion rate to cause cracking on the protective oxide film. So the generated Al(OH)₃ promotes corrosion.

(b) Reaction on cathode side

As the sealed resin absorbs moisture, the hydroxide ion concentration will be increased near the electrode due to oxygen reduction by application of bias and reaction generates hydrogen as

O2 + 2H2O + 4e- → 4(OH)-                                   (5)

H2O + e- → (OH)- + (1/2)H2                                 (6)

The OH- ions generated by the above reaction are diffused from the defect such as pinhole, void, crack, etc. on the Al protective oxide film to the substrate Al, and then react as:

Al + 3(OH)- → Al(OH)3 + 3e-                                (7)

The reaction on the cathode side also generates aluminum hydroxide.

The graph in above figure shows the relationship between the lifetime and the operation temperature of semiconductor devices. The red slash line is the activity energy of 0.7 eV, which represents Al wire corrosion mechanism and the red vertical line represents the typical operation temperature of the water thermal flow sensors. This means that the lifetime of the sensors is expected to be very high when compare with other semiconductor devices which need to be operated at least at 125⁰C.

Optical Coherence Topography with Tunable Cavity Surface Emitting Laser

Tu Xiang Zheng

  

US Patent 6,602,427 issued to the present author describes a micromachined optical mechanical modulator based WDM transmitter/receiver module. The Fabry-Perot cavity of the mechanical modulator is structured from a three-polysilicon-layer stack formed on the surface of a single crystalline silicon substrate. The polysilicon membrane and its supporting polysilicon beams of the cavity are cut from the top polysilicon layer of the stack and are released by selective etching of their underlying polysilicon. The etched underlying polysilicon layer is heavily doped and then converted into porous polysilicon by anodization in HF solution. The polysilicon membrane and its supporting polysilicon are finally released using a reactive ion etch process to avoid stiction often generated in a wet etch process. A conic hole is formed on the backside of the single crystalline silicon substrate for receiving an optical fiber that can be passively aligned with the Fabry-Perot cavity.

Optical coherence tomography (OCT) is a non-invasive imaging test that uses light waves to take cross-section pictures of your retina, the light-sensitive tissue lining the back of the eye. With OCT, each of the retina’s distinctive layers can be seen, allowing your ophthalmologist to map and measure their thickness. These measurements help with diagnosis and provide treatment guidance for glaucoma and retinal diseases, such as age-related macular degeneration and diabetic eye disease. OCT can also be used for intravascular imaging of plaque to assess heart disease, cancer biopsy imaging, developmental biology research, art preservation, and industrial inspection.

As shown in the above figure, a called swept-source OCT uses a wavelength-swept laser light source, that is, one whose emission sweeps back and forth across a range of wavelengths. A detector and a high speed analog-to-digital (A/D) converter complete the imaging system. The OCT has several fundamental advantages including ultrahigh imaging speeds, deep tissue penetration, Doppler OCT flow analysis, and long imaging range. With such a compact, high-performance, low-cost swept source for OCT it is possible to achieve a combination of ultrahigh sweep speeds, wide spectral tuning range, adjustability in sweep trajectory, and extremely long coherence length.

Wavelength tuning of the micromachined cavity is accomplished by applying a voltage between the top membrane and bottom membrane, across the air gap. A reverse bias voltage is used to provide the electrostatic force, which attracts the top membrane downward to the bottom membrane and shortens the air gap, thus tuning the laser wavelength toward a shorter wavelength (blue shift). It has been shown that the cavity using electrostatic force follows a 1/3 gap size rule. As the voltage is applied, the top membrane is attracted downwards with a displacement approximately equaling to 1/3 gap size. As increases further, the attractive force cannot be balanced by the mechanical spring force, and the membrane collapse onto the bottom membrane. Increasing voltage further at this point results either no movement or capacitor discharge. The top membrane can be brought back to its original position when the voltage is removed if an appropriate mechanical design is used.

The incident light to the micromachined cavity is emitted by a vertical cavity surface emitting laser. The micromachined cavity transmits a narrow band of wavelengths and rejects wavelengths outside of that band. The cavity will resonate when the following condition is met:

nd cosθ = mλ/2                         (1)

where θ is the incident light angle normal to the mirror, λ is wavelength, d is the micromachined cavity length, n is the refractive index of the medium, and m is the fringe order number. For normal incident light, with air as the medium (n = 1), the resonating micromachined cavity equals multiples of a half wavelength.

By driving the micromachined cavity with specially shaped voltage, the wavelength can be swept in time as required for swept source OCT. In classical physics, where the speeds of the top membrane of the micromachined cavity relative to the bottom membrane are lower than the velocity of laser light, the relationship between observed micromachined cavity transmitted light frequency f and the incident light frequency f₀ is expressed as

f = [(c+υ_r)/(c + υ_s)] *f₀                     (2)

  

Where c is the velocity of light, υ_r is the velocity of the top membrane relative to bottom membrane or air and υ_s is the velocity of the incident light relative to air. It can be seem that the transmitted light frequency or wavelength is decreased if two membranes of the cavity is moving away from the other.

It has been reported that the micromachined cavity can be move very fast, allowing the micron-scale cavity length to be tuned rapidly. It has demonstrated a fundamental repetition rate of 600 kHz, which for OCT purposes allows its individual scans to be acquired at rates as high as 1.2 MHz through the use of both forwards and backwards sweeps.